Cambridge IGCSE Combined and Co-ordinated Sciences

Sexually transmitted infections (STIs)

Oxygen and nutrients such as glucose and amino embryo’s blood to that of the mother. Blood
acids pass across the placenta to the embryo’s entering the placenta from the mother does not mix
bloodstream. Carbon dioxide passes from the with the embryo’s blood.

oviduct placenta placenta placenta placenta

implanted umbilical umbilical embryo
embryo cord cord (470 mm)
(12 mm) embryo uterus
uterus (60 mm) amnion
cervix embryo amnion
amnion (250 mm)
vagina (water sac) amniotic cervix
cavity

uterus

a 5 weeks b 10 weeks c 5 months d 35 weeks (a few weeks before birth)

Figure B10.28  Growth and development in the uterus (not to scale).

●● Sexually transmitted product that contains a clotting factor. Before the
infections (STIs) risks were recognised, infected carriers sometimes
donated blood, which was used to produce the
A sexually transmitted infection is an infection that clotting factor.
is transmitted via body fluids through sexual contact.
Babies born to HIV carriers may become infected
AIDS and HIV with HIV, either in the uterus or during birth
or from the mother’s milk. The rate of infection
The initials of AIDS stand for acquired immune varies from about 40% in parts of Africa to 14% in
deficiency syndrome. (A ‘syndrome’ is a pattern Europe. If the mother is given drug therapy during
of symptoms associated with a particular disease.) labour and the baby within 3 days, this method of
The virus that causes AIDS is the human transmission is reduced.
immunodeficiency virus (HIV).
There is no evidence to suggest that the disease
After a person has been infected, years may pass can be passed on by droplets, by saliva or by normal
before symptoms develop. So people may carry the everyday contact.
virus yet not show any symptoms. They can still infect
other people, however. It is not known for certain Controlling the spread of STIs
what proportion of HIV carriers will eventually
develop AIDS: perhaps 30–50%, or more. The best way to avoid sexually transmitted infections
is to avoid having sexual intercourse with an infected
HIV is transmitted by direct infection of the blood. person. However, the symptoms of the disease are
Drug users who share needles contaminated with often not obvious and it is difficult to recognise an
infected blood run a high risk of the disease. It can infected individual. So the disease is avoided by not
also be transmitted sexually, both between men and having sexual intercourse with a person who might
women and, especially, between homosexual men have the disease.
who practise anal intercourse. Prostitutes, who have
many sexual partners, are at risk of being infected The risk of catching a sexually transmitted disease
if they have sex without using condoms and are, can be greatly reduced if the man uses a condom or
therefore, a potential source of HIV to others. if a woman uses a femidom. These act as barriers to
bacteria or viruses.
Haemophiliacs have also fallen victim to AIDS.
Haemophiliacs have to inject themselves with a blood If a person suspects that he or she has caught
a sexually transmitted disease, treatment must be

137

B10 RePRODuCtiOn

sought at once. Information about treatment can be STIs which are caused by a bacterium, such
obtained by phoning one of the numbers listed under as syphilis and gonorrhoea, can be treated with
‘Venereal Disease’ or ‘Health Information Service’ antibiotics if the symptoms are recognised early
in the telephone directory. Treatment is always enough. However, HIV is viral so antibiotics are not
confidential. The patients must, however, ensure that effective.
anyone they have had sexual contact with also gets
treatment. There is no point in one partner being Blood used for transfusions should be screened for
cured if the other is still infected. HIV. Mothers with HIV should not breast-feed their
babies.

Questions place where fertilisation occurs Mammals
zygote grows into
9 List the structures, in the correct order, through which the Now complete the other two columns.
sperm must pass from the time they are produced in the
testis, to the time they leave the urethra. Flowering plants

10 What structures are shown in Figure B10.24, but are not male reproductive
shown in Figure B10.23? organs

11 In what ways does a zygote differ from any other cell in female reproductive
the body? organs

12 List, in the correct order, the parts of the female male gamete, etc.
reproductive system through which sperm must pass
before reaching and fertilising an ovum. 15 How do sperm differ from ova in their structure (see
Figure B10.21)?
13 State exactly what happens at the moment of fertilisation.
14 Draw up a table with three columns as shown below. In 16 In what ways will the composition of the blood in the
umbilical vein differ from that in the umbilical artery?
the first column write:
male reproductive organs 17 An embryo is surrounded with fluid, its lungs are
female reproductive organs filled with fluid and it cannot breathe. Why doesn’t it
male gamete suffocate?
female gamete

Checklist Sexual reproduction in plants

After studying Chapter B10 you should know and understand ● Flowers contain the reproductive organs of plants.
the following. ● The stamens are the male organs. They produce pollen grains

Asexual and sexual reproduction which contain the male gamete.
● The carpels are the female organs. They produce ovules
● Asexual reproduction is the process resulting in the
production of genetically identical offspring from one parent. which contain the female gamete and will form the seeds.
● The flowers of most plant species contain male and female
● Asexual reproduction occurs without gametes or fertilisation.
● Fungi can reproduce asexually by single-celled spores. organs. A few species have unisexual flowers.
● Many flowering plants reproduce asexually by vegetative ● Brightly coloured petals attract insects, which pollinate the

propagation. flower.
● Pollination is the transfer of pollen from the anthers of one
● Asexual reproduction keeps the characteristics of the
organism the same from one generation to the next, but flower to the stigma of the same or another plant.
does not result in variation, to cope with environmental ● Pollination may be done by insects or by the wind.
change. ● Flowers which are pollinated by insects are usually brightly

● Sexual reproduction is the process involving the fusion of the coloured and have nectar.
nuclei of two gametes (sex cells) to form a zygote and the ● Fertilisation occurs when a pollen tube grows from a pollen
production of offspring that are genetically different from
each other. grain into the ovary and up to an ovule. The pollen nucleus
passes down the tube and fuses with the ovule nucleus.
● The nuclei of the gametes are haploid, while the nucleus of ● Temperature and the amount of water and oxygen available
a zygote is diploid. have an effect on germination.

● There are advantages and disadvantages of sexual ● Flowers which are pollinated by the wind are usually small
reproduction to a species. and green. Their stigmas and anthers hang outside the
flower where they are exposed to air movements.

138

Sexually transmitted infections (STIs)

● The pollen grains of insect-pollinated flowers are usually ● In the menstrual cycle, each month the uterus lining thickens
light and smooth, while those of wind-pollinated flowers up in readiness to receive the fertilised ovum. If an ovum is
are usually round and sticky, or covered in spikes. not fertilised, the lining and some blood is lost through the
vagina. This is menstruation.
● Flowers which are pollinated by insects are usually large
and brightly coloured. ● The fertilised ovum (zygote) divides into many cells and
becomes embedded in the lining of the uterus. Here it
● Their stigmas and anthers are found inside the flower. grows into an embryo.

Sexual reproduction in humans ● The embryo gets its food and oxygen from its mother.
● The placenta and umbilical cord are involved in exchange
● The male reproductive system includes the testes, scrotum,
sperm ducts, prostate glands, urethra and penis and each of materials between the mother and fetus. Some toxins
part has a specific function. and viruses can also be passed across and affect the fetus.
● The amniotic sac surrounds the fetus and prevents the
● The female reproductive system includes the ovaries, entry of bacteria.
oviducts, uterus, cervix and vagina and each part has a ● The sac contains amniotic fluid. The fluid supports the
specific function. fetus, protecting it from physical damage. It absorbs
excretory materials released by the fetus.
● Fertilisation happens when a sperm enters an ovum and the
sperm and egg nuclei join up (fuse). ● A sexually transmitted infection is an infection transmitted via
bodily fluids through sexual contact.
● Eggs and sperm are different in size, structure, mobility
and numbers produced. ● HIV is an example of an STI and HIV infection may lead to AIDS.
● HIV can be transmitted in a number of ways.
● Sperm and eggs have special features to adapt them for ● The spread of HIV can be controlled.
their functions.

139

B11 Inheritance

Co-ordinated ● Distinguish between phenotypic variation and genetic
variation
● Define inheritance
● Define chromosome ● S tate that phenotypic variation is caused by both genetic
● Define gene and environmental factors
● Define allele
● Describe the inheritance of sex in humans with reference to ● State that continuous variation results in a range of
phenotypes between two extremes, e.g. height in humans
XX and XY chromosomes
● State that discontinuous variation is mostly caused by
● Define a haploid nucleus genes alone, e.g. A, B, AB and O blood groups in humans
● Define a diploid nucleus
● S tate that in a diploid cell, chromosomes are arranged in ● State that discontinuous variation results in a limited number
of phenotypes with no intermediates, e.g. tongue rolling
pairs and in a human diploid cell there are 23 pairs
● D efine mitosis ● Record and present the results of investigations into
● S tate that the exact duplication of chromosomes occurs continuous and discontinuous variation

before mitosis ● D efine mutation
● State the role of mitosis in growth, repair of damaged ● State that ionising radiation and some chemicals increase

tissues, replacement of cells and asexual reproduction the rate of mutation
● Define meiosis
● State that meiosis is involved in the production of gametes ● Describe natural selection with reference to: variation within
populations; production of many offspring; competition for
● Define genotype resources; struggle for survival; reproduction by individuals
● Define phenotype that are better adapted to the environment than others;
● Define homozygous passing on of their alleles to the next generation
● State that two identical homozygous individuals that breed
● Describe evolution as the change in adaptive features of a
together will be pure-breeding population over time as the result of natural selection
● Define heterozygous
● State that a heterozygous individual will not be pure-breeding ● D efine the process of adaptation
● Define dominant ● Describe the development of strains of antibiotic resistant
● Define recessive
● Use genetic diagrams to predict the results of monohybrid bacteria as an example of evolution by natural selection

crosses and calculate phenotypic ratios, limited to 1:1 and ● Describe selective breeding with reference to: selection by
3:1 ratios humans of individuals with desirable features; crossing these
● Use Punnett squares in crosses which result in more than individuals to produce the next generation; selection of
one genotype to work out and show the possible different offspring showing the desirable features
genotypes
● S tate the differences between natural and artificial
● Interpret pedigree diagrams for the inheritance of a given selection
characteristic
● Outline how selective breeding by artificial selection is
● Define variation carried out over many generations to improve crop plants
and domesticated animals

●● Inheritance ●● Chromosomes and genes

Key definition Key definitions
Inheritance is the transmission of genetic information from A chromosome is a thread of DNA, carrying genetic information

generation to generation. in the form of genes.
A gene is a length of DNA that codes for a protein.
We often talk about people inheriting certain An allele is a version of a gene.
characteristics: ‘Nathan has inherited his father’s curly
hair’, or ‘Fatima has inherited her mother’s brown Inside a nucleus are thread-like structures called
eyes’. We expect tall parents to have tall children. The chromosomes which can be seen most clearly at the
inheritance of such characteristics is called heredity time when the cell is dividing. Each chromosome
and the branch of biology that studies how heredity has certain characteristics when ready to divide:
works is called genetics. there are two chromatids, joined at one point called

140

Chromosomes and genes

centromere The inheritance of sex

chromatid Whether you are a male or female depends on the
pair of chromosomes called the ‘sex chromosomes’.
gene for hair In females, the two sex chromosomes, called
colour the X chromosomes, are the same size as each
other. In males, the two sex chromosomes are of
Figure B11.1  Structure of a chromosome. different sizes. One corresponds to the female sex
chromosomes and is called the X chromosome. The
a centromere (Figure B11.1). Each chromatid other is smaller and is called the Y chromosome. So
is a string of genes, coding for the person’s the female cells contain XX and male cells contain XY.
characteristics. The other chromatid carries the same
genes in the same order. A process takes place in the female’s ovaries
The genes that occupy corresponding positions on and the male’s testes which makes gametes (sex
homologous chromosomes and control the same cells). These contain half the normal number of
characteristic are called allelomorphic genes, or chromosomes. During the process in the female’s
alleles. The word ‘allelomorph ’ means ‘alternative ovaries, each ovum receives one of the chromosomes,
form’. For example, there are two alternative forms so all the ova are the same for this. The process in
of a gene for eye colour. One allele produces brown the male’s testes results in 50% of the sperms getting
eyes and one allele produces blue eyes. an X chromosome and 50% getting a Y chromosome
(Figure B11.2). If an X sperm fertilises the ovum,
the zygote will be XX and will grow into a girl. If a Y
sperm fertilises the ovum, the zygote will be XY and
will grow into a boy. There is an equal chance of an
X or a Y sperm fertilising an ovum, so the numbers of
girl and boy babies are more or less the same.

Figure B11.3 on the next page shows how sex is
inherited.

FEMALE MALE

XX XY
XY
ovum- sperm-
producing producing
cell cell

X process of
sex cell
ova
formation,
involving halving of

the chromosome
number

X

sperms

all ova will contain one X chromosome half the sperms
will contain an
Figure B11.2  Determination of sex. Note that: X chromosome and
i only the X and Y chromosomes are shown half will carry
ii details of meiosis have been omitted a Y chromosome
iii i n fact, four gametes are produced in each
case, but two are sufficient to show the
distribution of X and Y chromosomes.

141

B11 inheRitanCe

parents male × female
XY XX
sex chromosomes
of parents XY × XX

gametes (sex cells)

sex chromosomes XX XX XY XY
of children

sex of female female male male
children
The ratio is 1 female : 1 male Figure B11.4 Human karyotype.
Figure B11.3 Determination of sex.

Number of chromosomes

Key definitions
A haploid nucleus is a nucleus containing a single set of

unpaired chromosomes present, for example, in sperm

and egg cells.
A diploid nucleus is a nucleus containing two sets of

chromosomes present, for example, in body cells.

Figure B11.4 is a karyotype of a human body kangaroo (12) human (46)
cell because there are 23 pairs of chromosomes
present (they come from a diploid cell). Because domestic fowl (36) fruit fly (8)
the chromosomes are in pairs, the diploid number is
always an even number. The karyotype of a sperm cell Figure B11.5 Chromosomes of different species. Note that the
would show 23 single chromosomes (they come from chromosomes are always in pairs.
a haploid cell). The sex chromosome would be either
X or Y. The chromosomes have different shapes and
sizes and can be recognised by a trained observer.

There is a fixed number of chromosomes in
each species. Human body cells each contain
46 chromosomes, mouse cells contain 40 and
garden pea cells 14 (see also Figure B11.5).

The number of chromosomes in a species
is the same in all of its body cells. There are
46 chromosomes in each of your liver cells, in
every nerve cell, skin cell and so on.

Questions 4 What is the diploid number in humans?
5 Which of the following cells would be haploid and which
1 A married couple has four girl children but no boys. This
does not mean that the husband produces only X sperms. diploid: white blood cell, male cell in pollen grain, guard
Explain why not.
cell, root hair, ovum, sperm, skin cell, egg cell in ovule?
2 Which sex chromosome determines the sex of a baby? 6 How many chromosomes would be present in:
Explain your answer.
a a mouse sperm cell
3 How many chromosomes would there be in the nucleus of: b a mouse ovum?
a a human muscle cell
b a mouse kidney cell
c a human skin cell that has just been produced by mitosis
d a kangaroo sperm cell?

142

mitosis

●● Mitosis

Key definitions replacement of cells all rely on mitosis. Organisms
Mitosis is nuclear division giving rise to genetically that reproduce asexually (see Chapter B10) also use
mitosis to create more cells.
identical cells.
The process of cell division in an animal cell is shown
The process of mitosis is important in growth. in Figure B11.6. The events in a plant cells are shown
We all started off as a single cell (a zygote). That in Figure B11.7. Because of the cell wall, the cytoplasm
cell divided into two cells, then four and so on, to cannot simply pinch off in the middle, and a new cell
create the organism we are now, made up of millions wall has to be laid down between the daughter cells.
of cells. Cells have a finite life: they wear out or Also, a new vacuole has to form. Organelles such as
become damaged, so they need to be replaced chloroplasts are able to divide and are shared more or
constantly. The processes of growth, repair and less equally between the daughter cells at cell division.

a Animal cell about to b The nucleus divides first. c The daughter nuclei separate d Two cells are formed – one
divide. and the cytoplasm pinches may keep the ability to
off between the nuclei. divide, and the other may
become specialised.

Figure B11.6 Cell division in an animal cell.

a A plant cell b The nucleus divides c The cytoplasm adds layers of d The vacuoles join up to form one
about to divide first. A new cell wall cellulose on each side of the new vacuole. This takes in water and makes
has a large develops and cell wall. Vacuoles form in the the cell bigger. The other cell will divide
nucleus and separates the two cytoplasm of one cell. again.
no vacuole. cells.

Figure B11.7 Cell division in a plant cell.

The process of mitosis

To understand how the ‘instructions’ are passed chromosomes. Although they are present in the nucleus
from cell to cell, we need to look in more detail at all the time, they show up clearly only at cell division
what happens when the zygote divides and produces because at this time they get shorter and thicker.
an organism consisting of thousands of cells. This
type of cell division is called mitosis. It takes place Each chromosome duplicates itself and is seen
not only in a zygote but in all growing tissues. to be made up of two parallel strands, called
chromatids (Figure B11.1). When the nucleus
When a cell is not dividing, there is very little detailed divides into two, one chromatid from each
structure to be seen in the nucleus even if it is treated chromosome goes into each daughter nucleus.
with special dyes called stains. Just before cell division, The chromatids in each nucleus now become
however, a number of long, thread-like structures chromosomes and later they will make copies
appear in the nucleus and show up very clearly when of themselves ready for the next cell division.
the nucleus is stained (Figures B11.8 and B11.9 on The process of copying is called replication
the next page). These thread-like structures are called because each chromosome makes a replica (an exact

143

B11 inheRitanCe

copy) of itself. As Figure B11.8 is a simplified (b) (f)
diagram of mitosis, only two chromosomes are
shown, but there are always more than this.
Human cells contain 46 chromosomes.

nucleus with two cell nuclear (d)
membrane
chromosomes membrane

a Just before the cell divides, b The chromosomes get Figure B11.9 Mitosis in a root tip (×500). The letters refer to the
chromosomes appear in the shorter and thicker. stages described in Figure B11.8. (The tissue has been squashed to
nucleus. fibres pull separate the cells.)
two two chromatids apart
chromatids chromatids ●● Meiosis

c Each chromosome is d The nuclear membrane Key definition
now seen to consist of disappears and the Meiosis is reduction division in which the chromosome
two chromatids. chromatids are pulled apart
to opposite ends of the cell. number is halved from diploid to haploid, resulting in
nuclear membrane genetically different cells.
forms nucleus with two ‘daughter’
chromosomes cell The process of meiosis takes place in the gonads
of animals (e.g. the testes and ovaries of mammals,
e A nuclear membrane forms f Cell division completed, and the anthers and ovules of flowering plants).
round each set of chromatids, giving two ‘daughter’ cells, The cells formed are gametes (sperm and egg cells
and the cell starts to divide. each containing the same in mammals; egg cells and pollen grain nuclei in
number of chromosomes flowering plants). Gametes are different from other
as the parent cell. cells because they have half the normal number of
chromosomes (they are haploid).
Figure B11.8 Mitosis. Only two chromosomes are shown. Three
of the stages described here are shown in Figure B11.9. Details of Gamete production and chromosomes
the stages are not required.
The genes on the chromosomes carry the instructions
that turn a single-cell zygote into a bird or a rabbit
or an oak tree. The zygote is formed at fertilisation,
when a male gamete fuses with a female gamete. Each
gamete brings a set of chromosomes to the zygote.
The gametes, therefore, must each contain only half
the diploid number of chromosomes, otherwise the
chromosome number would double each time an
organism reproduced sexually. Each human sperm cell
contains 23 chromosomes and each human ovum has
23 chromosomes. When the sperm and ovum fuse at
fertilisation (Chapter B10), the diploid number of 46
(23 + 23) chromosomes is produced (Figure B11.10).

The process of cell division that gives rise to
gametes is different from mitosis because it results
in the cells containing only half the diploid number
of chromosomes. This number is called the haploid
number and the process of cell division that gives rise
to gametes is called meiosis.

Meiosis takes place only in reproductive organs.

144

monohybrid inheritance

23 Patterns of inheritance

46 meiosis meiosis 46 A knowledge of mitosis and meiosis allows us to
ovum- explain, at least to some extent, how heredity works.
sperm- 23 23 producing The allele in a mother’s body cells that causes her
producing sperms ova (only cell to have brown eyes may be present on one of the
cell one develops) chromosomes in each ovum she produces. If the
23 23 father’s sperm cell contains an allele for brown eyes
fertilisation on the corresponding chromosome, the zygote will
receive an allele for brown eyes from each parent.
46 zygote These alleles will be reproduced by mitosis in all the
embryo’s body cells and when the embryo’s eyes
46 46 cell division develop, the alleles will make the cells of the iris
by mitosis produce brown pigment (melanin) and the child
will have brown eyes. In a similar way, the child may
46 46 46 46 embryo receive alleles for curly hair.
46 46 46 46
Figure B11.11 shows this happening, but it does
46 46 46 46 not, of course, show all the other chromosomes with
46 46 46 46 thousands of genes for producing the enzymes, making
different types of cell and all the other processes that
control the development of the organism.

allele for curly hair

Figure B11.10 Chromosomes in gamete production and fertilisation. C C
B
allele for B
brown eyes sperm ovum

Question C

7 In which human tissues would you expect mitosis to be CB
going on, in:
a a 5-year-old child B
b an adult?
zygote
●● Monohybrid inheritance Figure B11.11 Fertilisation. Fertilisation restores the
diploid number of chromosomes and combines the alleles
Key definitions from the mother and father.
Genotype is the genetic make-up of an organism in terms of
Monohybrid inheritance
the alleles present.
Phenotype is the observable features of an organism. Because it is impossible to follow the inheritance
Homozygous means having two identical alleles of a of the thousands of characteristics controlled by
genes, it is usual to start with the study of a single
particular gene. gene that controls one characteristic. We have used
Heterozygous means having two different alleles of a eye colour as an example so far. Probably more
than one allele pair is involved, but the simplified
particular gene. example will serve our purpose. It has already
Dominant means an allele that is expressed if it is present. been explained how an allele for brown eyes from
Recessive means an allele that is only expressed when there is each parent results in the child having brown eyes.
Suppose, however, that the mother has blue eyes
no dominant allele of the gene present. and the father brown eyes. The child might receive
an allele for blue eyes from its mother and an allele
for brown eyes from its father (Figure B11.12).

145

B11 Inheritance

allele for brown eyes partners. In the same way L could represent the allele
for long fur and l the allele for short fur.
B b allele for
blue eyes Breeding true
sperm
from ovum from A white rabbit must have both the recessive alleles b and
father mother b. If it had B and b, the dominant allele for black (B)
would override the allele for white (b) and produce a
B black rabbit. A black rabbit, on the other hand, could be
b either BB or Bb and, by just looking at the rabbit, you
could not tell the difference. When a male black rabbit
zygote BB produces sperm, each one of the pair of chromosomes
Figure B11.12  Combination of alleles in the zygote (only one carrying the B alleles will end up in different sperm
chromosome is shown). The zygote has both alleles for eye colour; cells. Since the alleles are the same, all the sperm will
the child will have brown eyes. have the B allele for black fur (Figure B11.13a).

If this happens, the child will have brown eyes. The A black rabbit BB is called a pure-breeding black and is
allele for brown eyes is said to be dominant to the said to be homozygous for black coat colour (‘homo-’
allele for blue eyes. Although the allele for blue eyes means ‘the same’). If this rabbit mates with another
is present in all the child’s cells, it is not expressed. black (BB) rabbit, all the babies will be black because all
It is said to be recessive to brown. will receive a dominant allele for black fur. When all the
offspring have the same characteristic as the parents, this
Eye colour is a useful ‘model’ for explaining is called ‘breeding true’ for this characteristic. In other
inheritance but it is not wholly reliable because ‘blue’ words, two identical homozygous individuals that
eyes vary in colour and sometimes contain small breed together will be pure-breeding.
amounts of brown pigment.
When a Bb black rabbit produces gametes by
This example illustrates the following important meiosis, the chromosomes with the B allele and the
points: chromosomes with the b allele will end up in different
gametes. So 50% of the sperm cells will carry B
l There is a pair of alleles for each characteristic, one alleles and 50% will carry b alleles (Figure B11.13b).
allele from each parent. Similarly, in the female, 50% of the ova will have a B
allele and 50% will have a b allele. So, a heterozygous
l Although the allele pairs control the same individual will not be pure breeding. If a b sperm
characteristic, e.g. eye colour, they may have fertilises a b ovum, the offspring, with two b alleles
different effects. One tries to produce blue eyes, (bb), will be white. The black Bb rabbits are not
the other tries to produce brown eyes. true-breeding because they may produce some white
babies as well as black ones. The Bb rabbits are called
l Often one allele is dominant over the other. heterozygous (‘hetero-’ means ‘different’).
l The alleles of each pair are on corresponding chromo­
The black BB rabbits are homozygous dominant.
somes and occupy corresponding positions. For The white bb rabbits are homozygous recessive.
example, in Figure B11.11 the alleles for eye colour
are shown in the corresponding position on the two BB black male Bb
short chromosomes and the alleles for hair curliness are rabbits meiosis
in corresponding positions on the two long chromo­ 50 %
somes. In diagrams and explanations of heredity: meiosis sperms
l alleles are represented by letters have B
l alleles controlling the same characteristic are B all sperms B B 50 % b
carry B have b
given the same letter
l the dominant allele is given the capital letter. a pure-breeding b not pure-breeding
Figure B11.13  Pure breeding.
For example, in rabbits, the dominant allele for black
fur is labelled B. The recessive allele for white fur
is labelled b to show that it corresponds to B for
black fur. If it were labelled w, we would not see any
connection between B and w. B and b are obvious

146

Monohybrid inheritance

Genotype and phenotype and what we really expect is that the ratio (proportion)
of black to brown will be close to 3:1.
The two kinds of black rabbit BB and Bb are said to
have the same phenotype. This is because their coat If the mouse had 13 babies, you might expect nine
colours look exactly the same. However, because they black and four brown, or eight black and five brown.
have different allele pairs for coat colour they are said to Even if she had 16 babies you would not expect to find
have different genotypes, i.e. different combinations of exactly 12 black and four brown because whether a B or
alleles. One genotype is BB and the other is Bb. b sperm fertilises a B or b ovum is a matter of chance. If
you spun ten coins, you would not expect to get exactly
You and your brother might both be brown-eyed five heads and five tails. You would not be surprised at
phenotypes but your genotype could be BB and his six heads and four tails or even seven heads and three
could be Bb. You would be homozygous dominant for tails. In the same way, we would not be surprised at 14
brown eyes; he would be heterozygous for eye colour. black and two brown mice in a litter of 16.

The three to one ratio BB bb
homozygous black male homozygous brown female
The result of a mating between a true-breeding
(homozygous) black mouse (BB) and a true-breeding cell BB bb cell
(homozygous) brown mouse (bb) is shown in in in
Figure B11.14a. The illustration is greatly simplified testis ovary
because it shows only one pair of the 20 pairs of
mouse chromosomes and only one pair of alleles on BB bb
the chromosomes.
sperms ova
Because black is dominant to brown, all the
offspring from this mating will be black phenotypes, b
because they all receive the dominant allele for black B
fur from the father. Their genotypes, however, will be
Bb because they all receive the recessive b allele from (all possible
the mother. They are heterozygous for coat colour. combinations
The offspring resulting from this first mating are are the same)
called the F1 generation.
Bb
Figure B11.14b (on the next page) shows what
happens when these heterozygous, F1 black mice Bb Bb Bb Bb
are mated together to produce what is called the F2
generation. Each sperm or ovum produced by meiosis a all the F1 generation are heterozygous black
can contain only one of the alleles for coat colour, either Figure B11.14  Inheritance of coat colour in mice.
B or b. So there are two kinds of sperm cell, one kind
with the B allele and one kind with the b allele. There are
also two kinds of ovum, with either B or b alleles. When
fertilisation occurs, there is no way of telling whether a
b or a B sperm will fertilise a B or a b ovum, so we have
to look at all the possible combinations as follows:

l A b sperm fertilises a B ovum. Result: bB zygote.
l A b sperm fertilises a b ovum. Result: bb zygote.
l A B sperm fertilises a B ovum. Result: BB zygote.
l A B sperm fertilises a b ovum. Result: Bb zygote.

There is no difference between bB and Bb, so there
are three possible genotypes in the offspring – BB,
Bb and bb. There are only two phenotypes – black
(BB or Bb) and brown (bb). So, according to the laws
of chance, we would expect three black baby mice and
one brown. Mice usually have more than four offspring

147

B11 Inheritance Bb Bb
heterozygous black male heterozygous black female
PARENTS
cell Bb Bb cell
MEIOSIS in in
testis ovary

GAMETES sperms (two B b B b ova (two
possibilities) possibilities)

FERTILISATION B b B b b (four possible
B B b combinations)

POSSIBLE BB bB Bb bb
ZYGOTES

OFFSPRING

BB bB Bb bb

b the probable ratio of coat colours in the F2 generation is 3 black : 1 brown

Figure B11.14  Inheritance of coat colour in mice (continued). Tt

Tt

T TT Tt
Tt Tt tt

t

Figure B11.15  F2 hybrids in maize. In the two left-hand cobs, the grain Figure B11.16  Using a Punnett square to predict the outcomes of a
colour phenotypes appear in a 3:1 ratio (try counting single rows in the lighter genetic cross.
cob). What was the colour of the parental grains for each of these cobs?

148

Monohybrid inheritance

To decide whether there really is a 3:1 ratio, we homozygous recessive (bb). Figure B11.17 shows what
need a lot of results. These may come either from happens. Half the gametes from a black heterozygous
breeding the same pair of mice together for a year mouse would carry the B allele and half would have the
or so to produce many litters, or from mating 20 b allele. The bb mouse will produce gametes with only
black and 20 brown mice, crossing the offspring and the recessive b allele. Half the offspring from the cross
adding up the number of black and brown babies in will, on average, be brown homozygotes, bb, and half
the F2 families (see also Figure B11.15). will be black heterozygotes, Bb.

When working out the results of a genetic cross, phenotypes of parents black mouse brown mouse
it is useful to display the outcomes in a ‘Punnett genotypes of parents Bb bb
square’ (Figure B11.16). This a box divided into
four compartments. The two boxes along the top gametes Bb bb
are labelled with the genotypes of the gametes of Bb
one parent. The genotypes are circled to show they
are gametes. The parent’s genotype is written above Bb
the gametes. The boxes down the left-hand side are
labelled with the genotypes of the gametes of the Punnett square
other parent. The parent’s genotype is written to
the left. The genotypes of the offspring can then be b Bb bb
predicted by completing the four boxes, as shown. Bb bb
In this example, two heterozygous tall organisms bb
(Tt) are the parents. The genotypes of the offspring b
are TT, Tt, Tt and tt. We know that the allele T is
dominant because the parents are tall, although they F1 genotypes 2 Bb , 2 bb
carry both tall and dwarf alleles. So, the phenotypes
of the offspring will be three tall to one dwarf. F1 phenotypes black brown

The one to one ratio Ratio 1: 1

This is achieved when a heterozygous organism such as Figure B11.17 Crossing a heterozygous organism with a
a black mouse (Bb) is crossed with an organism which is homozygous recessive organism.

Pedigree diagrams and inheritance A sufferer of cystic fibrosis has received two
recessive alleles (cc); one from each parent. A carrier
The term pedigree often refers to the pure breeding of the disease has one normal allele and one recessive
nature of animals, but is also used to describe human allele (Cc). This condition is not uncommon: 1 in
inheritance. Pedigree diagrams are similar to family trees 25 people of European descent is a carrier. A normal
and can be used to demonstrate how genetic diseases person has two normal alleles (CC).
can be inherited. They include symbols to indicate
whether individuals are male or female and what their A normal man (CC) who has children with a
genotype is for a particular genetic characteristic. normal woman (CC) will produce 100% normal
children (all CC).
One genetic disease is called cystic fibrosis. The
symptom of cystic fibrosis is the production of If one parent is a carrier for cystic fibrosis (Cc)
very sticky mucus in the lungs. This makes gas and the other parent is normal (CC), 50% of their
exchange more difficult and traps microbes so children are likely to be carriers (Cc) and 50% will be
the cystic fibrosis sufferer gets respiratory diseases normal (CC).
more frequently. Sticky mucus can also block ducts
responsible for transferring enzymes into the small However, if both parents are carriers, then the
intestine and can block reproductive ducts, resulting likely ratio of offspring is one normal (CC): two
in lower fertility or sterility. Cystic fibrosis sufferers carriers (Cc): one cystic fibrosis sufferer (cc). So
tend to have a much shorter life span than normal, there is a 1 in 4 chance of a child born to these
although treatment of the disease is becoming more parents having cystic fibrosis.
effective and average life expectancies are improving.
The pedigree diagram (Figure B11.18 on the next
page) shows the inheritance of cystic fibrosis in a

149

B11 inheRitanCe

family. Parents Alfred and Mildred are married and inheritance of cystic fibrosis. Alternatives may be to
both are cystic fibrosis carriers. However, because foster or adopt children instead.
carriers have no symptoms of the disease, they may
be unaware that they have defective alleles for cystic Alfred Mildred
fibrosis. They go on to have four children. Three
of these children eventually get married and have Sarah Andrew Kate Peter Helen Mary Anthony
children of their own. One child, Helen, suffers from
cystic fibrosis. The pedigree diagram shows that she David James Gary Rita Paul Joanne Zoe Frank Oliver Pat
does not get married and has no children.
normal male normal female
It is possible for individuals in a family with a
history of cystic fibrosis to have genetic counselling. carrier male carrier female
In this process, carriers and sufferers would be given
an explanation of the risk of having children with cystic fibrosis male cystic fibrosis female
cystic fibrosis. Genetic screening can be carried out
on developing embryos to identify their genotype. Figure B11.18 Pedigree diagram to show the inheritance of
A carrier with a partner who is also a carrier (such as cystic fibrosis in a family.
Zoe and Frank in Figure B11.18) may choose not
to have children because of the 1 in 4 chance of the

Questions pigs, half of the matings result in all black litters and the

8 Some plants occur in one of two sizes, tall or dwarf. This other half produce litters containing equal numbers of black
characteristic is controlled by one pair of genes. Tallness
is dominant to shortness. Choose suitable letters for the and white babies. From these results, deduce the genotypes
gene pair.
of the parents and explain the results of the various
9 Why are there two types of gene controlling one
characteristic? Do the two types affect the characteristic in matings, assuming that colour in this case is determined by
the same way as each other?
a single pair of alleles. Use punnett squares to illustrate your
10 The allele for red hair is recessive to the allele for black hair.
What colour hair will a person have if he inherits an allele answer.
for red hair from his mother and an allele for black hair 13 Figure B11.18 is a pedigree diagram to show the inheritance
from his father?
in a family of a disease called cystic fibrosis.
11 Use the words ‘homozygous’, ‘heterozygous’, ‘dominant’ Using the letters C (dominant allele) and c (recessive allele):
and ‘recessive’ (where suitable) to describe the following a Identify the genotypes of
allele combinations: Aa, AA, aa.
i Alfred
12 Two black guinea-pigs are mated together on several ii Helen
occasions and their offspring are invariably black. However, iii Sarah
when their black offspring are mated with white guinea- iv Zoe.
b Explain how Joanne can be born as a normal female

when both her parents are carriers of the disease. Identify

Joanne in a Punnett square to support your answer.

●● Variation inherited are determined by genes. They are genetic
variations. Phenotypic variations may be brought
Key definition about by genes, but can also be caused by the
Variation is the differences between individuals of the same environment, or a combination of both genes and
the environment.
species.
So, there are variations that are not heritable, but
The term ‘variation’ refers to observable differences determined by factors in the environment. A kitten
within a species. All domestic cats belong to the that gets insufficient food will not grow to the same
same species, i.e. they can all interbreed, but size as its litter mates. A cat with a skin disease may
there are many variations of size, coat colour, eye have bald patches in its coat. These conditions are not
colour, fur length, etc. Those variations that can be heritable. They are caused by environmental effects.

150

Variation

Similarly, a fair-skinned person may be able to change 20
the colour of his or her skin by exposing it to the Sun, 18
so getting a tan. The tan is an acquired characteristic.
You cannot inherit a suntan. Black skin, on the other 16
hand, is an inherited characteristic.
frequency 14
Many features in plants and animals are a mixture of 12
acquired and inherited characteristics (Figure B11.19). 10
For example, some fair-skinned people never go brown
in the Sun, they only become sunburned. They have 8
not inherited the genes for producing the extra brown
pigment in their skin. A fair-skinned person with the 6
genes for producing pigment will only go brown if he
or she exposes themselves to sunlight. So the tan is a 4 non-rollers
result of both inherited and acquired characteristics. 2
0 rollers

Figure B11.20   Discontinuous variation. Tongue rollers and non-rollers
in a class.

north side, upper branches south side, upper branches Discontinuous variation is under the control of a
single pair of alleles or a small number of genes. An
example is human blood groups (Figure B11.21).

A person is one of four blood groups: A, B, AB
or O. There are no groups in between.

46
42

percentage in population
of Britain
north side, lower branches south side, lower branches 9
3
Figure B11.19  Acquired characteristics. These apples have all been
picked from different parts of the same tree. All the apples have AB A O B
similar genotypes, so the differences in size must have been caused by blood group
environmental effects.
Figure B11.21  Discontinuous variation. Frequencies of ABO
Discontinuous variation blood groups in Britain. The figures could not be adjusted to fit a
smooth curve because there are no intermediates.
In discontinuous variation, the variations take the
form of distinct, alternative phenotypes with no Continuous variation
intermediates (Figures B11.20 and B11.21). The
mice in Figure B11.14 are either black or brown; An example of continuous variation is height. There
there are no intermediates. You are either male or are no distinct categories of height; people are not
female. Apart from a small number of abnormalities, either tall or short. There are all possible intermediates
sex is inherited in a discontinuous way. Some people between very short and very tall (Figure B11.22 on
can roll their tongue into a tube. Others are unable the next page).
to do it. They are known as non-tongue rollers.
Again, there are no intermediates (Figure B11.20). There are many characteristics that are difficult to
classify as either wholly continuous or discontinuous
Discontinuous variation cannot usually be altered variations. Human eye colour has already been
by the environment. You cannot change your eye mentioned. People can be classified roughly as having
colour by altering your diet. A genetic dwarf cannot blue eyes or brown eyes, but there are also categories
grow taller by eating more food. You cannot learn
how to roll your tongue. 151

B11 Inheritance

thousands of mendescribed as grey, hazel or green. It is likely that ●● Recording and presenting
there are a small number of genes for eye colour and the results of investigations
a dominant gene for brown eyes, which overrides all
the others when it is present. Similarly, red hair is a When carrying out any investigation it is very
discontinuous variation but it is masked by genes for important to record the results as you collect them.
other colours and there is a continuous range of hair The recording is usually presented in the form of a
colour from blond to black. table, which may have the data in columns or rows.
It is often best to plan the headings for a table before
14 starting the investigation, remembering to include a
description of what the data represents (the physical
12 quantity) and also the units used in the measurement,
if appropriate. For example, initial data collected to
10 produce a graph of the heights of students in your
class could be recorded using the table headings:
8
Name of student Height/cm
6
The outline of the table should be drawn using a
4 ruler. The number of rows would depend on the
number of students in the class. Note that the units
2 (in this case cm) are only written in the heading, not
the body of the table.
0
145 150 155 160 165 170 175 180 185 190 Once all the data has been collected and recorded,
height/cm a second table would be useful, containing a tally of
groups of heights:
Figure B11.22  Continuous variation. Heights of 90 000 army recruits.
The apparent ‘steps’ in the distribution are the result of arbitrarily chosen Height/cm Tally Total number of students
categories, differing in height by 1 cm. But heights do not differ by 145–149
exactly 1 cm. If measurements could be made accurately to the nearest 150–154
millimetre there would be a smooth curve like the one shown in colour. 155–159
160–164
Phenotypic variation is influenced by a combination etc.
of both genetic and environmental factors.
Continuously variable characteristics are usually Note that none of the ranges in the groups of heights
controlled by several pairs of alleles. There might overlaps with the next one, to prevent one student’s
be five pairs of alleles for height – (Hh), (Tt), (Ll), data being tallied twice. This processed data can then
(Ee) and (Gg) – each dominant allele adding 4 cm be used to plot a graph, similar to the one shown
to your height. If you inherited all ten dominant in Figure B11.22. This is a histogram – each of the
genes (HH, TT, etc.) you could be 40 cm taller blocks touches the next and all the blocks have the
than a person who inherited all ten recessive genes same width. The numerical characteristic, in this case
(hh, tt, etc.). height, is the independent variable and is displayed in
the horizontal axis and the frequency (the dependent
The actual number of genes that control height, variable) is displayed on the vertical axis. The graph
intelligence, and even the colour of hair and skin, is should have a title that describes what it represents.
not known.
Data collected for discontinuous variation, such as
Continuously variable characteristics are greatly blood groups or tongue rolling, is categoric rather
influenced by the environment. A person may inherit than numerical. There are distinct categories. The
genes for tallness and yet not get enough food to data would be displayed in a bar chart, as shown in
grow tall. A plant may have the genes for large fruits Figures B11.20 and B11.21. The horizontal axis is
but not get enough water, minerals or sunlight a list of the categories, and the blocks are of equal
to produce large fruits. Continuous variations in width. There is an equal space between each block.
human populations, such as height, physique and
intelligence, are always the result of interaction
between the genotype and the environment.

152

Selection

●● Mutations

Key definition a chromosome during mitosis or meiosis, or even the
A mutation is a change in a gene or chromosome. gain of an extra chromosome, as in Down’s syndrome.

Many of the coat variations seen in cats may have An abrupt change in a gene or chromosome is
arisen, in the first place, as mutations in a wild stock likely to result in a defective enzyme and will usually
of cats. A recent variant produced by a mutation is disrupt the complex reactions in the cells. Most
the ‘rex’ variety, in which the coat has curly hairs. mutations, therefore, are harmful to the organism.

Many of our high-yielding crop plants have Surprisingly, only about 3% of human DNA
arisen as a result of mutations in which the whole consists of genes. The rest consists of repeated
chromosome set has been doubled. sequences of nucleotides that do not code for
proteins. This is sometimes called ‘junk DNA’,
Exposure to mutagens, namely certain but that term only means that we do not know its
chemicals and radiation, is known to increase function. If mutations occur in these non-coding
the rate of mutation. Some of the substances in sequences they are unlikely to have any effect on the
tobacco smoke, such as tar, are mutagens, which organism and are, therefore, described as ‘neutral’.
can cause cancer.
Rarely, a gene or chromosome mutation produces
Ionising radiation from X-rays and radioactive a beneficial effect and this may contribute to the
compounds, and ultraviolet radiation from success of the organism (see ‘Selection’ later in this
sunlight, can both increase the mutation rate. It chapter).
is uncertain whether there is a minimum dose of
radiation below which there is negligible risk. It If a mutation occurs in a gamete, it will affect all
is possible that repeated exposure to low doses of the cells of the individual that develops from the
radiation is as harmful as one exposure to a high zygote. Thus the whole organism will be affected. If
dose. It has become clear in recent years that, in the mutation occurs in a somatic cell (body cell), it
light-skinned people, unprotected exposure to will affect only those cells produced, by mitosis, from
ultraviolet radiation from the Sun can cause a form the affected cell.
of skin cancer.
Thus, a mutation in a gamete may result in a
Generally speaking, however, exposure to natural genetic disorder, e.g. haemophilia or cystic fibrosis.
and medical sources of radiation carries less risk than Mutations in somatic cells may give rise to cancers
smoking cigarettes or driving a car, but it is sensible by promoting uncontrolled cell division in the
to keep exposure to a minimum. affected tissue. For example, skin cancer results from
uncontrolled cell division in the basal layer of the skin.
A mutation may occur in a gene or a chromosome.
In a gene mutation it may be that one or more A mutation may be as small as the change within a
genes are not replicated correctly. A chromosome single gene or as large as the breakage, loss or gain
mutation may result from damage to or loss of part of of a chromosome.

●● Selection ● ­Individuals within a species are all slightly different
from each other (Figure B11.23 on the next page).
Natural selection These differences are called variations.

Theories of evolution have been put forward in ● If the climate or food supply changes, individuals
various forms for hundreds of years. In 1858, Charles possessing some of these variations may be better
Darwin and Alfred Russel Wallace published a theory able to survive than others. For example, a variety
of evolution by natural selection, which is still an of animal that could eat the leaves of shrubs as well
acceptable theory today. as grass would be more likely to survive a drought
than one that fed only on grass.
The theory of evolution by natural selection is as
follows: ● If one variety lives longer than others, it is also
likely to leave behind more offspring. A mouse
that lives for 12 months may have ten litters of five

153

B11 Inheritance

For example, if a pair of rabbits had eight offspring
that grew up and formed four pairs, eventually having
eight offspring per pair, in four generations the number
of rabbits stemming from the original pair would be
512 (i.e. 2 → 8 → 32 → 128 → 512). The population
of rabbits, however, remains more or less constant.
Many of the offspring in each generation must,
therefore, have failed to survive to reproductive age.

Figure B11.23  Variation. The garden tiger moths in this picture are all Competition and selection
from the same family. There is a lot of variation in the pattern on the wings. There will be competition between members of
the rabbit population for food, burrows and mates.
babies (50 in all). A mouse that lives for 6 months If food is scarce, space is short and the number of
may have only five litters of five babies (25 in all). potential mates limited, then only the healthiest,
● If some of the offspring inherit alleles responsible most vigorous, most fertile and otherwise well-
for the variation that helped the parent survive adapted rabbits will survive and breed.
better, they too will live longer and have more
offspring. The competition does not necessarily involve
● In time, this particular variety will outnumber and direct conflict. The best adapted rabbits may be
finally replace the original variety. able to run faster from predators, digest their food
This is sometimes called ‘the survival of the fittest’. more efficiently, have larger litters or grow coats
However, ‘fitness’, in this case, does not mean good that camouflage them better or more effectively
health but implies that the organism is well fitted to reduce heat losses. These rabbits will survive longer
the conditions in which it lives. and leave more offspring. If the offspring inherit
Thomas Malthus, in 1798, suggested that the the advantageous characteristics of their parents,
increase in the size of the human population would they may give rise to a new race of faster, different
outstrip the rate of food production. He predicted coloured, thicker furred and more fertile rabbits,
that the number of people would eventually be which gradually replace the original, less well-
regulated by famine, disease and war. When Darwin adapted varieties. The new variations are said to have
read the Malthus essay, he applied its principles to survival value.
other populations of living organisms.
He observed that animals and plants produce vastly This is natural selection; the better adapted
more offspring than can possibly survive to maturity varieties are ‘selected’ by the pressures of the
and he reasoned that, therefore, there must be a environment (selection pressures).
‘struggle for survival’.
For natural selection to be effective, the variations
have to be heritable. Variations that are not heritable
are of no value in natural selection. Training may give
athletes more efficient muscles, but this characteristic
will not be passed on to their children.

The peppered moth
A possible example of natural selection is provided by
a species of moth called the peppered moth, found
in Great Britain. The common form is speckled but
there is also a variety that is black. The black variety
was rare in 1850, but by 1895 in the Manchester
area of England its numbers had risen to 98% of the
population of peppered moths. Observation showed
that the light variety was concealed better than the
dark variety when they rested on tree-trunks covered
with lichens (Figure B11.24). In the Manchester
area of England, pollution had caused the death

154

selection

ab c d
Figure B11.24 Selection for varieties of the peppered moth.

of the lichens and the darkening of the tree-trunks Although this is an attractive and plausible
with soot. In this industrial area the dark variety hypothesis of how natural selection could occur, some
was the better camouflaged (hidden) of the two and of the evidence does not support the hypothesis or
was not picked off so often by birds. So the dark has been called into question.
variety survived better, left more offspring and nearly
replaced the light form. For example, the moths settle most frequently on
the underside of branches rather than conspicuously
The selection pressure, in this case, was presumed on tree trunks, as in Figure B11.24. Also, in several
to be mainly predation by birds. The adaptive unpolluted areas the dark form is quite abundant,
variation that produced the selective advantage was for example 80% in East Anglia in England. Research
the dark colour. is continuing in order to test the hypothesis.

Adaptation of a water beetle are adaptations to locomotion in
water when compared with the corresponding legs of
Key definition a land-living relative (Figure B11.25).
Adaptation is the process, resulting from natural
Similarly, in Figure B11.26 (on the next page) it
selection, by which populations become more seems reasonable to suppose that, compared with the
suited to their environment over many generations. generalised mammalian limb, the forelimbs of whales
are adapted for locomotion in water.
When biologists say that a plant or animal is adapted
to its habitat they usually mean that, in the course of By studying animals which live in extreme habitats,
evolution, changes have occurred in the organism, it is possible to suggest ways in which they might be
which make it more successful in exploiting its adapted to these habitats especially if the observations
habitat, e.g. animals finding and digesting food, are supported by physiological evidence.
selecting nest sites or hiding places, or plants
exploiting limited mineral resources or tolerating long legs
salinity or drought. It is tempting to assume that with fringe
because we find a plant or animal in a particular of bristles
habitat it must be adapted to its habitat. There is
some logic in this; if an organism was not adapted a water beetle b ground beetle
to its habitat, presumably it would be eliminated
by natural selection. However, it is best to look for Figure B11.25 Adaptation to locomotion in water and on land.
positive evidence of adaptation.

Sometimes, just by looking at an organism and
comparing it with related species, it is possible to make
reasoned guesses about adaptation. For example, there
seems little doubt that the long, hair-fringed hind legs

155

B11 Inheritance

ball and hinge five groups of bones,
socket joint joint
(shoulder) (elbow) each arranged in a ‘chain’1 2
(hand and fingers)

3

one bone two bones group of 5 4
(humerus) (radius and ulna)
small bones (wrist)

a pattern of bones in human forelimb

humerus radius wrist
1

2

3

ulna 54 Figure B11.27  Protection against wind-blown sand. The nostrils are
slit-like and can be closed. The long eyelashes protect the eyes.
b whale
energy-giving food. However, when the fat is
Figure B11.26  Skeletons of the forelimbs of human and whale. metabolised during respiration, carbon dioxide and
water (metabolic water) are produced. The water
T he camel enters the blood circulation and would normally be
Camels are adapted to survive in a hot, dry and lost by evaporation from the lungs, but the water-
sandy environment. Adaptive physical features are conserving nasal mucus will trap at least a proportion
closable nostrils and long eyelashes, which help keep of it.
out wind-blown sand (Figure B11.27). Their feet are
broad and splay out under pressure, so reducing the The polar bear
tendency to sink into the sand. Thick fur insulates Polar bears live in the Arctic, spending much
the body against heat gain in the intense sunlight. of their time on snow and ice. Several physical
features contribute to their adaptation to this cold
Physiologically, a camel is able to survive without environment.
water for 6–8 days. Its stomach has a large water-
holding capacity, though it drinks to replace water It is a very large bear (Figure B11.28), which
lost by evaporation rather than in anticipation of means that the ratio of its surface area to its volume
water deprivation. is relatively small. The relatively small surface area
means that the polar bear loses proportionately less
The body temperature of a ‘thirsty’ camel rises to heat than its more southerly relatives. Also its ears
as much as 40 °C during the day and falls to about are small, another feature that reduces heat loss
35 °C at night. The elevated daytime temperature (Figure B11.29).
reduces the heat gradient between the body and
the surroundings, so less heat is absorbed. A camel It has a thick coat with long, loosely packed coarse
is able to tolerate water loss equivalent to 25% of hairs (guard hairs) and a denser layer of shorter
its body weight, compared with humans for whom woolly hairs forming an insulating layer. The long
a 12% loss may be fatal. The blood volume and hairs are oily and water-repellent and enable the bear
concentration are maintained by withdrawing water to shake off water when it emerges from a spell of
from the body tissues. swimming.

The nasal passages are lined with mucus. During The principal thermal insulation comes from a
exhalation, the dry mucus absorbs water vapour. 10 cm layer of fat (blubber) beneath the skin. The
During inhalation the now moist mucus adds water thermal conductivity of fat is little different from any
vapour to the inhaled air. In this way, water is other tissue but it has a limited blood supply. This
conserved. means that very little warm blood circulates close to
the skin surface.
The role of the camel’s humps in water
conservation is more complex. The humps contain
fat and are therefore an important reserve of

156

selection

key
warm blood

cool blood

Figure B11.28 The polar bear and the sun bear (from SE Asia). The heat is transferred
smaller surface area/volume ratio in the polar bear helps conserve heat. from the artery to
the vein
the blood supply
to the foot is
maintained but
heat loss is
minimised

Figure B11.30 The heat-exchange mechanism in the polar bear’s limb.

Figure B11.29 The heavy coat and small ears also help the polar bear and rear her two cubs. In this way the cubs are
to reduce heat losses. protected from the extreme cold.
The female remains in the den for about 140 days,
The hollow hairs of the white fur are thought to suckling her young on the rich milk, which is formed
transmit the Sun’s heat to the black skin below. from her fat reserves.
Black is an efficient colour for absorbing heat. The
white colour is also probably an effective camouflage Venus flytrap
when hunting its prey, mainly seals. Many plants show adaptions as well as animals.
Insectivorous plants such as the Venus flytrap
A specific adaptation to walking on snow and ice (Figure B11.31 on the next page) live in habitats
is the heat-exchange arrangement in the limbs. The where there is often a shortage of nitrates for
arteries supplying the feet run very close to the veins growth. They have developed pairs of leaves with
returning blood to the heart. Heat from the arteries tooth-like edges. The leaves have sensitive hairs on
is transferred to the veins before the blood reaches their surface. When an insect walks inside the leaves,
the feet (Figure B11.30). So, little heat is lost from the hairs are triggered, causing the leaves to close
the feet but their temperature is maintained above very rapidly – trapping the animal. The leaves then
freezing point, preventing frost-bite. secrete protease enzymes, which digest the insect’s
protein and produce soluble amino acids. These are
Polar bears breed in winter when temperatures absorbed by the leaf and used to build new proteins.
fall well below zero. However, the pregnant female It is unusual for a photosynthetic plant to show such
excavates a den in the snow in which to give birth rapid movement or to gain nourishment other than
by photosynthesis.

157

B11 Inheritance

Figure B11.31  Venus flytrap with trapped insect, which will eventually
be digested.

Other adaptations
Adaptive features of the long-eared bat and the hare
are illustrated in Figures B11.32 and B11.33.

Figure B11.32  Long-eared bat. The bat gives out high-pitched sounds, Figure B11.33  Hare. This animal is a herbivore and is hunted by
which are reflected back from its prey and from obstacles, to its ears and predators such as foxes. Its fur is a good insulator and its colour provides
sensitive patches on its face. By timing these echoes the bat can judge excellent camouflage. The long ears help to pick up and locate sound
its distance from the obstacle or prey. This allows it to fly and feed in the vibrations. The eyes at the side of the head give the hare good all-round
dark. Its body is covered in fur for insulation. Its forearms are covered by vision. The hind legs are very long to enable the animal to run away
a membrane of skin to form a wing. The fingers are very long to stretch from predators and its kick is a good defence mechanism. Some species
out the membrane to increase the surface area of the wing. of hare change the colour of their fur in winter from brown to white to
provide better camouflage in snow.

Evolution to appear in a large population of bacteria. If a
population of bacteria containing one or two drug-
Key definition resistant mutants is subjected to that particular drug,
Evolution can be described as the change in adaptive features the non-resistant bacteria will be killed but the drug-
resistant mutants survive (Figure B11.34). Mutant
of a population over time as the result of natural selection. genes are inherited in the same way as normal genes,
so when the surviving mutant bacteria reproduce, all
Most biologists believe that natural selection, among their offspring will be resistant to the drug.
other processes, contributes to the evolution of new
species and that the great variety of living organisms If a course of antibiotics is not completed, some
on the Earth is the product of millions of years of of the bacteria it is being used to destroy will not be
evolution involving natural selection. killed, but will have been exposed to the drug. Some
A ntibiotic-resistant bacteria of the survivors may be drug-resistant mutants.
Antibiotics are drugs used to treat infections caused When they reproduce, all their offspring will have
by bacteria. Bacterial cells reproduce very rapidly, the drug resistance, so the antibiotic will become less
perhaps as often as once every 20 minutes. Thus effective (Figure B11.34).
a mutation, even if it occurs only rarely, is likely

158

Selection

ANTIBIOTIC

all these are killed
by antibiotic

bacterium
reproducing

mutation mutants survive population
(resistance to antibiotic of bacteria
an antibiotic) treatment now all
resistant
Figure B11.34  Mutation in bacteria can lead to drug resistance. to the
antibiotic

One type of bacteria that has developed resistance allowing it to infect internal organs and causing
to a number of widely used antibiotics is called blood poisoning. This can happen in hospitals with
MRSA (methicillin-resistant Staphylococcus aureus). infection during operations, especially if hygiene
These types of bacteria are sometime referred precautions are not adequate.
to as ‘superbugs’ because they are so difficult to
treat. Staphylococcus aureus is very common and Doctors now have to be much more cautious
is found living harmlessly on the skin, the nose about prescribing antibiotics, to reduce the risk of
and throat, sometimes causing mild infections. It resistant strains developing. Patients need to be
becomes dangerous if there is a break in the skin, aware of the importance of completing a course of
antibiotics, again to reduce the risk of development
of resistant strains.

Selective breeding today have been produced by selecting individuals
with pointed ears, particular fur colour or length, or
The process of selective breeding involves humans even no tail, etc. One of the kittens in a litter might
selecting individuals with desirable features. These vary from the others by having distinctly pointed
individuals are then cross-bred to produce the next ears. This individual, when mature, is allowed to
generation. Offspring with the most desirable features breed. From the offspring, another very pointed-
are chosen to continue the breeding programme and eared variant is selected for the next breeding stock,
the process is repeated over a number of generations. and so on, until the desired or ‘fashionable’ ear
shape is established in a true-breeding population
Human communities practise this form of selection (Figure B11.35 on the next page).
when they breed plants and animals for specific
characteristics. The many varieties of cat that you see

159

B11 Inheritance

Figure B11.35  Selective breeding. The Siamese cat, produced by
artificial selection over many years.

More important are the breeding programmes to Figure B11.36  Selective breeding in tomatoes. Different breeding
improve agricultural livestock or crop plants. Animal- programmes have selected genes for fruit size, colour and shape.
breeders will select cows for their high milk yield and Similar processes have given rise to most of our cultivated plants and
sheep for their wool quality. Plant-breeders will select domesticated animals.
varieties for their high yield and resistance to fungus
diseases (Figure B11.36).

An important part of any breeding programme is the Comparing natural and artificial
selection of the desired varieties. The largest fruit on selection
a tomato plant might be picked and its seeds planted
next year. In the next generation, once again only Natural selection occurs in groups of living
seeds from the largest tomatoes are planted. Eventually organisms through the passing on of genes to the
it is possible to produce a true-breeding variety of next generation by the best adapted organisms,
tomato plant that forms large fruits. Figure B11.36 without human interference. Those with genes
shows the result of such selective breeding. The same that provide an advantage, to cope with changes
technique can be used for selecting other desirable in environmental conditions for example, are more
qualities, such as flavour and disease resistance. likely to survive, while others die before they can
breed and pass on their genes. However, variation
Similar principles can be applied to farm animals. within the population remains.
Desirable characteristics, such as high milk yield and
resistance to disease, may be combined. Stock-breeders Artificial selection is used by humans to produce
will select calves from cows that give large quantities varieties of animals and plants that have an increased
of milk. These calves will be used as breeding stock economic importance. It is considered a safe way of
to build a herd of high yielders. A characteristic such developing new strains of organisms, compared with
as milk yield is probably under the control of many genetic engineering, and is a much faster process
genes. At each stage of selective breeding the farmer, than natural selection. However, artificial selection
in effect, is keeping the beneficial genes and discarding removes variation from a population, leaving it
the less useful genes from his or her animals. susceptible to disease and unable to cope with
changes in environmental conditions. Potentially,
Selective breeding in farm stock can be slow and therefore, artificial selection puts a species at risk
expensive because the animals often have small of extinction.
numbers of offspring and breed only once a year.

160

selection
In agriculture and horticulture, asexual
reproduction (vegetative propagation) is exploited
to preserve desirable qualities in crops: the good
characteristics of the parent are passed on to all the
offspring. With a flower such as a daffodil, the bulbs
produced can be guaranteed to produce the same
shape and colour of flower from one generation to
the next. In some cases, such as tissue culture, the
young plants grown can be transported much more
cheaply than, for example, potato tubers as the latter
are much heavier and more bulky. Growth of new
plants by asexual reproduction tends to be a quick
process.

In natural conditions in the wild it might be a
disadvantage to have no variation in a species. If the
climate or other conditions change and a vegetatively
produced plant has no resistance to a particular
disease, the whole population could be wiped out.

a b c de Checklist

Figure B11.37 The genetics of bread wheat. A primitive wheat (a) was After studying Chapter B11 you should know and understand
crossed with a wild grass (b) to produce a better-yielding hybrid wheat the following.
(c). The hybrid wheat (c) was crossed with another wild grass (d) to
produce one of the varieties of wheat (e) which is used for making flour Chromosomes and genes
and bread.
● Inheritance is the transmission of genetic information from
Questions generation to generation.

14 Study the following photographs and captions, then ● A chromosome is a thread-like structure of DNA, carrying
make a list of the adaptations of each animal. genetic information in the form of genes.
a long-eared bat (Figure B11.32)
b hare (Figure B11.33) ● A gene is a length of DNA that codes for a protein.
c polar bear (Figure B11.29) (See also details in the text.) ● An allele is a version of a gene.
● Sex, in mammals, is determined by the X and Y
15 Suggest some good characteristics that an animal-breeder
might try to combine in sheep by mating different varieties chromosomes. Males are XY; females are XX.
together.
● A haploid nucleus is a nucleus containing a single set of
16 A variety of barley has a good ear of seed but has a long unpaired chromosomes e.g. in gametes.
stalk and is easily blown over. Another variety has a short,
sturdy stalk but a poor ear of seed. ● A diploid nucleus is a nucleus containing two sets of
a Suggest a breeding programme to obtain and select chromosomes e.g. in body cells.
a new variety that combines both of the useful
characteristics. ● In a diploid cell, the chromosomes are in pairs; in a human
b Choose letters to represent the genes and show the diploid cell there are 23 pairs.
genotypes of the parent plants and their offspring.
Cell division

● Mitosis is nuclear division giving rise to genetically
identical cells.

● Before mitosis, the exact duplication of chromosomes
occurs.

● Mitosis is important in growth, repair of damaged tissues,
replacement of cells and in asexual reproduction.

161

B11 Inheritance

● Meiosis is reduction division in which the chromosome ● Discontinuous variation results in a limited number of
number is halved from diploid to haploid resulting in phenotypes with no intermediates, e.g. tongue rolling.
genetically different cells.
● Phenotypic variations are usually caused by both genetic
● Meiosis is involved in the production of gametes. and environmental factors.

Monohybrid inheritance ● Discontinuous variation is mostly caused by genes alone
e.g. A, B, AB and O blood groups in humans.
● The genotype of an organism is its genetic make-up in terms
of the alleles present. ● A mutation is a change in a gene or a chromosome.
● Increases in the rate of mutation can be caused by ionising
● The phenotype of an organism is its observable features.
● Homozygous means having two identical alleles of a radiation and some chemicals.

particular gene. ● Some members of a species may have variations which
● Two identical homozygous individuals that breed together enable them to compete more effectively.

will be pure-breeding. ● Within a population there will be competition for resources.
● Heterozygous means having two different alleles of a ● There will be a struggle for survival.
● Those variants that are better adapted to the environment
particular gene.
● A heterozygous individual will not be pure-breeding. will live longer and leave more offspring.
● A dominant allele is one that is that is expressed if it is ● These variants pass on their alleles to the next generation.

present. ● Evolution is the change in adaptive features of a
● A recessive allele is one that is only expressed when there is population over time as the result of natural selection.

no dominant allele of the gene present. ● Adaptation is a process, resulting from natural selection,
● Genetic diagrams are used to predict the results of by which populations become more suited to their
environment over many generations.
monohybrid crosses and calculate phenotypic ratios.
● Punnett squares can be used in crosses to work out and show ● The development of strains of antibiotic resistant bacteria
is an example of evolution by natural selection.
the possible different genotypes.
● Selective breeding is used to improve commercially useful
● Pedigree diagrams can be used to interpret the inheritance plants and animals.
of particular characteristics.
● Humans select individuals with desirable features.
Variation ● These individuals are crossed to produce the next generation.
● Humans select offspring with the desirable features.
● Variation is the differences between individuals of the same
species. ● There are a number of differences between natural and
artificial selection.
● The difference between phenotypic and genetic variation.
● Continuous variation results in a range of phenotypes ● Selective breeding by artificial selection is carried out
over many generations to improve crop plants and
between two extremes, e.g. height in humans. domesticated animals.

162

B12 Organisms and their environment

Combined Co-ordinated

● State that the Sun is the principal source of energy input to ● State that the Sun is the principal source of energy input to
biological systems biological systems

● Define the terms: food chain; food web; producer; ● Define the terms: food chain; food web; producer;
consumer; herbivore; carnivore; decomposer consumer; herbivore; carnivore; decomposer

● D efine the terms: ecosystem; trophic level ● Define the terms: ecosystem; trophic level

● D escribe how energy is transferred between trophic levels ● D escribe how energy is transferred between trophic levels

● Explain why food chains usually have fewer than five ● Explain why food chains usually have fewer than five
trophic levels trophic levels

● Construct simple food chains ● Construct simple food chains
● Interpret food chains and food webs in terms of identifying ● Interpret food chains and food webs in terms of identifying

producers and consumers producers and consumers
● State that consumers may be classed as primary, secondary ● State that consumers may be classed as primary, secondary

and tertiary according to their position in a food chain and tertiary according to their position in a food chain

● Identify producers, primary consumers, secondary ● Identify producers, primary consumers, secondary
consumers, tertiary consumers and quaternary consumers consumers, tertiary consumers and quaternary consumers
as the trophic levels in food webs and food chains as the trophic levels in food webs and food chains

●● Energy flow photosynthesis sunlight photosynthesis
in wheat flowering plants
Nearly all living things depend on the Sun to provide photosynthesis
energy. This is harnessed by photosynthesising plants in grass
and the energy is then passed through food chains.
wheat grains cow nectar
Dependence on sunlight
flour milk bees
With the exception of atomic energy and tidal power,
all the energy released on Earth is derived from bread cheese honey
sunlight. The energy released by animals comes,
ultimately, from plants that they or their prey eat and Nearly all the energy released on the Earth can
the plants depend on sunlight for making their food. be traced back to sunlight. Coal comes from tree-
Photosynthesis is a process in which light energy is like plants, buried millions of years ago. These
trapped by plants and converted into chemical energy plants absorbed sunlight for their photosynthesis
(stored in molecules such as carbohydrates, fats and when they were alive. Petroleum was formed, also
proteins). Since all animals depend, in the end, on millions of years ago, probably from the partly
plants for their food, they therefore depend indirectly decayed bodies of microscopic algae that lived
on sunlight. A few examples of our own dependence in the sea. These, too, had absorbed sunlight for
on photosynthesis are given below. photosynthesis.

163

B12 Organisms and their environment

●● Food chains and food
webs

Key definitions
A food chain shows the transfer of energy from one organism

to the next, beginning with a producer.
A food web is a network of interconnected food chains.
A producer is an organism that makes its own organic nutrients,

usually using energy from sunlight, through photosynthesis.
A consumer is an organism that gets its energy from feeding

on other organisms.
A herbivore is an animal that gets its energy by eating plants.
A carnivore is an animal that gets its energy by eating other

animals.

‘Interdependence’ means the way in which living
organisms depend on each other in order to remain
alive, grow and reproduce. For example, bees depend
for their food on pollen and nectar from flowers.
Flowers depend on bees for pollination (Chapter B10).
Bees and flowers are, therefore, interdependent.

Food chains Figure B12.1  A food chain. The caterpillar eats the leaf; the blue
tit eats the caterpillar but may fall prey to the kestrel.
One important way in which organisms depend on
each other is for their food. Many animals, such The food chains in Figure B12.2 show this
as rabbits, feed on plants. Such animals are called relationship. There will be millions of microscopic,
herbivores. Animals that eat other animals are called single-celled algae in a pond (Figure B12.3a). These
carnivores. A predator is a carnivore that kills and will be eaten by the larger but less numerous water
eats other animals. A fox is a predator that preys fleas and other crustacea (Figure B12.3b), which
on rabbits. Scavengers are carnivores that eat the in turn will become the food of small fish such as
dead remains of animals killed by predators. These minnow. The hundreds of small fish may be able
are not hard and fast definitions. Predators will to provide enough food for only four or five large
sometimes scavenge for their food and scavengers carnivores, like perch.
may occasionally kill living animals. Animals obtain
their energy by ingestion. The organisms at the beginning of the food chains
in Figure B12.2 are plants. Plants produce food from
Basically, all animals depend on plants for their carbon dioxide, water and salts (see ‘Photosynthesis’,
food. Foxes may eat rabbits, but rabbits feed on Chapter B5), and are, therefore, called producers.
grass. A hawk eats a lizard, the lizard has just eaten The animals that eat the plants are called primary
a grasshopper but the grasshopper was feeding on a consumers, e.g. grasshoppers. Animals that prey on
grass blade. This relationship is called a food chain the plant-eaters are called secondary consumers,
(Figure B12.1). e.g. shrews, and these may be eaten by tertiary
consumers, e.g. weasels or kestrels.
The organisms at the beginning of a food chain are
usually very numerous while the animals at the end of
the chain are often large and few in number.

164

food chains and food webs

(a) land plant leaves caterpillars blue tits kestrel

(b) water microscopic water fleas minnows perch
algae

producers consumers
Figure B12.2 Examples of food chains.

aphypthoyptloapnlakntoktnon(×(×110000) )Tthheessee mmiiccrroossccooppicicaalglgaaeefofromrmthtehbeabsiassoisf oaf bzoozpoloapnlkantoknto(n×(2×02)0T) htheesseeccrruussttaacceeaa wwiilllleeaat tmmicircorsocsocpoicpiaclgaalgeae

foofdoopdyrpaymraimd iidninthtehewwataeterr

Figure B12.3 Plankton. The microscopic organisms that live in the surface waters of the sea or fresh water are called, collectively, plankton. The
single-celled algae are the phytoplankton. They are surrounded by water, salts and dissolved carbon dioxide. Their chloroplasts absorb sunlight and
use its energy for making food by photosynthesis. Phytoplankton is eaten by small animals in the zooplankton, mainly crustacea. Small fish will eat the
crustacea.

When constructing a food chain, always start a food web can be drawn up (Figure B12.4 on the
with the name of the producer, usually a type of next page).
plant. It is not necessary to include an illustration. The food webs for land, sea and fresh water, or for
The instruction used in exam questions ‘Draw a ponds, rivers and streams, will all be different. Food
food chain’ should not be taken literally. It just webs will also change with the seasons when the food
means that the food chain should be completed supply changes.
using arrows to link the names of the organisms
involved. There should then be an arrow, pointing If some event interferes with a food web, all the
right towards a named herbivore (the animal organisms in it are affected in some way. For example,
which eats the plant). This is linked by another if the rabbits in Figure B12.4 were to die out,
arrow to a named carnivore (the animal which eats the foxes, owls and stoats would eat more beetles
the herbivore). There may be higher carnivores and rats. Something like this happened in 1954
involved, also linked by arrows. when the disease myxomatosis wiped out nearly all
the rabbits in England. Foxes ate more voles, beetles
Food webs and blackberries, and attacks on lambs and chickens
increased. Even the vegetation was affected because
Food chains are not really as straightforward as the tree seedlings that the rabbits used to nibble
described above, because most animals eat more than on were able to grow. As a result, woody scrubland
one type of food. A fox, for example, does not feed started to develop on what had been grassy downs.
entirely on rabbits but takes beetles, rats and voles in A similar effect is shown in Figure B12.5 on the
its diet. To show these relationships more accurately, next page.

165

B12 Organisms and their environment stoat

fox
owl

beetle rat rabbit
Figure B12.4  A food web. vegetation

sah  esehpeehpavheaveeaeteantenanaynyseseededlilninggsswthhaitchgrgewrewunudnerdtehretthreeetsrees bstteeh enedteltyirnneegayesresshaelraasevtdelealrtgi,enrtrog,hwstehnhefaefvneencecgerhohawassnkkeepptt tthhee sshheeeeppooffffaannddthe tree
Figure B12.5  Effect of grazing.

●● Energy transfer

Study Figure B12.1. When an herbivorous animal blue tit) eats the herbivore, the carnivore gains the
eats a plant (the caterpillar feeding on a leaf), the energy stored in the herbivore. If the carnivore is
chemical energy stored in that plant leaf is transferred eaten by another carnivore (the kestrel), the energy is
to the herbivore. Similarly, when a carnivore (the transferred again.

166

Energy transfer

Use of sunlight study the efficiency of transmission of energy from
To try to estimate just how much life the Earth can plant products to primary consumers. On land,
support it is necessary to examine how efficiently primary consumers eat only a small proportion
the Sun’s energy is used. The amount of energy of the available vegetation. In a deciduous forest
from the Sun reaching the Earth’s surface in 1 year only about 2% is eaten; in grazing land, 40% of
ranges from 2 million to 8 million kilojoules per m2 the grass may be eaten by cows. In open water,
(2–8 ×109 J/m2/yr) depending on the latitude. When however, where the producers are microscopic
this energy falls onto grassland, about 20% is reflected plants (phytoplankton, see Figure B12.3a) and are
by the vegetation, 39% is used in evaporating water swallowed whole by the primary consumers in the
from the leaves (transpiration), 40% warms up the zooplankton (see Figure B12.3b), 90% or more may
plants, the soil and the air, leaving only about 1% to be eaten. In the land communities, the parts of the
be used in photosynthesis for making new organic vegetation not eaten by the primary consumers will
matter in the leaves of the plants (Figure B12.6). eventually die and be used as a source of energy by
the decomposers.
SUNLIGHT 20% 39% evaporates water
39% from plant A cow is a primary consumer; over 60% of the
40% grass it eats passes through its alimentary canal
20% (Chapter B6) without being digested. Another 30%
reflected is used in the cow’s respiration to provide energy for
from plant its movement and other life processes. Less than 10%
of the plant material is converted into new animal
only 1% used in 40% warms up soil, tissue to contribute to growth (Figure B12.7).
photosynthesis air and vegetation This figure will vary with the diet and the age of
the animal. In a fully grown animal all the digested
Figure B12.6  Absorption of Sun’s energy by plants. food will be used for energy and replacement and
none will contribute to growth. Economically it is
This figure of 1% will vary with the type of vegetation desirable to harvest the primary consumers before
being considered and with climatic factors, such as their rate of growth starts to fall off.
availability of water and the soil temperature. Sugar-
cane grown in ideal conditions can convert 3% of the 30% lost as energy
Sun’s energy into photosynthetic products; sugar-beet
at the height of its growth has nearly a 9% efficiency. 10% for
Tropical forests and swamps are far more productive growth
than grassland but it is difficult, and, in some cases
undesirable, to harvest and utilise their products. FOOD

In order to allow crop plants to approach their 60% not
maximum efficiency they must be provided with digested
sufficient water and mineral salts. This can be
achieved by irrigation and the application of fertiliser. Figure B12.7  Energy transfer from plants to animals.

E nergy transfer between organisms The transfer of energy from primary to secondary
Having considered the energy conversion from consumers is probably more efficient, since a greater
sunlight to plant products, the next step is to proportion of the animal food is digested and
absorbed than is the case with plant material.

167

B12 ORganisms anD theiR enViROnment

Key definitions
The trophic level of an organism is its position in a food

chain or food web.

maize locust lizard snake eagle
producer primary secondary tertiary quaternary
consumer consumer consumer consumer

Figure B12.8 A food chain containing five trophic levels.

Trophic levels in food chains The producer is grass. Locusts and impala are
and food webs primary consumers (herbivores) and the baboon is a
tertiary consumer (carnivore). The leopard is acting
Food chains are lists of organisms that show the as a secondary and quaternary consumer, depending
feeding relationship between them, as in the example on what it is feeding on.
in Figure B12.8.
In the food web shown in Figure B12.4, there are
The arrows used to link each organism to the next eight food chains. The producer is the vegetation.
represent the transfer of energy. The feeding level is The beetle, rat and rabbit are primary consumers
known as the trophic level. (herbivores) and the fox, owl and stoat are tertiary
consumers (carnivores). In reality, this food web
Food webs are more realistic than food chains would be even more complex because the primary
because they show the range of organisms that the consumers may select particular species of vegetation
consumers rely on for food. For example, in Figure to feed on and avoid others.
B12.9, the leopard feeds on baboons and impala.
It is very unusual for food chains to have more
leopard than five trophic levels because, on average, about
90% of the energy is lost at each level. Consequently,
baboon very little of the energy entering the chain through
the producer is available to the top consumer. The
scorpion impala food chain below shows how the energy reduces
through the chain. It is based on grass obtaining
100 units of energy.

locust grass → locust → lizard → snake → mongoose
Figure B12.9 A food web.
grass 100 10 1 0.1 0.01

units units unit unit unit

Questions 4 Outline the events that might happen to a carbon atom
in a molecule of carbon dioxide, which entered the
1 Construct a simple food web using the following: stoma in the leaf of a potato plant and became part of a
sparrow, fox, wheat seeds, cat, kestrel, mouse. starch molecule in a potato tuber, which was then eaten
by a man. Finally the carbon atom is breathed out again
2 Construct a simple food chain involving the following in a molecule of carbon dioxide.
organisms:
blackbird, earthworm, grass, hawk.
On your food chain, identify the producer and the levels of
the consumers.

3 An electric motor, a car engine and a race horse can all
produce energy.
a Show how this energy could come, originally, from sunlight.
b What forms of energy on the Earth are not derived from
sunlight?

168

Energy transfer

Checklist ● Energy from the sun flows through living organisms.
● Energy is transferred between trophic levels through feeding.
After studying Chapter B12 you should know and understand ● First, light energy is converted into chemical energy
the following.
in photosynthetic organisms. Then they are eaten by
Organisms and their environment herbivores. Carnivores eat herbivores.
● As organisms die, the energy is transferred to the
● The Sun is the principal source of energy input to biological environment.
systems. ● The transfer of energy from one trophic level to another is
inefficient.
● A food chain shows the transfer of energy from one ● Only about 1% of the Sun’s energy which reaches the
organism to the next, beginning with a producer. Earth is trapped by plants during photosynthesis.
● At each step in a food chain, only a small proportion of
● A food web is a network of interconnected food chains. the food is used for growth. The rest is used for energy to
● Producers are organisms that make their own organic keep the organism alive.
● Food chains usually have less than five trophic levels because,
nutrients, usually using energy from sunlight, through on average, 90% of the energy is lost at each level.
photosynthesis.
● Consumers are organisms that get their energy from feeding ● All animals depend, ultimately, on plants for their source of
on other organisms. food.
● A herbivore is an animal that gets its energy by eating plants.
● A carnivore is an animal that gets its energy by eating other ● Plants are the producers in a food chain or food web; animals
animals. may be primary, secondary or tertiary consumers.
● A decomposer is an organism that gets its energy from dead
or waste organic material. ● Plants are the producers in a food chain or food web;
animals may be primary, secondary, tertiary or quaternary
● An ecosystem is a unit containing all of the organisms and consumers. These represent trophic levels.
their environment, interacting together, in a given area.

● A trophic level is the position of an organism in a food
chain or food web.

169

B13 Human influences on ecosystems

Combined Co-ordinated

● Describe the carbon cycle, limited to photosynthesis, ● Describe the carbon cycle, limited to photosynthesis,
respiration, feeding, decomposition, fossilisation and respiration, feeding, decomposition, fossilisation and
combustion combustion

● D iscuss the effects of the combustion of fossil fuels and ● Discuss the effects of the combustion of fossil fuels and
the cutting down of forests on the oxygen and carbon the cutting down of forests on the oxygen and carbon
dioxide concentrations in the atmosphere dioxide concentrations in the atmosphere

● List the undesirable effects of deforestation as an example of ● List the undesirable effects of deforestation as an example of
habitat destruction, to include extinction, loss of soil, flooding habitat destruction, to include extinction, loss of soil, flooding
and increase of carbon dioxide in the atmosphere and increase of carbon dioxide in the atmosphere

● Explain the process of eutrophication of water in terms of: ● Explain the undesirable effects of deforestation on the
increased availability of nitrate and other ions; increased environment
growth of producers; increased decomposition after
death of producers; increased aerobic respiration by ● State the sources and effects of pollution of water (rivers,
decomposers; reduction in dissolved oxygen; death of lakes and the sea) by chemical waste, discarded rubbish,
organisms requiring dissolved oxygen in water untreated sewage and fertilisers

● E xplain the process of eutrophication of water in terms of: ●
i●ncreased availability of nitrate and other ions; increased
g● rowth of producers; increased decomposition after
d● eath of producers; increased aerobic respiration by
d● ecomposers; reduction in dissolved oxygen; death of
o● rganisms requiring dissolved oxygen in water

●● The carbon cycle photosynthesis

Carbon is an element that occurs in all the compounds decay GREEN PLANTS
which make up living organisms. Plants get their
carbon from carbon dioxide in the atmosphere and ATMOSPHERIC respiration build CO2 into organic
animals get their carbon from plants. The carbon CARBON DIOXIDE compounds by photosynthesis
cycle, therefore, is mainly concerned with what
happens to carbon dioxide (Figure B13.1).

Removal of carbon dioxide from the atmosphere dec ay ANIMALS eaten
Photosynthesis respir combustion ation convert plant fossilisation
Green plants remove carbon dioxide from the material into
atmosphere as a result of their photosynthesis. The animal tissue
carbon from the carbon dioxide is built first into a
carbohydrate such as sugar. Some of this is changed into PREHISTORIC PLANTS
starch or the cellulose of cell walls, and the proteins, form deposits of coal,
pigments and other compounds of a plant. When the petroleum and natural gas
plants are eaten by animals, the organic plant material Figure B13.1  The carbon cycle.
is digested, absorbed and built into the compounds
making up the animals’ tissues. Thus the carbon atoms becomes trapped and compressed and can remain
from the plant become part of the animal. there for millions of years. The carbon may form
fossil fuels such as coal, oil and natural gas. Some
Fossilisation animals make shells or exoskeletons containing
Any environment that prevents rapid decay may carbon and these can become fossils.
produce fossils. The carbon in the dead organisms

170

The carbon cycle

Addition of carbon dioxide to the atmosphere Figure B13.2  Mould fungus growing on over-ripe oranges.
Respiration
Plants and animals obtain energy by oxidising anaerobic conditions but it is slow and incomplete, as
carbohydrates in their cells to carbon dioxide and in the waterlogged conditions of peat bogs.
water (Chapter B8). The carbon dioxide and water Combustion (burning)
are excreted so the carbon dioxide returns once again When carbon-containing fuels such as wood, coal,
to the atmosphere. petroleum and natural gas are burned, the carbon is
oxidised to carbon dioxide (C + O2 → CO2). The
Decomposition hydrocarbon fuels, such as coal and petroleum, come
A crucial factor in carbon recycling is the process of from ancient plants, which have only partly decomposed
decomposition, or decay. If it were not for decay, over the millions of years since they were buried.
essential materials would not be released from dead
organisms. When an organism dies, the enzymes in its So, an atom of carbon which today is in a molecule
cells, freed from normal controls, start to digest its own of carbon dioxide in the air may tomorrow be in a
tissues (auto-digestion). Soon, scavengers appear on molecule of cellulose in the cell wall of a blade of
the scene and eat much of the remains; blowfly larvae grass. When the grass is eaten by a cow, the carbon
devour carcases, earthworms consume dead leaves. atom may become part of a glucose molecule in the
cow’s bloodstream. When the glucose molecule is
Finally the decomposers, fungi and bacteria used for respiration, the carbon atom will be breathed
(collectively called micro-organisms), arrive and out into the air once again as carbon dioxide.
invade the remaining tissues (Figure B13.2).
These saprophytes secrete extracellular enzymes The same kind of cycling applies to nearly all the
(Chapter B4) into the tissues and reabsorb the liquid elements of the Earth. No new matter is created, but
products of digestion. When the micro-organisms it is repeatedly rearranged. A great proportion of the
themselves die, auto-digestion takes place, releasing atoms of which you are composed will, at one time,
the products such as nitrates, sulfates, phosphates, have been part of other organisms.
etc. into the soil or the surrounding water to be taken
up again by the producers in the ecosystem.

The speed of decay depends on the abundance
of micro-organisms, temperature, the presence of
water and, in many cases, oxygen. High temperatures
speed up decay because they speed up respiration of
the micro-organisms. Water is necessary for all living
processes and oxygen is needed for aerobic respiration
of the bacteria and fungi. Decay can take place in

The effects of the combustion of fossil fuels temperature, i.e. global warming, the majority
If you look back at the carbon cycle, you will see that of scientists and climatologists agree that it is
the natural processes of photosynthesis, respiration happening now and will get worse unless we take
and decomposition would be expected to keep the drastic action to reduce the output of these gases.
CO2 concentration at a steady level. However, since
the Industrial Revolution, we have been burning the Another factor contributing to the increase
fossil fuels such as coal and petroleum and releasing in atmospheric CO2 is deforestation. Trees are
extra CO2 into the atmosphere. As a result, the responsible for removing gaseous CO2 and trapping the
concentration of CO2 has increased from 0.029% to carbon in organic molecules (carbohydrates, proteins
0.035% since 1860. It is likely to go on increasing as and fats – see Chapter B3). When they are cut down
we burn more and more fossil fuel. the amount of photosynthesis globally is reduced.
Often deforestation is achieved by a process called
Although it is not possible to prove beyond all ‘slash and burn’, where the felled trees are burned to
reasonable doubt that production of CO2 and other provide land for agriculture and this releases even more
‘greenhouse gases’ is causing a rise in the Earth’s atmospheric CO2.

171

B13 Human influences on ecosystems ● Soil erosion is more likely to happen as there are no
roots to hold the soil in place. The soil can end up
●● Deforestation in rivers and lakes, destroying habitats there.

The removal of large numbers of trees results in ● Flooding becomes more frequent as there is no soil
habitat destruction on a massive scale. to absorb and hold rainwater. Plant roots rot and
● Animals living in the forest lose their homes and animals drown, destroying food chains and webs.

sources of food; species of plant become extinct ● Carbon dioxide builds up in the atmosphere as
as the land is used for other purposes such as there are fewer trees to photosynthesise, increasing
agriculture, mining, housing and roads. global warming. Climate change affects habitats.

Figure B13.3  Cutting a road through a tropical rainforest. The road Figure B13.4  Soil erosion. Removal of forest trees from steeply sloping
not only destroys the natural vegetation, it also opens up the forest to ground has allowed the rain to wash away the topsoil.
further exploitation.

The undesirable effects of Removal of forests allows soil erosion, silting up
deforestation on the environment of lakes and rivers, floods and the loss for ever of
thousands of species of animals and plants.
Forests have a profound effect on climate, water
supply and soil maintenance. They have been Trees can grow on hillsides even when the soil
described as environmental buffers. For example, layer is quite thin. When the trees are cut down and
they intercept heavy rainfall and release the water the soil is ploughed, there is less protection from the
steadily and slowly to the soil beneath and to the wind and rain. Heavy rainfall washes the soil off the
streams and rivers that start in or flow through them. hillsides into the rivers. The hillsides are left bare and
The tree roots hold the soil in place. useless and the rivers become choked up with mud
and silt, which can cause floods (Figures B13.4 and
At present, we are destroying forests, particularly B13.5). For example, Argentina spends 10 million
tropical forests, at a rapid rate (1) for their timber, dollars a year on dredging silt from the River Plate
(2) to make way for agriculture, roads (Figure B13.3) estuary to keep the port of Buenos Aires open to
and settlements, and (3) for firewood. The Food and shipping. It has been found that 80% of this sediment
Agriculture Organisation, run by the United Nations, comes from a deforested and overgrazed region
reported that the overall tropical deforestation rates 1800 km upstream, which represents only 4% of the
in the decade up to 2010 were 8.5% higher than river’s total catchment area. Similar sedimentation
during the 1990s. At the current rate of destruction, has halved the lives of reservoirs, hydroelectric
it is estimated that all tropical rainforests will have schemes and irrigation programmes. The disastrous
disappeared in the next 75 years. floods in India and Bangladesh in recent years may
be attributed largely to deforestation.

172

Deforestation

trees cut down
thin soil washed
away by rain

pastures over-grazed

sloping land ploughed removal of grass cover
cereal crops grown every year allows soil to be washed away

streams carry away topsoil water runs down furrows
causing gulley erosion

soil structure deteriorates
and it is blown or washed away

lakes and rivers choked with
silt, resulting in floods
Figure B13.5  The causes of soil erosion.

The soil of tropical forests is usually very poor in removed from the atmosphere in the process of
nutrients. Most of the organic matter is in the leafy photosynthesis. Most scientists agree that the
canopy of the tree tops. For a year or two after build-up of CO2 in the atmosphere contributes to
felling and burning, the forest soil yields good crops global warming.
but the nutrients are soon depleted and the soil
eroded. The agricultural benefit from cutting down Forests and biodiversity
forests is very short-lived, and the forest does not
recover even if the impoverished land is abandoned. One of the most characteristic features of tropical
rainforests is the enormous diversity of species
Forests and climate they contain. In Britain, a forest or wood may
consist of only one or two species of tree such
About half the rain that falls in tropical forests as oak, ash, beech or pine. In tropical forests
comes from the transpiration of the trees there are many more species and they are widely
themselves. The clouds that form from this dispersed throughout the habitat. It follows that
transpired water help to reflect sunlight and there is also a wide diversity of animals that live
so keep the region relatively cool and humid. in such habitats. In fact, it has been estimated
When areas of forest are cleared, this source of that half of the world’s 10 million species live in
rain is removed, cloud cover is reduced and the tropical forests.
local climate changes quite dramatically. The
temperature range from day to night is more Destruction of tropical forest, therefore, destroys
extreme and the rainfall diminishes. a large number of different species, driving many of
them to the verge of extinction, and also drives out
In North Eastern Brazil, for example, an area which the indigenous populations of humans. In addition,
was once rainforest is now an arid wasteland. If more we may be depriving ourselves of many valuable
than 60% of a forest is cleared, it may cause irreversible sources of chemical compounds that the plants and
changes in the climate of the whole region. This could animals produce. The US National Cancer Institute
turn the region into an unproductive desert. has identified 3000 plants having products active
against cancer cells and 70% of them come from
Removal of trees on such a large scale rainforests (Figure B13.6 on the next page).
also reduces the amount of carbon dioxide

173

B13 Human influences on ecosystems

Equator In 1971, 45 people in Minamata Bay in Japan died
and 120 were seriously ill as a result of mercury
Figure B13.6  The world’s rainforests. poisoning. It was found that a factory had been
discharging a compound of mercury into the bay as
●● Pollution of water part of its waste. Although the mercury concentration
in the sea was very low, its concentration was
Chemical waste increased as it passed through the food chain. By the
time it reached the people of Minamata Bay in the
Many industrial processes produce poisonous waste fish and other sea food that formed a large part of
products. Electroplating, for example, produces waste their diet, it was concentrated enough to cause brain
containing copper and cyanide. If these chemicals are damage, deformity and death.
released into rivers they poison the animals and plants
and could poison humans who drink the water. It is High levels of mercury have also been detected
estimated that the River Trent receives 850 tonnes of in the Baltic Sea and in the Great Lakes of North
zinc, 4000 tonnes of nickel and 300 tonnes of copper America.
each year from industrial processes.
Any factory getting rid of its effluent into water systems Oil pollution of the sea has become a familiar
risks damaging the environment (Figure B13.7). event. In 1989, a tanker called the Exxon Valdez ran
Some detergents contain a lot of phosphate. This is on to Bligh Reef in Prince William Sound, Alaska,
not removed by sewage treatment and is discharged and 11 million gallons of crude oil spilled into the
into rivers. The large amount of phosphate encourages sea. Around 400 000 sea birds were killed by the
growth of microscopic plants (algae). oil (Figure B13.8) and the populations of killer
whales, sea otters and harbour seals among others,
were badly affected. The hot water high-pressure
hosing techniques and chemicals used to clean up the
shoreline killed many more birds and sea creatures
living on the coast. Since 1989, there have continued
to be major spillages of crude oil from tankers and
off-shore oil wells.

Figure B13.8  Oil pollution. Oiled sea birds like this long-tailed duck
cannot fly to reach their feeding grounds. They also poison themselves
by trying to clean the oil from their feathers.

Figure B13.7  River pollution. The river is badly polluted by the effluent Discarded rubbish
from a paper mill.
The development of towns and cities, and the
crowding of growing populations into them, leads to
problems of waste disposal. The domestic waste from

174

Pollution of water

a town of several thousand people can cause disease
and pollution in the absence of effective means of
disposal. Much ends up in landfill sites, taking up
valuable space, polluting the ground and attracting
vermin and insects, which can spread disease. Most
consumable items come in packaging, which, if not
recycled, ends up in landfill sites or is burned, causing
air pollution. Discarded rubbish that ends up in the
sea can cause severe problems for marine animals.

Sewage Figure B13.9  Growth of algae in a lake. Abundant nitrate and phosphate
from treated sewage and from farmland make this growth possible.
Diseases like typhoid and cholera are caused by
certain bacteria when they get into the human Fertilisers
intestine. The faeces passed by people suffering from
these diseases will contain the harmful bacteria. When nitrates and phosphates from farmland and
If the bacteria get into drinking water they may sewage escape into water they cause excessive growth
spread the disease to hundreds of other people. of microscopic green plants. This may result in
For this reason, among others, untreated sewage a serious oxygen shortage in the water, resulting
must not be emptied into rivers. It is treated at the in the death of aquatic animals – a process called
sewage works so that all the solids are removed. The eutrophication.
human waste is broken down by bacteria and made
harmless (free from harmful bacteria and poisonous
chemicals), but the breakdown products include
phosphates and nitrates. When the water from the
sewage treatment is discharged into rivers it contains
large quantities of phosphate and nitrate, which
allow the microscopic plant life to grow very rapidly
(Figure B13.9).

Eutrophication Figure B13.10  Fish killed by pollution. The water may look clear but is
Nitrates and phosphates are present from a number so short of oxygen that the fish have died from suffocation.
of sources, including untreated sewage, detergents
from manufacturing and washing processes, arable
farming and factory farming.

If these nitrates or phosphates enter a water
system, they become available for algae (aquatic
plants) to absorb. The plants need these nutrients
to grow. More nutrients result in faster growth
(Figure B13.9). As the plants die, some through lack
of light because of overcrowding, aerobic bacteria
decompose them and respire, taking oxygen out of
the water. As oxygen levels drop, animals such as fish
cannot breathe, so they die and the whole ecosystem
is destroyed (Figure B13.10).

175

B13 human influenCes On eCOsystems

Figure B13.11 shows this sequence of events as a Checklist
flow chart.
After studying Chapter B13 you should know and understand
nitrates or phosphates from raw sewage, fertilisers or the following.
other sources enter a water system (river or lake) ● The materials which make up living organisms are constantly

algae absorb the nutrients and grow rapidly recycled.
(called an algal bloom) ● Plants take up carbon dioxide during photosynthesis; all

algae form a blanket on the surface of the water, living organisms give out carbon dioxide during respiration;
blocking light from the reaching algae below the burning of carbon-containing fuels produces carbon
dioxide.
algae die without light ● The uptake of carbon dioxide by plants balances the
production of carbon dioxide from respiration and
bacteria decompose the dead algae, using up combustion.
the oxygen in the water for respiration
● The combustion of fossil fuels and the cutting down of
animals in the water die through lack of oxygen forests increases the carbon dioxide concentrations in the
atmosphere.
Figure B13.11 The sequence of events leading to eutrophication.
● Deforestation is an example of habitat destruction: it can
Questions lead to extinction, soil erosion, flooding and carbon dioxide
build-up in the atmosphere.
1 Write three chemical equations:
a to illustrate that respiration produces carbon dioxide ● The conversion of tropical forest to agricultural land usually
(see Chapter B8) results in failure because forest soils are poor in nutrients.
b to show that burning produces carbon dioxide
c to show that photosynthesis uses up carbon dioxide ● Deforestation has many undesirable effects on the
(see Chapter B5). environment.

2 What kinds of human activity can lead to the extinction of ● We pollute our rivers, lakes and the sea with chemical,
a species? discarded rubbish, untreated sewage and fertilisers.

3 a What pressures lead to destruction of tropical forest? ● The process of eutrophication of water involves:
b Give three important reasons for trying to preserve ● increased availability of nitrate and other ions
tropical forests. ● increased growth of producers
● increased decomposition after death of the producers
4 What is the possible connection between: ● increased aerobic respiration by bacteria, resulting in a
a cutting down trees on hillsides and flooding in the
valleys, and reduction in dissolved oxygen
b clear-felling (logging) in tropical forests and local ● the death of organisms requiring dissolved oxygen in
climate change?
water.

176

Chemistry

Chapters C8 Acids, bases and salt
C9 The periodic table
C1 The particulate nature of matter C10 Metals
C2 Experimental techniques C11 Air and water
C3 Atoms, elements and compounds C12 Sulfur
C4 Stoichiometry C13 Carbonates
C5 Electricity and chemistry C14 Organic chemistry
C6 Energy changes in chemical reactions
C7 Chemical reactions

177

C1 The particulate nature of matter

Combined Co-ordinated

● State the distinguishing properties of solids, liquids and gases ● State the distinguishing properties of solids, liquids and gases
● Describe the structure of solids, liquids and gases in terms of ● Describe the structure of solids, liquids and gases in terms of

particle separation, arrangement and types of motion particle separation, arrangement and types of motion
● Describe the changes of state in terms of melting, boiling, ● Describe changes of state in terms of melting, boiling,

evaporation, freezing and condensation evaporation, freezing and condensation
● Demonstrate understanding of the terms atom, molecule
● E xplain changes of state in terms of particle theory and
the energy changes involved and ion

● Describe qualitatively the pressure and temperature of a gas ● E xplain changes of state in terms of particle theory and
in terms of the motion of its particles the energy changes involved

● Demonstrate understanding of the terms atom, molecule ● Describe and explain diffusion in terms of the movement of
and ion particles (atoms, molecules or ions)

● D escribe and explain dependence of rate of diffusion on
molecular mass

Chemistry is about what matter is like and how it
behaves, and our explanations and predictions of its
behaviour. What is matter? This word is used to cover
all the substances and materials from which the physical
universe is composed. There are many millions of
different substances known, and all of them can be
categorised as solids, liquids or gases (Figure C1.1).
These are what we call the three states of matter.

b l iquid

a solid c gas
Figure C1.1 W ater in three different states.

178

The kinetic theory of matter

●● Solids, liquids and gases The main points of the theory are:

A solid, at a given temperature, has a definite volume l All matter is made up of tiny, moving particles,
and shape which may be affected by changes in invisible to the naked eye. Different substances
temperature. Solids usually increase slightly in size have different types of particles (atoms, molecules
when heated (expansion) (Figure C1.2) and usually or ions) which have different sizes.
decrease in size if cooled (contraction).
l The particles move all the time. The higher the
A liquid, at a given temperature, has a temperature, the faster they move on average.
fixed volume and will take up the shape of any
container into which it is poured. Like a solid, a l Heavier particles move more slowly than lighter
liquid’s volume is slightly affected by changes in ones at a given temperature.
temperature.
The kinetic theory can be used as a scientific model
A gas, at a given temperature, has neither a definite to explain how the arrangement of particles relates to
shape nor a definite volume. It will take up the shape the properties of the three states of matter.
of any container into which it is placed and will
spread out evenly within it. Unlike those of solids Explaining the states of matter
and liquids, the volumes of gases are affected quite
markedly by changes in temperature. In a solid the particles attract one another. There are
attractive forces between the particles which hold
Liquids and gases, unlike solids, are relatively them close together. The particles have little freedom
compressible. This means that their volume can be of movement and can only vibrate about a fixed
reduced by the application of pressure. Gases are position. They are arranged in a regular manner,
much more compressible than liquids. which explains why many solids form crystals.

It is possible to model such crystals by using
spheres to represent the particles (Figure C1.3a).
If the spheres are built up in a regular way then the
shape compares very closely with that of a part of a
chrome alum crystal (Figure C1.3b).

Figure C1.2  Without expansion gaps between the rails, the track would a a model of a chrome alum crystal b  an actual chrome alum crystal
buckle in hot weather. Figure C1.3

●● The kinetic theory In a liquid the particles are still close together but
of matter they move around in a random way and often collide
with one another. The forces of attraction between
The kinetic theory helps to explain the way in which the particles in a liquid are weaker than those in a
matter behaves. The evidence is consistent with the solid. Particles in the liquid form of a substance have
idea that all matter is made up of tiny particles. This more energy on average than the particles in the solid
theory explains the physical properties of matter in form of the same substance.
terms of the movement of its constituent particles.
In a gas the particles are relatively far apart. They
are free to move anywhere within the container in
which they are held. They move randomly at very high
velocities, much more rapidly than those in a liquid. They

179

C1 the PaRtiCulate natuRe Of matteR

collide with each other, but less often than in a liquid, ●● Changes of state
and they also collide with the walls of the container.
They exert virtually no forces of attraction on each The kinetic theory model can be used to explain
other because they are relatively far apart. Such forces, how a substance changes from one state to
however, are very significant. If they did not exist we another. If a solid is heated the particles vibrate
could not have solids or liquids (see Changes of state). faster as they gain energy. This makes them ‘push’
their neighbouring particles further away from
The pressure inside a balloon is caused by the gas themselves. This causes an increase in the volume
particles striking the inside surface of the balloon. of the solid, and the solid expands. Expansion has
There is increased pressure inside the balloon at higher taken place.
temperatures as the gas particles have more energy
and therefore move around faster, striking the inside Eventually, the heat energy causes the forces
surface of the balloon more frequently. Since the of attraction to weaken. The regular pattern of
balloon is an elastic envelope, the increased pressure the structure breaks down. The particles can now
causes the skin to stretch and the volume to increase. move around each other. The solid has melted. The
temperature at which this takes place is called the
The arrangement of particles in solids, liquids and melting point of the substance. The temperature
gases is shown in Figure C1.4. of a pure melting solid will not rise until it has all
melted. When the substance has become a liquid
solid there are still very significant forces of attraction
particles only vibrate about fixed positions. between the particles, which is why it is a liquid and
Regular structure not a gas.

liquid Solids which have high melting points have
particles have some freedom and can move stronger forces of attraction between their particles
around each other. Collide often than those which have low melting points. A list of
some substances with their corresponding melting
gas and boiling points is shown in Table C1.1.
particles move freely and at random in all
the space available; they collide less often Table C1.1 Melting point/°C Boiling point/°C
than in liquid also Substance 661 2467
Aluminium
Figure C1.4 The arrangement of particles in solids, liquids and gases.
Ethanol −117 79
Questions
Magnesium oxide 827 3627
1 When a metal such as copper is heated it expands. Explain
what happens to the metal particles as the solid metal expands. Mercury −30 357

2 a Draw diagrams to show the arrangement of particles in: Methane −182 −164
i solid lead
ii molten lead Oxygen −218 −183
iii gaseous lead.
Sodium chloride 801 1413
b Explain how the particles move in these three states of matter.
c Explain, using the kinetic theory, what happens to the Sulfur 113 445

particles in oxygen as it is cooled down. Water 0 100

If the liquid is heated the particles will move around
even faster as their average energy increases. Some
particles at the surface of the liquid have enough
energy to overcome the forces of attraction between
themselves and the other particles in the liquid and
they escape to form a gas. The liquid begins to
evaporate as a gas is formed.

Eventually, a temperature is reached at which
the particles are trying to escape from the liquid so
quickly that bubbles of gas actually start to form
inside the bulk of the liquid. This temperature is

180

Changes of state

called the boiling point of the substance. At the 111000 liquid and all
boiling point the pressure of the gas created above gas (liquid gas
the liquid equals that in the air – atmospheric temperature/ЊC water and
pressure. water
vapour)
Liquids with high boiling points have stronger
forces between their particles than liquids with low 0 all solid and liquid all
boiling points. 15 solid (ice and liquid liquid
water) (liquid
When a gas is cooled the average energy of the water)
particles decreases and the particles move closer
together. The forces of attraction between the (ice)
particles now become significant and cause the gas
to condense into a liquid. When a liquid is cooled time/minutes
it freezes to form a solid. In each of these changes
energy is given out. Figure C1.6 G raph of temperature against time for the change from
Changes of state are summarised in Figure C1.5 ice at −15 °C to water to steam.
and are examples of physical changes. Whenever
a physical change of state occurs, the temperature The temperature will begin to rise again only after
remains constant during the change (see Heating and all the ice has melted. Generally, the heating curve
cooling curves). During a physical change no new for a pure solid always stops rising at its melting
substance is formed. point and gives rise to a sharp melting point. A
sharp melting point indicates a pure sample. The
heat heat addition or presence of impurities lowers the
(melt) (boil) melting point. You can try to find the melting
point of a substance using the apparatus shown in
cool cool Figure C1.7.
(freeze) (condense)
In the same way, if you want to boil a liquid such
solid liquid gas as water you have to give it some extra energy.
This can be seen on the graph (Figure C1.6) where
Figure C1.5 S ummary of the changes of state. the curve levels out at 100 °C – the boiling point
of water.
Heating and cooling curves
Solids and liquids can be identified from their
The graph shown in Figure C1.6 was drawn by characteristic melting and boiling points.
plotting the temperature of water as it was heated
steadily from −15 °C to 110 °C. You can see from The reverse processes of condensing and freezing
the curve that changes of state have taken place. occur on cooling. This time, however, energy is
When the temperature was first measured only ice given out when the gas condenses to the liquid and
was present. After a short time the curve flattens, the liquid freezes to give the solid.
showing that even though heat energy is being put
in, the temperature remains constant. thermometer

In ice the particles of water are close together rubber band melting point
and are attracted to one another. For ice to melt the oil tube
particles must obtain sufficient energy to overcome solid
the forces of attraction between the water particles
to allow relative movement to take place. This is heat
where the heat energy is going.
Figure C1.7  Apparatus shown here if heated slowly can be used
to find the melting point of a substance such as the solid in the melting
point tube.

181

C1 the PaRtiCulate natuRe Of matteR

Questions (Figure C1.9). This shows the position at which
the two gases meet and react. The white cloud
3 Draw and label the graph you would expect to produce if forms in the position shown because the ammonia
water at 100 °C was allowed to cool to −5 °C. particles are lighter and have a smaller relative
formula mass (Chapter C4, p.221) than the
4 Ice on a car windscreen will disappear as you drive hydrogen chloride particles (released from the
along, even without the heater on. Explain why this hydrochloric acid) and so move faster.
happens.

●● Diffusion – evidence for
moving particles

When you walk past a cosmetics counter in a
department store you can usually smell the perfumes.
For this to happen gas particles must be leaving open
perfume bottles and be spreading out through the
air in the store. This spreading out of a gas is called
diffusion and it takes place in a haphazard and
random way.

All gases diffuse to fill the space available. In
Figure C1.8, after a day the brown–red fumes of
gaseous bromine have spread evenly throughout
both gas jars from the liquid present in the lower
gas jar.

Figure C1.8 After 24 hours the bromine fumes have diffused throughout
both gas jars.

Gases diffuse at different rates. If one piece of Figure C1.9 Hydrochloric acid (left) and ammonia (right) diffuse at
cotton wool is soaked in concentrated ammonia different rates.
solution and another is soaked in concentrated
hydrochloric acid and these are put at opposite
ends of a dry glass tube, then after a few minutes
a white cloud of ammonium chloride appears

182

atoms, molecules and ions

Diffusion also takes place in liquids (Figure C1.10)
but it is a much slower process than in gases. This
is because the particles of a liquid move much more
slowly.

When diffusion takes place between a liquid and a gas
it is known as intimate mixing. The kinetic theory can
be used to explain this process. It states that collisions
are taking place randomly between particles in a liquid
or a gas and that there is sufficient space between the
particles of one substance for the particles of the other
substance to move into.

ab Figure C1.11 John Dalton (1766–1844).
Figure C1.10 Diffusion within nickel(ii) sulfate solution can take days to
reach the stage shown on the right. Atoms – the smallest particles

Questions Everything is made up of billions of atoms. The
atoms of all elements are extremely small; in fact they
5 When a jar of coffee is opened, people in all parts of the are too small to be seen. The smallest atom known
room soon notice the smell. Use the kinetic theory to is hydrogen, with each atom being represented as
explain how this happens. a sphere having a diameter of 0.000 000 07 mm
(or 7 × 10−8 mm) (Table C1.2). Atoms of different
6 Describe, with the aid of diagrams, the diffusion of elements have different diameters as well as different
copper(ii) sulfate solution. masses. How many atoms of hydrogen would have
to be placed side by side along the edge of your ruler
7 Explain why diffusion is faster in gases than in liquids. to fill just one of the 1 mm divisions?

●● Atoms, molecules Table C1.2 Sizes of atoms
and ions
Atom Diameter of atom/mm
In 1803, John Dalton (Figure C1.11) suggested Hydrogen 7 × 10−8
that each element was composed of its own kind of Oxygen 12 × 10−8
particles, which he called atoms. Atoms are much too Sulfur 20.8 × 10−8
small to be seen. We now know that about 20 × 106
of them would stretch over a length of only 1 cm.

Molecules

The atoms of some elements are joined together
in small groups. These small groups of atoms are
called molecules. For example, the atoms of the
elements hydrogen, oxygen, nitrogen, fluorine,
chlorine (Figure C1.12 on the next page),

183

C1 the PaRtiCulate natuRe Of matteR

Figure C1.12 A chlorine molecule. Checklist

bromine and iodine are each joined in pairs and they are After studying Chapter C1 you should know and understand
known as diatomic molecules. In the case of phosphorus the following.
and sulfur the atoms are joined in larger numbers, four
and eight respectively (P4, S8). ● Atmospheric pressure is the pressure exerted by the
atmosphere on the surface of the Earth due to the weight of
Molecules are not always formed by atoms of the the air.
same type joining together. For example, water exists
as molecules containing oxygen and hydrogen atoms. ● Boiling point is the temperature at which the pressure of the
gas created above a liquid equals atmospheric pressure.
Ions
● Condensation is the change of a vapour or a gas into a
An ion is another type of small particle. Unlike atoms liquid. This process is accompanied by the evolution of heat.
and molecules, however, ions have either a positive or
a negative electrical charge. These opposite electrical ● Diffusion is the process by which different substances mix as
charges cause the ions to attract one another strongly a result of the random motions of their particles.
to form hard solids with high melting and boiling
points. ● Evaporation is a process occurring at the surface of a liquid
involving the change of state of a liquid into a vapour at a
temperature below the boiling point.

● Kinetic theory is a theory which accounts for the bulk
properties of matter in terms of the constituent particles.

● Matter is anything which occupies space and has a mass.
● Melting point is the temperature at which a solid begins to

liquefy. Pure substances have a sharp melting point.
● Solids, liquids and gases are the three states of matter to

which all substances belong.

Questions

8 Are the following composed of atoms, molecules or ions?
a carbon, C
b water H2O
c sodium chloride Na+Cl−
d carbon dioxide, CO2
e iron, Fe

9 How would you use chemical shorthand to write a
representation of the molecules of iodine and fluorine?

184

C2 Experimental techniques

Combined Co-ordinated

● Name and suggest appropriate apparatus for the ● Name and suggest appropriate apparatus for the
measurement of time, temperature, mass and volume, measurement of time, temperature, mass and volume
including burettes, pipettes and measuring cylinders including burettes, pipettes and measuring cylinders

● Interpret simple chromatograms ● Demonstrate knowledge and understanding of paper
chromatography
● I nterpret simple chromatograms, including the use of Rf
values ● Interpret simple chromatograms

● Describe and explain methods of separation and purification ● Interpret simple chromatograms, including the use of Rf
by the use of a suitable solvent, filtration, crystallisation, values
distillation, fractional distillation, paper chromatography
● Understand the importance of purity in substances for use in
● Suggest suitable separation and purification techniques, given everyday life, e.g. in the manufacture of compounds to use in
information about the substances involved drugs and food additives

● Recognise that mixtures melt and boil over a range of
temperatures

● Identify substances and assess their purity from melting
point and boiling point information

● Describe and explain methods of separation and purification
by the use of a suitable solvent, filtration, crystallisation,
distillation, fractional distillation, paper chromatography

● Suggest suitable separation and purification techniques, given
information about the substances involved

●● Accuracy in Figure C2.1 T his stopwatch can be used to measure the time passed in a
experimental work in chemical reaction.
the laboratory
Measurement of temperature
Scientists find out about the nature of materials by The most commonly used thermometers in a
carrying out experiments in a laboratory. Many of laboratory are alcohol-in-glass. However, mercury
these experiments require apparatus that you have
used in your study of chemistry to date. Certainly Figure C2.2 A thermometer can be used to measure temperature.
a knowledge and understanding of the use of
this scientific apparatus is required for successful
experimentation and investigations that you may
carry out in your further study of chemistry. Much
of the work involves accurate measurements
with particular pieces of apparatus in particular
experiments, many of which are shown in the
section below.

Apparatus used for measurement
in chemistry

Measurement of time
Experiments involving rates of reaction will require
the use of an accurate stopwatch – one that measures
to a hundredth of a second. The units of time are
hours (h), minutes (min) and seconds (s).

185

C2 exPeRimental teChniques

in-glass thermometers can be used but should When using an electronic balance you should wait
be handled with great care. The mercury inside until the reading is steady before taking it.
them is poisonous and should not be handled if a Measurement of volume
thermometer breaks. The units of temperature are Different experiments involving liquids will require
those of the Celsius scale. This scale is based on the one or other or all the various measuring apparatus
temperature at which water freezes and boils, that is: for volume. The volume of a liquid is a measure of
●● the freezing point of water is 0 °C whilst the amount of space that it takes up. The units of
●● the boiling point of water is 100 °C. volume are litres (l) and cubic centimetres (cm3).
For accuracy the thermometer should be capable of
being read to a tenth of a degree Celsius. The usual 1 litre = 1000 cm3
thermometer used is that shown in the photograph
that measures accurately between –10 ° and 110 °C However, some of the manufacturers of apparatus
(Figure C2.2). When reading the thermometer always used for measuring volume use millilitres (ml). This
ensure that your eye is at the same level as the liquid is not a problem, however, since 1 cm3 = 1 ml.
meniscus in the thermometer to ensure there are
no parallax effects. The meniscus is the way that the When reading the volume using one of the pieces of
liquid curves at the edges of the capillary in which the apparatus (Figure C2.4) it is important to ensure that
liquid is held in the thermometer. the apparatus is vertical and that your eye is level with
the top of the meniscus of the liquid being measured.
Measurement of mass
There are many different electronic balances which
can be used. The important detail with any of them is
that they are accurate to one hundredth of a gram.

Figure C2.4 The apparatus shown in the photograph is generally used in
different experiments to measure volume accurately.

Questions

Figure C2.3 An electronic balance can be used to measure the mass of 1 What measuring device would you use to obtain values for:
reagents. a the volume of liquid in a coffee mug
b the mass of an apple
The units for measuring mass are grams (g) and c the temperature of a cup of tea
kilograms (kg). d the time taken to run up 20 stairs
e the time taken by an apple to fall through one metre?
1 kg = 1000 g
2 Complete the table below by stating the typical accuracy of
186 each of the measuring devices listed.

Device Accuracy

stopwatch

digital timer

digital balance

liquid in glass thermometer

100 ml measuring cylinder