bio form 4 chap 4 notes

CHAPTER 4 CHEMICAL COMPOSITION OF THE CELL

Importance of Water in the Cell

1. The human body consists of approximately 70% water.
2. If more than 5% of water is lost from your body, you will become unconscious. However, if more than

8% of water is lost from your body, it may lead to death.

Polarity of water

1. A polar molecule is a molecule with an unequal distribution of charges, that is, each molecule has a
partial positive and negative charge.

2. In a water molecule, the nucleus of oxygen attracts electrons stronger than the nucleus of hydrogen.
This causes the irregular distribution of electrons.

3. The irregular distribution of electrons causes water molecules to have partial charges/a few negative
charges ( − ) at the end of the oxygen atom and partial charges/a few positive charges (  + ) at both
ends of the hydrogen atoms that make the polarised water molecules.

4. The polarity property of water molecules causes the water molecules to pull one another, that is the
end of hydrogen (  + ) is attracted to the end of oxygen (  − ) electrostatically to form a weak
hydrogen bond.

5. This allows water to act as a universal solvent which can dissolve ionic compounds (salts) and other
polar molecules such as sugar and amino acid.

Specific heat capacity of water

1. The specific heat capacity of water is 4200 J kg -1 °C-1 or 4.2 kJ/kg/°C. This means that 4200 J of
heat energy is needed to raise the temperature of one kilogram of water by 1°C.

2. Water has a high specific heat capacity. Water can absorb a lot of heat energy with a small increase
in temperature. On the contrary, the high specific heat capacity of water causes the water to take a
longer time to cool.

3. This property is important in maintaining the body temperature of organisms.

Cohesive force and adhesive force of water

1. Water molecules stick to one another through cohesive force. For example, a drop of water on a
smooth surface appears spherical because of the attraction between the water molecules.

2. At the same time, water molecules stick to other surfaces through adhesive force.

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3. Both these forces result in capillary action that draws water u within the long and narrow column of
the xylem to the branches and leaves

Carbohydrates
Types of Carbohydrates

1. Carbohydrates contain the elements carbon, hydrogen and oxygen.
2. The ratio of C:H:O in the molecules is 1:2:1.
3. Many carbohydrates have the general formula (CH2O) n, or CnH2nOn
4. There are three main groups of carbohydrates:

a) monosaccharides
b) disaccharides
c) polysaccharides

Monosaccharide
1. Are the monomers of carbohydrates which are the simplest type of carbohydrates.
2. Glucose (grape sugar) is the most common monosaccharides and respiratory substrate.
3. Fructose (fruit sugar) are found in sweet fruits and honey. Galactose is present in milk.
4. All monosaccharides are reducing sugars because of their ability to transfer hydrogens or electrons to

other compounds, a process known as reduction.
5. The presence of reducing can be detected by using Benedict’s test.

a) When a simple sugar is boiled with Benedict’s reagent (alkaline solution of CuSO4), the colour
changes from blue to green, yellow, orange and finally, a brick red precipitate is produced.

b) The simple sugar reduces the blue copper (II) sulphate, CuSO4 (Cu2+) in the Benedict’s reagent to
insoluble copper (I) oxide, Cu2O (Cu+)
(A colour change from blue to green, yellow or brick-red indicates that a reducing sugar is present.
The difference in colour is due to the difference in the quantity of reducing sugar present.)

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Amount of reducing sugar present Colour of solution or precipitate
Increasing quantity Green
Of reducing sugar Yellow

Brick-red

Known as Monosaccharides Disaccharides Polysaccharides
Simple sugar Complex sugar / double Large complex sugar
Molecular sugar
formula C6H12O6 (C6H10O5) n
Glucose, fructose, C12H22O11
Examples galactose Glycogen, starch, cellulose
Maltose, sucrose (sugar No taste, cannot be
cane), lactose (milk sugar) crystallized
Insoluble
Taste Sweet, can be crystallized Sweet, can be crystallized
Non-reducing
Solubility in Dissolve Dissolve
water Yes Iodine solution turns dark
Reducing – maltose and blue in the presence of
Reducing lactose starch.
sugar Non-reducing – sucrose

Confirmation
test

Disaccharides

1. A disaccharide consists of two monosaccharides combined together chemically through a process called
condensation. In this process, one water molecule is eliminated. Are also known as complex sugar.

2. Disaccharides are soluble in water and form crystals.
3. Examples of disaccharides are maltose (malt sugar, presents in germinating seeds), sucrose (cane sugar,

occurs in fruits, sugar cane, sugar beet) and lactose (milk sugar, occurs in all mammalian milk).
4. Maltose is made up of 2 molecules of glucose; sucrose consists of one glucose and one fructose; lactose

is made up of one glucose and one galactose.

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5. Disaccharides can be broken down into monosaccharides through a process called hydrolysis (hydro:
water, lysis: break). This process requires a molecule of water to break down disaccharides.
Maltose + water → glucose + glucose
Sucrose + water → glucose + fructose
Lactose + water → glucose + galactose

6. All disaccharides are reducing sugars except sucrose which is a non-reducing sugar.
7. When sucrose is heated with Benedict's solution or Fehling's solution, a brick-red precipitate does not

form. Sucrose must first be broken down to fructose and glucose by boiling sucrose in a dilute acid
(hydrolysis). The solution is neutralized with caustic soda. When the products of hydrolysis are tested, a
brick-red precipitate is formed.

Polysaccharides

1. Are polymers of monosaccharide monomers.
2. Consists of hundreds of monosaccharides that are joined together by condensation to form a long chain

simple sugar.
3. Examples are starch, glycogen, and cellulose.
4. Are insoluble in water due to the large molecular size. Do not taste sweet and do not crystallise
5. Polysaccharides can be broken down into smaller molecules through hydrolysis using dilute acid and

enzymatic reaction.
6. Polysaccharide + water ------► monosaccharides
6. Glycogen is the storage form of glucose in animal cells whereas starch is the storage form of glucose in

plant cells. This is because glycogen and starch are insoluble in water, easily hydrolysed into glucose
for cellular respiration and too large to diffuse out of the cells, and thus remain within the cells.

Polysaccharide Sub unit Structure Occurrence
Starch
Unbranched, helical chains Found in plants such as wheat, rice,
of glucose potatoes, bread and corn. As a major
food storage of carbohydrate in plants.
Glucose Starch granules can be found in
chloroplasts, cereal and beans.

Highly branched short chain
of glucose units

Glycogen Glucose Known as animal starch. Store in liver
and muscle cells in humans and animals.

Straight unbranched chains
of glucose units.

Cellulose Glucose Plant cell walls. Provide support for plant
cells.

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Importance of carbohydrates in cells
1. As a support structure
i. Cellulose is the main component that builds up the cell wall.
ii. Cellulose is also the main source of fibre in our diet.
iii. Chitin forms the outer skeleton of insects and the cell wall of most fungi.
2. As a source of energy and food reserve
i. Starch is the main polysaccharide stored in plants. It is also found in chloroplasts. Sources of
starch includes wheat, rice, potatoes and bread.
ii. Glycogen is the main polysaccharide stored in muscle cells and liver cells.
iii. Glucose is the main source of energy in the cell (each gram of glucose can produce
approximately 17 kJ of energy when oxidised during respiration).

Proteins
1. About two-thirds of the total dry mass of a cell is composed of proteins.
2. Proteins are complex and large organic compounds.
3. Proteins are organic macromolecules found in all organisms.
4. Proteins contain carbon, hydrogen, oxygen and nitrogen. Sulphur is often present and sometimes

phosphorus and other elements too.
5. The monomer (building blocks) of protein is called amino acid. Amino acid is soluble in water but not in

organic solvent.
6. Each amino acid carries two functional groups: a carboxyl group (-COOH) which is acidic and an amino

group (-NH2) which is basic.

7. Two amino acids can combine to form a dipeptide by a condensation reaction. The bond linking the two
amino acids is called peptide bond.

8. Polypeptides are formed when many molecules of amino acids are joined together to form long chains of
amino acids.

9. Polypeptides are broken down through hydrolysis reaction to become dipeptides and finally amino acids.
The peptide bond can be broken by hydrolysis with heat, dilute acids or by enzymes.

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10. The loss of the three-dimensional structure of a protein is known as denaturation. The protein loses its
biological functions.

11. Factors that lead to the denaturing of proteins are: heat, pH, pressure, chemical and heavy metal.
12. Heating proteins usually denatures protein irreversibly. For example, the transparent egg white

irreversibly solidifies and becomes opaque on boiling.
13. There are 20 different types of amino acids found in the proteins of living cells.
14. Even though there are 20 types of amino acids, millions of combination amino acids can occur to form

various types of protein molecules.

Importance of Proteins in a Cell

1. Proteins are important:
(a) For the formation of cells for growth and for replacing damaged cells or tissues
(b) For producing enzymes to catalyse biochemical reactions
(c) For producing hormones to regulate a balanced internal environment in the body
(d) For the production of antibodies to eliminate pathogens and fight infections
(e) For forming haemoglobin to carry oxygen to the whole body
(f) As a replacement source of energy when carbohydrates are not enough
(g) Protein also forms structural components such as keratin in skin, collagen in bones and
myosin in muscle tissues.
(h) Histone is a type of protein that is important in packaging the DNA threads and also helps to
strengthen the chromosome structures.
(i) Glycoproteins (proteins that bind to carbohydrate) on the surface of the cell plasma membrane
strengthen and stabilise the cell plasma membrane and also help in the introduction and
communication between cells
(j) Carrier protein and pore protein help in the movement of essential substances across the
plasma membrane into the cell.

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Lipids

1. Lipids are organic compounds of carbon, hydrogen and oxygen.
2. Lipids have a high hydrogen to oxygen ratio (much higher than that of carbohydrate which is at 2:1).
3. Some lipids contain phosphorus and nitrogen.
4. The main types of lipids are:

Lipids Characteristic Function

1. Made up of one glycerol and three fatty 1. Serve as a good energy store.
acids. Is formed through condensation and 2. Stored under the skin as a heat
broken down by hydrolysis.
insulator.
3. Protect the organs
4. Transport fat-soluble vitamins (A, D,

E, K)

Triglycerides
(Fats and
oils)

2. Fats are solid at room temperature (20oC)
whereas oils are liquid. They are
chemically very similar.

3. Are insoluble in water but soluble in
alcohol

Phospholipids Composed of diglyceride that is bonded to a Most abundant lipids in plasma
phosphate group. membrane. Controls cell permeability.

1. Waxes are long-chained esters that give wax Are used to waterproof the external

its water repellent property. surfaces of plants and animals (furs of
mammals, feathers of birds). The cuticle
2. Are insoluble.
of the leaf and the protective covering on
3. Are usually hard solids at room temperature. an insect’s body are made of waxes.

Waxes 4. Wax also exists in sebum, a substance that
Steroids
is excreted from the oil glands inside the Sebum provides protection to our skin
skin epidermis. by preventing excessive loss of water,

preventing bacterial growth and it

softens the skin.

1. Steroids are lipid compounds that do not Cholesterol is mainly synthesised in the
have fatty acids and has a complex ring liver and is the precursor of many steroid
structure. hormones, such as testosterone,
oestrogen and progesterone.
2. Examples are vitamins, bile salts,

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cholesterol, sex hormones such as Cholesterol makes the membrane more
testosterone, oestrogen and progesterone. rigid and stable.

Sex hormones controls sexual
development and body physiology

Saturated and unsaturated Fats

1. There are two types of fats:
a) saturated fats
b) unsaturated fats

Saturated fat Unsaturated fat

Similarity Both are triglycerides that contain fatty acids and glycerol.

Both are non-polar molecules. They yield 38kJ per gram.

Is a triglyceride that contains saturated Is a triglyceride that contains un saturated
fatty acid. fatty acid.

Differences Has only single bond. The carbon atoms Has double bond in the form of -CH=CH–
are bonded to maximum number of other
atoms. Liquid at room temperature.

Solid at room temperature. Has a low melting point
unstable at room temperature and readily
Has a high melting point become rancid
More reactive because of the double bond.
More stable at room temperature and less
readily become rancid Less likely to cause diseases of the heart
and arteries
Less reactive
Example: mostly in plant product such as
More likely to cause diseases of the heart vegetable oils, palm oil, corn oil, olive oil,
and arteries because excess cholesterol peanut oil
will be deposited on the wall of arteries
and blocking the flow of the blood.
Example: mostly in animal product such
as red meat, chicken fat, chicken skin,
butter. Coconut oil

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Importance of Lipids in Cells and in Multicellular Organisms

In cells and multicellular organisms, lipids are
important:
For storing energy

(a) If taken in excess, the extra available energy will
be stored in the adipose tissue.

(b) For protection and as a heat insulator
(c) Lipids under the skin layer (subcutaneous fat)

regulate body temperature and absorbs shock
from impact.
(d) The lipid storage around organs such as the
kidneys and heart give mechanical protection to
the organs.
For digestion and absorption
(a) Bile juices is made up of lipid. It emulsifies fat
and aids in the digestion process.
For building the plasma membrane
(a) The presence of cholesterol within the plasma
membrane gives flexibility to an otherwise rigid
structure.
For transport
(a) Lipid is needed to transport the fat-soluble
vitamins A, D. E and K.
For the production of hormones
1. Cholesterol is needed in the production of
oestrogen, testosterone and progesterone
necessary for the development of secondary sex
characteristics.
Buoyancy - Multicellular living things such as marine
fish (and many single-celled aquatic organisms) store
lipids in their body to obtain buoyancy.

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Nucleic Acids

1. Nucleic acids are complex macromolecules that store genetic information in the form of codes.
2. The elements in a nucleic acid are carbon, hydrogen, oxygen, nitrogen and phosphorus.
3. Nucleic acids are single or double- chain polymers made up of nucleotide monomers.
4. Each nucleotide consists of a pentose sugar (5-carbon sugar), a phosphate group and a nitrogenous

base that combine through the process of condensation.
5. There are two types of pentose sugars; ribose and deoxyribose.
6. Nitrogenous bases consist of purine and pyrimidine.

(a) Purine consists of the bases adenine (A) and guanine (G).
(b) Pyrimidine consists of the bases cytosine (C), thymine (T) and uracil (U).
7. There are two types of nucleic acids:
(a) Ribonucleic acid (RNA) which contains ribose.
(b) Deoxyribonucleic acid (DNA) which contains deoxyribose.

Deoxyribonucleic acid (DNA)

1. DNA consists of two polynucleotide strands that coil around each other in opposite directions to
form a double helix (Figure 4.4).

2. When a chromosome is uncoiled, it forms a very long thread that is made up of one DNA molecule
and protein.

3. DNA is made up of units called nucleotide. Each nucleotide consists of three units:
(a) A sugar (pentose, deoxyribose
sugar)
(b) A phosphate group
(c) A nitrogenous base

4. The nitrogenous bases on both polynucleotide chains complement each other and are held together
by hydrogen bonds.

5. The nitrogenous bases for DNA are adenine (A), guanine (G), thymine (T) and cytosine (C). A
pair with T while G pairs with C.

3. The phosphate group and the deoxyribose sugar are the backbone in the DNA molecule as they are a
fixed structure.

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Ribonucleic acid (RNA)

1. The structure of RNA is a single
polynucleotide chain that is shorter than DNA.

2. RNA nucleotide has:
(a) Ribose sugar (DNA has deoxyribose
sugar)
(b) Four nitrogenous bases which are
adenine (A), guanine (G), cytosine (S) or
uracil (U) - Uracil replaces thymine (T)
available in DNA
(c) Phosphate group

3. Most RNA is present in the cytoplasm,
ribosomes and nucleus.

4. There are three types of RNA that is
messenger RNA (mRNA), ribosomal RNA
(rRNA) and transfer RNA (tRNA) which are
involved in the synthesis of proteins.

Importance of Nucleic Acids in a Cell

1. Nucleic acids (DNA and RNA) in cells are important because of their role in:
(a) carrying genetic information
(b) the production of protein

2. DNA carries genetic information in the form of genetic codes (in sequence of four different
nucleotide bases A, G, C and T) on a polypeptide chain.

3. Production of protein
(a) Both types of nucleic acids complement each other to produce proteins.
(b) DNA is in the nucleus, but the synthesis of protein occurs in the ribosome in the cell cytoplasm.

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4. Genetic codes are written as a series of three nucleotides that determine the sequence of amino
acids in the protein that will be synthesised. For example, the codon ACC (base sequence: adenine,
cytosine, cytosine) on mRNA is the code for the amino acid tryptophan.

5. There are two stages in the production of protein:
(a) Transcription
i. Transcription is the process by which the information in a strand of DNA is copied into a
new molecule of messenger RNA (mRNA).
ii. The mRNA leaves the nucleus after this process.
(b) Translation
i. Translation is the process in which a message carried by mRNA is decoded into a
polypeptide chain (protein)
ii. The mRNA attaches to a ribosome. The mRNA interacts with ribosomes to direct the
synthesis of the protein it encodes during this process.
iii. The mRNA is “read” according to the genetic code.
iv. Each group of three bases in mRNA constitutes a codon which specifies a particular amino
acid.
v. Transfer ribonucleic acid (tRNA) is a type of RNA molecule that helps decode a messenger
RNA (mRNA) sequence into a protein. tRNAs function at specific sites in the ribosome
during translation, which is a process that synthesizes a protein from an mRNA molecule.

6. The sequence of nucleotides in DNA determines the amino acid sequence in a polypeptide chain
that builds the corresponding protein.

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Formation of chromosome from DNA and protein
(a) DNA is a long polymer up to thousands of metres long, tightly packed up, to fit in the nucleus of
every cell.
(b) DNA molecule wraps around histone proteins forming tight loops known as nucleosomes.
(c) These nucleosomes coil and stack together to form fibres known as chromatin.
(d) Chromatin will then loop and fold with the help of additional proteins to form chromosomes.
(e) DNA is condensed into chromosomes to prevent DNA from getting tangled and damaged during cell
division.

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