ii) Digestion of food: The whole process of breaking down complex compounds in
the food into simpler compounds that our body can utilise is called digestion.
iii) Absorption: The process by which the essential nutrients are transported from the
intestine into the bloodstream is called absorption.
iv) Assimilation: Assimilation is the process by which the absorbed nutrients are
used up in the cells to release energy.
v) Egestion: Egestion is the process by which the undigested and unabsorbed food
mass is eliminated from the body.
Digestive enzymes and their function
The digestion is followed by several juices and enzymes that act on several chemicals and
gradually convert them into absorbable form. Some of the digestive juices and enzymes and
their function are given below:
a) Saliva : It is secreted by the salivary and parotid glands present in the buccal cavity.
The ptyalin enzyme present in saliva converts starch into maltose.
b) Bile juice : The bile juice does not contain enzyme but helps to neutralize acid and
emulsify fat. It is secreted by the liver.
FACT WITH REASON
A person whose gall bladder is removed should not eat spicy and oily food, why?
A person whose gall bladder is removed should not eat spicy and oily food because gall bladder stores bile
juice and supplies for the digestion of fat and oily food.
c) Pancreatic juice : The pancreatic juice is secreted by the pancreas. It contains three
enzymes: trypsin, amylase and lipase. The trypsin converts the proteins into peptones,
amylase acts on starch to form maltose and lipase converts fats into fatty acid and
glycerol.
d) Intestinal juice : The main intestinal juices are: The erepsin that converts peptones and
peptides into amino acids, the maltase that acts on maltose and converts it into glucose,
the sucrase that converts the sucrose into glucose and fructose and the lactase that acts
on lactose and converts it into glucose and galactose.
Importance of the digestive system:
i) Digestion is the only process that can convert the food into absorbable nutrients.
ii) It helps to extract essential nutrients like the carbohydrates, fats proteins, vitamins,
minerals and water from the food we eat.
iii) It controls the entry of germs from food to vital organs.
iv) It promotes growth, repairing and maintenance of the body parts.
v) It helps to excrete the unwanted and harmful substances by egestion and vomiting.
Respiratory system
The human body simply needs two basic raw materials to form energy. One is the glucose
that we obtain from food. The other is the oxygen that we breathe in. The oxygen breaks the
Modern Concept Science - 9 343
glucose and forms carbon dioxide and water which get expelled out. Both the entry of oxygen
and exit of carbon dioxide in the cells are possible by a system of organs called the respiratory
system.
The respiratory system is the system of organs that helps us to inhale oxygen, break food,
release energy in the cells and excrete carbon dioxide out of the body. The process of inhalation
of oxygen, production of energy and exhalation of carbon dioxide is known as respiration.
The process of respiration and the organs of the respiratory system are explained below:
Nasal cavity Pharynx
Mouth cavity Larynx
Trachea
Bronchiole Bronchus
Right lung Heart
Diaphragm Left lung
Respiratory system
a) Nose
The nose is the organ of the respiratory system. It is a part which is seen externally. It is
a protruding organ at the centre of the face that has two holes known as nasal passages
or nostrils. The nostrils are separated by a central cartilage and upper nasal bone. The
upper nasal cavities contain sticky substance called mucus which forms a thin lining
known as mucous lining. The foreign materials like pollen, dust etc. are checked by
small hairs and mucus present in the nasal cavity.
b) Pharynx
Pharynx is the funnel shaped tube that connects the food pipe and the windpipe with
the larynx. The part of pharynx that originates from the oral cavity is called oropharynx.
Similarly, nasopharynx is the part that originates from nasal cavity. Near the epiglottis,
a voice box is present which produces sound by forcing air vibrations through it. It is
called the larynx. On the top of larynx, a cartilaginous flap called the epiglottis guards
the entry of the food and water. It prevents swallowed food and liquid from entering
into the windpipe by blocking the way to the windpipe. On this entry level, there are
also muscular rings called the tonsils. They help to check the germs from entering the
body. When they are infected, they get swollen. This condition is called tonsillitis.
344 System
c) Trachea or windpipe
The trachea is a long air transporting tube that extends from the larynx to the bronchus
of the lung. It consists of flexible ring like cartilages that provides flexibility in the neck
movement. The walls of trachea are filled with sticky mucus to decrease friction during
breathing. The air that we breathe in moves into trachea for its journey into the lungs.
d) Bronchi
The larger tracheal tube that receives air is split into two small tubes of uneven lengths
and enters the lungs. These tubes are known as primary brochi (sing. bronchus). The
right primary bronchus goes into the right lung while the left primary bronchus enters
the left lung. The right bronchus is slightly longer than the left bronchus (5 cm and
2.2 cm respectively). Each bronchus further divides into sub branch to form secondary
bronchus. The right primary bronchus subdivides into three secondary bronchi while
the left primary bronchus divides into two secondary bronchi.
e) Bronchioles
After about 12-25 sub branching, when the diameter of the tubes in the lungs reaches
1mm, these tubes are known as the bronchioles.
f) Lungs and alveoli
The lungs are the two mechanical organs present inside the thoracic
cavity for breathing. They draw oxygen from the atmosphere for blood
purification and expel out carbon dioxide. The process of taking in air
inside the lungs is called inhalation or inspiration. Similarly, the process
of throwing carbon dioxide out of the lungs is known as expiration or
exhalation. The volume of lungs increases during on inhalation and Alveoli
decrease during exhalation. For smooth contraction and relaxation of
the lungs, there is a thin cavity outside the lungs and the inside the chest wall known
as the pleural cavity.
At the terminal part of bronchioles, small cherry like air sacs are present where the
gaseous exchange takes place. These air sacs are known as alveoli (sing. alveolus). In
other words, alveoli are the structural and functional units of lungs. There are about
300 million alveoli smaller than the grains of salt in the lungs. These tiny air sacs are
surrounded by blood vessels called capillaries. When we breathe in oxygen into the
lungs, its concentration increases which causes it to be pushed forcibly inside the blood
capillaries by diffusion process. Then, in the same way, the carbon dioxide escapes from
the blood into the air sacs and is expired out.
The simplest objective of the respiratory system is to breathe in oxygen and pass out
carbon dioxide. A healthy adult breathes about 18-20 times a minute. There are two
types of respiration: External and Internal respiration. The external respiration is simply
Modern Concept Science - 9 345
breathing in and breathing out. These processes are called inspiration and expiration
respectively.
Inspiration or inhalation
The process of breathing in air into the lungs is called inspiration or inhalation. Inhalation is
made easier by the diaphragm and the intercostal muscles. The diaphragm is a dome-shaped
muscular wall that separates the thorax from the abdomen. Similarly, the intercostal muscles
are the muscles present in between the ribs for the chest movement. When we inspire air, the
diaphragm and the intercostal muscles contract and pull the abdomen inwards. At the same
time, the chest expands and takes in the air.
Expiration or exhalation
The process of breathing out air from the lungs is called expiration or exhalation. During the
exhalation, the diaphragm and intercostal muscles relax causing the abdomen to increase in
size and chest to contract. This pushes carbon dioxide out of the lungs.
The internal respiration occurs in the cells of the human body. It is the process in which the
oxygen is used to break down glucose and release energy. The energy released is in the form of
chemical called ATP (adenosine triphosphate) molecules. A single glucose molecule can yield
29-30 molecules of ATP per reaction. The following chemical reaction explains the process of
internal respiration.
Glucose + Oxygen Enzymes Water + Carbon dioxide
C6H12O6 + 6O2 Enzymes 6H2O + 6CO2
Importance of respiratory system Memory Tips
i) It makes cellular oxidation possible by which energy Respiration is important because
it continuously releases energy to
is released.
keep us alive.
ii) It helps to remove waste CO2 from the body.
iii) It is the part of organs that defends us from germs (tonsils).
iv) It forces air into the larynx and helps us deliver speech.
v) It helps to take in air which carries chemical signals for smell.
Excretory system
There are about 37 trillion cells in the human body of an adult. Every second, the useful
nutrients and chemicals are being used up to form and break the other chemicals. During this
process, the body uses up substances as much as it wants, deposits the rest and throws out
the excess.
Excretion is the total process of getting rid of wasteful and toxic substances from the body.
Usually, the human body excretes out excess water, carbon dioxide, urea, foreign materials
like dust, dirt etc.
The Urinary system
The system of organs which is responsible for the purification of blood and filtration of toxic
substances like ammonia, urea etc. through urine is known as the urinary system. It contains
the following major organs:
346 System
a) The kidneys
Kidneys are the pair of bean-shaped organs that are located in the back of upper
abdomen. They are responsible for filtration of blood and removal of toxic substances
that get collected over time. The structural and functional unit of kidneys is called
nephron. These nephrons are small structures in the kidney that filtrate waste materials
in the blood.
Kidneys Cortex Renal artery
Medulla Renal vein
Ureter
Ureter
Urinary bladder
Urethra
Urinary system Detailed structure of kidney
b) Ureter
The waste liquid produced in the kidneys is rich in urea, excess electrolytes and water.
This passes through a pair of hollow muscular pipes known as the ureter. The ureter
transports the urine from the kidneys to the urinary bladder. Ureters are normally about
25 cm long.
c) Urinary bladder
It is the reservoir for urine storage for excretion. Its general capacity is half a litre and
maximum capacity is about one litre. When the volume of urine in the urinary bladder
increases generally above 250 ml, the muscles called urinary sphincters open up in the
bladder and pass the urine through excretory duct or urethra.
d) Urethra
Urethra is the internal excretory duct extending from urinary bladder to reproductive
organs. It is longer in males compared to females.It is the passage through which the
liquid from the urinary bladder is emptied out.
Process of Urine formation
The process of formation of urine starts in the part of kidney called the lobule. Inside the
lobules, there are functional units called nephrons. The nephron consists of a swollen structure
called the glomerulus which is enclosed inside a capsule called the Bowman’s capsule. At first,
the blood flows into the glomerulus through an arteriole which enters the Bowman’s space.
Then the blood is absorbed by tiny uriniferous tubules where important minerals and water
are absorbed. The final liquid is collected in a tube called the collection tubule in which wastes
like the urea, uric acid and creatinine pass through the liquid called urine. The collected urine
passes from the kidney to the bladder through the ureter which then excretes out through the
urethra.
Modern Concept Science - 9 347
FACT WITH REASON
We should drink lot of water to prevent stone, why?
Kidney filters blood to remove uric acid and other nitrogen compounds. If we drink less water waste will
change into stone in kidney, urinary bladder, urethra etc. So we must drink plenty of water.
Functions of the excretory system
i) It removes harmful substances like urea, excess salts, uric acid, creatinine etc. from the
body.
ii) It reabsorbs useful substances like Na+, K+ etc. in the kidney to prevent their rapid loss.
iii) It helps us to free from undigested materials.
iv) It maintains a balance between salt and water in the body.
v) It helps to control the overall body temperature.
vi) It prevents the building up of the toxic materials inside the body.
ANSWER WRITING SKILL
1. What is meristematic tissue?
Ans: Meristematic tissues are those tissues which are constantly growing due to active cell division
2. Name the two complex permanent tissues present in plants.
Ans: Xylem and phloem are wo complex permanent tissues present in plants.
3. Write a function of skull.
Ans: The function of skull is to provide a space to the brain and protect it from mechanical injuries.
4. Define internal respiration.
Ans: The process of oxidation of glucose to produce carbon dioxide, water and energy is called internal
respiration.
5. Write any two differences between xylem and phloem.
Ans: The two differences between xylem and phloem is tabulated below:
S.N. Xylem S.N. Phloem
1 Xylem conducts water and minerals from 1 Phloem conducts food materials from
the roots to the leaves. the leaves to all parts of the body.
2 Xylem tissues are dead cells except for 2 Phloem tissues are living cells except for
xylem parenchyma. phloem fibres.
6. Write any four functions of the excretory system.
Ans: The four functions of the excretory system are given below:
i) It removes harmful substances like urea, excess salts, uric acid, creatinine etc. from the body.
ii) It reabsorbs useful substances like Na+, K+ etc. in the kidney to prevent their rapid loss.
iii) It helps us to free from undigested materials.
iv) It maintains a balance between salt and water in the body.
348 System
7. Write any two functions of lungs.
Ans: The two function of lungs are:
i) It helps to take in oxygen and throw out carbon dioxide. A
ii) It makes platelets, a type of blood cell.
8. Label the following parts shown in the figure alongside. B
Ans: A – Frontal bone, B – Nasal bone, C – Maxila and D – Mandible C
9. Illustrate in brief the journey of food from mouth to anus. D
Ans: The journey of food starts from the mouth. Food gets chewed in the buccal cavity with the help
of teeth and tongue. Then it is swallowed with the assistance of epiglottis down to the esophagus
towards the stomach. The stomach makes the food acidic and treats the food with enzymes. After
an hour or two, the partially digested food passes to the small intestine through duodenum,
jejunum and ilium. The complex compounds in the small intestine is digested and converted into
small absorbable form. After digestion and absorption, the undigested matter moves to the large
intestine where water is absorbed. The undigested content is finally excreted out through the
rectum from the anus.
10. Observe the following figure and answer the questions that follow:
a) What is the tube that attaches the larynx and bronchus called?
Ans: The tube that connects the larynx with the bronchus is called
windpipe or trachea.
b) Is heart anterior or posterior to the lungs?
Ans: The heart is posterior to the lungs.
c) What happens in the alveoli?
Ans: Gases get exchanged in the air sacs. Oxygen gets diffused into the
blood capillaries and carbon dioxide is released in the air sacs.
STEPS EXERCISE
STEP 1
1. What is meristematic tissue? c) skull
2. What is permanent tissue?
3. Define xylem and phloem.
4. Mention the position of pith in the tree trunk.
5. Which tissue protects the leaves from the tearing effect?
6. Name the living part of xylem component.
7. What is latex?
8. Define cranium.
9. How many bones does the following body part have?
a) hand b) leg
d) rib cage e) vertebral column
10. Write the definition of peristalsis.
Modern Concept Science - 9 349
11. What is respiration?
12. Define nephron.
STEP 2
13. Write any two differences between xylem and phloem.
14. Mention any two similarities between simple permanent and complex permanent tissue.
15. Write the location of apical and lateral meristem.
16. Why are bones hard?
17. Bones dipped in an acid softens. Why?
18. Give a reason why the permanent tissue does not grow.
19. Name the smallest and largest bone of human body.
20. Write any two importances of digestive system.
21. What are the functions of kidneys?
22. Write the contents of urine.
STEP 3
23. Explain the types of meristematic tissues.
24. Name the types of simple permanent tissues and explain.
25. Write any two functions each of xylem and phloem.
26. Describe the physiology of a bone.
27. Write the names of cranial bones along with their numbers.
28. Name the bones of upper extremity along with their numbers.
29. How does the digestion of food take place in the small intestine?
30. What are alveoli? What are their functions?
31. List out any four waste materials secreted by human body and describe any two of
them.
32. Draw a labelled diagram to show:
a) human digestive system b) bones in upper limbs
c) bones in lower limbs d) bones in cranium
e) human respiratory system
STEP 4
33. Explain in brief the physiology of urine formation.
34. Write a short note on gaseous exchange in lungs.
35. Elaborate the process of digestion.
36. Mention any four digestive enzymes and their functions.
350 System
UNIT Estimated teaching periods Theory Practical
2 0
19
Sense Organs
Syllabus issued by CDC Eye
Introduction to sense organs of human body
Structure, function and care of eyes
Structure, function and care of nose
Structure, function and care of tongue
Structure, function and care of ear
Structure, function and care of skin
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
explain the structure of sense organs of human body and describe their functions.
Key terms and terminologies of the unit
1. Stimulus : The external factor that may cause organisms to respond towards or away from them is
called stimulus.
2. Reaction: The movement which is shown by an organism in response to the source of stimulus is
called reaction.
3. Receptors : The special cells present in the sense organs that detect changes in the environment are
called receptors.
4. Sense organs: The external organs which help to feel the changes in the surroundings are called sense
organs.
5. Conjunctiva: The conjunctiva is a tough covering of the sclera that can be seen from outside and
extends to the inner eyelid.
6. Iris: Iris is the coloured part of the eye which is an extension of choroid.
7. Pupil: The central circular opening of the iris is called the pupil
8. Eye lens: Eye lens is the crystalline biconvex structure present behind the pupil.
9. Suspensory ligaments: The lens is held in position by fibrous strands of tissues called the suspensory ligaments.
10. Accommodation : The process of focusing on objects of varying distances by our eye is called accommodation.
11. Aqueous humour: The fluid present in the anterior eye between the lens and cornea is called aqueous humour.
12. Vitreous humour: The fluid present in the posterior eye between the lens and the retina is called vitreous humour
13. Yellow spot: The yellow spot is the region on the retina where photoreceptor cells are present.
14. Blind spot: The blind spot is the region on the retina from where the optic nerve arises.
15. Rod cells: The cells which become activated in response to dim light are called rod cells.
16. Cone cells: The cells which become activated in response to bright light are called cone cells.
17. Retinol : A form of vitamin A in animals is called retinol.
Modern Concept Science - 9 351
18. Auditory canal: The tube that runs from the outer ear to the middle ear is called the auditory canal or ear canal.
19. Ear drum: The final end of the outer ear which picks up the sound vibrations and transfers them to
the middle ear is called the ear drum or tymphanic membrane.
20. Ear ossicles: The middle ear holds three small ear bones connected together are called the ear ossicles.
21. Middle ear infection : Infection of the ear oscicles which is common among children is called middle ear infection.
22. Tymphanic cavity: The air-filled cavity that encloses the ear ossicles is called the tymphanic cavity.
23. Eustachian tube: A tube that emerges from the tymphanic cavity and connects the middle ear to the throat
is called eustachian tube.
24. Taste buds: Taste buds are those structures on the tongue which contain taste receptor cells.
25. Olfactory cells: The sensitive cells in the mucus lining in upper nasal cavities are known as olfactory cells.
Introduction
The human body is an intelligent machine. It can react to the changes in the surroundings such
as heat, pressure, air, light etc. These external factors that may cause organisms to respond
towards or away from them are called stimuli (sing. stimulus).
The body may respond to the stimulus in different ways. The movement which is shown by
an organism in response to the source of stimulus is called reaction. After we detect the stimuli,
we become aware of it. All this happens inside our brain.
There are organs in our body which Sense organs Type of sense
help to detect the change in the Eye Sight and position
external world and internal world. Ear Hearing and equilibrium
The external organs help to detect Nose Smell and food sense
light, sound, touch, pain, chemicals Tongue Taste and chemical sense
etc. while the internal organs help
to detect blood pressure, heartbeat, Skin Touch, pain, pressure, temperature
sweating, urination etc. The external
organs which help to feel the changes in the surroundings are called sense organs. There are
five sense organs namely: eye, ear, nose, tongue and skin. Humans have five sense organs but
more than five senses. The organs and senses are shown in the given table.
Touch Skin Eyes Light
Pressure Ears
Temperature Sound
Balance
Sense
organs
Taste Tongue Nose Smell
352 Sense Organs
Sensory Receptors Memory Tips
The sense organs help to realise various senses with the help We need sensory organs for better
of specialised neurons or epithelial cells called receptors. understanding of surrounding,
Different organs have different receptors to detect the stimulus. communication and safety.
The organs and the respective receptors are as follows:
a) Eye : Photoreceptors- They help to sense light and enable vision.
b) Ear : Acoustic receptors (a type of Mechanoreceptor)- They help to sense different
frequencies of audible sound.
c) Nose : Olfactory receptors (a type of chemical receptors)- They help to sense smell of the
chemicals and food.
d) Tongue : Gustatory receptors (a type of chemical receptor)- They help to sense taste of
food and chemicals
e) Skin : Mechanoreceptors- They help to sense touch, pain, pressure, temperature etc.
Eyes
Eyes are the two delicate ball-like organs that help to sense External eye
the light around us. They are present in the anterior face
inside the body cavities called the orbital cavities. Each
eye moves around side to side to some extent with the
help of six muscles. The eyes work when light particles
called photons fall on them. They help to send the light
signals to the brain and thus we sense vision. Actually, the
image of the objects is formed in the brain rather than the
eye. We see with our eyes in the brain.
The eyeball is a very delicate and sensitive organ. Hence, it is protected inside the orbital
cavity cushioned with walls of fat tissues. The eyeball along with its accessory organs (helping
organs) help to achieve the ability of human to see.
External and Internal Morphology of Eye Sclera Choroid
The eyeball is roughly 2.6 cm in diameter with a near Cornea Retina
spherical shape. It is distinctly divided into three parts: Fovea
a) The outermost sclerotic layer Pupil
b) The middle black opaque layer called choroid Lens
Iris
c) The innermost layer called retina Ciliary body Optic nerve
The sclerotic layer
Internal morphology of eye
The sclerotic layer is the outermost tough coat of fibrous tissue that covers the entire eyeball.
It has blood capillaries on it. The sclerotic layer is divided into two parts:
a) The anterior sclerotic or cornea
The cornea is a transparent anterior part of sclerotic layer which bulges slightly outward.
Its multilayered tissue focuses most of the light rays inside the eye and reduces the
workload of the lens. Light rays enter into the eye through this part.
Modern Concept Science - 9 353
b) The posterior sclerotic or sclera
The white outer part of the sclerotic layer is called sclera. It has a tough fibrous coat that
protects the internal eye from injury and germs. It also supplies blood and nutrients to
the eye. Its outermost exposed layer is made up of thin semi-transparent membrane
called conjunctiva. The conjunctiva is a tough covering of the sclera that can be seen
from outside and extends to the inner eyelid. The conjunctiva fights bacteria, virus, dirt
etc. and keeps the eye healthy. Sometimes it might get swollen because of the allergic
reactions and infections. This condition is known as conjunctivitis.
The middle layer-choroid Memory Tips
The choroid is the middle thin layer of the eye that lies Choroid absorbs light so that
between the retina and the sclerotic layer. It contains a non- image will be clear.
reflective pigment and thus prevents unnecessary glare
inside the eye. The choroid extends in the anterior part of the eye in two ways. In front of the
lens, it forms the iris coat which has a hole at the centre called pupil. Similarly, at the upper
and the lower end of the lens, it forms the ciliary body.
a) Iris and pupil
Iris is the coloured part of the eye which is an extension Pupil
of choroid. It is also pigmented. Iris absorbs the glare
inside and outside the eye. It may be blue, brown or
hazel (reddish-brown or greenish brown) in colour
depending on the amount of pigment called melanin . Iris
The central circular opening of the iris is called the
pupil. It is highly light sensitive and is able to change its size. The size of the pupil
decreases when the light of higher intensity enters the eye. This protects the retina from
damaging. Likewise, the size of the pupil increases when the light of the lower intensity
enters the eye. This enables to have a clearer vision. Thus, the pupil controls the intensity
of light that enters the eyes.
FACT WITH REASON
We cannot see the objects inside a dim room immediately when we return back from the bright light out-
side, why?
The size of the pupil decreases when the light of higher intensity enters the eye. As we enter into a room
with dim light, the pupil does not adjust its size instantly. It becomes bigger after some time for entrance of
sufficient light inside the eye. So, we cannot see the objects inside a dim room immediately when we return
back from the bright light outside.
b) Lens, ciliary muscles and suspensory ligaments
The lens is the crystalline biconvex structure present behind the pupil. It focuses the
rays of light coming from objects into the screen of the eye i.e. retina. The lens is held in
position by fibrous strands of tissues called the suspensory ligaments. These suspensory
bodies are attached to the ciliary muscles that form the end of choroids. The function of
354 Sense Organs
suspensory ligaments is to hold the lens in place. Similarly, the ciliary muscles perform
two main functions:
i) They secrete a liquid called aqueous humour that maintains the shape of cornea
ii) They help to move the lens via the suspensory ligaments
c) Aqueous humour and vitreous humour
The gel like liquids that maintain the overall shape of the eyes are known as aqueous
humour and vitreous humour. The fluid present in the anterior eye between the lens
and cornea is called aqueous humour. The aqueous humour maintains the shape of the
cornea. It is present between the cornea and the lens. Likewise, the fluid present in the
posterior eye between the lens and the retina is vitreous humour. Both aqueous and
vitreous humour maintain the shape of the eye and act as a shock absorber.
FACT WITH REASON
Why do we see floaters?
We see floaters because blood cells mixed in aqueous humour blocks light and its shadow falls in retina.
The innermost layer : Retina
The retina is the third and the innermost layer of the eyeball. It is a multilayered structure which
is about 0.5mm thick. The rays of light entering the eyeball strike on this transparent layer where
several light sensitive cells are present. These light sensitive cells are called photoreceptors.
a) Yellow spot and blind spot
The yellow spot is the region on the retina where photoreceptor cells are present. It is
the spot where light signals are converted into electrical signals.
There are two kinds of photoreceptors cells:
i) Rod cells : The cells which become activated in response to dim light are called
rod cells. They contain a pigment called rhodopsin which is very sensitive to light.
Rhodopsin is formed by the action of retinol (vitamin A). Hence, the deficiency of
this vitamin may result in the failure of rod cells formation. If this happens, the
person suffers from night blindness in which he/she won’t be able to see in low
light situations. Rods do not distinguish other colours except shades of gray or
black-white.
FACT WITH REASON
Deficiency of vitamin A causes night blindness, why?
Rod cells response to dim light and make objects visible in dim light. Rhodopsin in the rod cells is formed by
the action of retinol (vitamin A). Hence, the deficiency of this vitamin may result in the failure of rod cells
formation and the person suffers from night blindness.
We can’t distinguish colours at night, why?
We cannot distinguish colours at night because our rod cells of the retina are activated during low light
situations and rod cells have no ability to distinguish colours.
Modern Concept Science - 9 355
ii) Cone cells : The cells which become activated in response to bright light are called
cone cells. They have various cells that are sensitive to different wavelengths of light.
The blind spot is the region on the retina from where the optic nerve arises. It doesn’t
contain any light sensitive cells and thus forms no image. This is why this region is called
blind spot. The optic nerve is a bundle of nerve cells that carries the electrical signal of
the image from the retina to the occipital lobe (just behind the eyes) of the brain.
FACT WITH REASON
Some people can’t distinguish colours, why?
Some people cannot distinguish colours because of colour blindness as a result of which their cone cells
respond to limited light colours.
b) The journey of light in the eye (Functioning of eye)
The working mechanism of the eye is very amazing. Light striking the various objects
enter the eye through the cornea. After that, the cornea refracts a two thirds of the light
passing into the eye which then enters the pupil that controls the intensity of the light.
The light then strikes the lens which then converges to the retina where the image is
formed. The nature of image so formed is real, inverted and small in size. The rods
and cones are responsible for the formation of the image. When light falls on them,
they become activated and convert the light signal into electrical signal. It then passes
through the optic nerve and reaches the brain. The vision is processed by the occipital
part of the brain where it connects to other parts to see the world around us. Amazingly,
it happens in a fraction of a second.
Achqaumeobuesr Iris Lens Vitreous chamber
Cornea Retina
Inverted image
of object
Object Pupil
Liquid
Optical nerve
Ligaments Jelly
Cilliary muscle
Working mechanism of human eye
Care of the eye
The eyes are the windows to the world that we live in. They are very sensitive to tough
situations. We can care them in the following ways:
i) Taking food rich in vitamin A from early childhood. (leafy vegetables and yellow fruits)
356 Sense Organs
ii) Avoiding works that cause strain to the eye e.g. long exposure to screens (cellphones,
TV, tablets, computers, laptops etc.)
iii) Washing eye regularly with clean tap water.
iv) Performing eye exercises and meditating
v) Avoiding reading, watching TV or writing in low light conditions
vi) Wearing sunglasses to avoid UV rays and glare that might harm our eyes.
vii) Visiting an ophthalmologist or a physician regularly for checking the conditions like
high BP, diabetes, defects of vision etc.
Ears
Sound is a form of energy that produces a sensation of hearing. Humans have two external
ear projections on either side of the head called pinnae. They are naturally formed in such a
shape which directs sound waves into the ear. The ear that we see externally is only a part of
it. Most of its parts with which we hear are present inside our head.
Ears are also the organs of balance. They help us to maintain posture, ride bicycles properly,
walk etc. The sense of position, rotation, motion, equilibrium etc. is all controlled by the ear.
Basically, there are three parts in the ear which are described below:
a) External ear : It directs sound waves into the ear, picks vibrations and controls foreign
particles.
b) Middle ear : It makes the vibrations strong enough to be sensed (amplifies).
c) Internal ear : It senses vibrations, senses position and converts them into electrical
signals.
External/outer ear (auricle, auditory canal and the outer ear drum surface)
Ear Ossicles Semicircular
Malleus Incus Stapes canals
Vestibular
nerve
Tympanic Tympanic cavity Cochlea Cochlear nerve
membrane Eustachian tube
Pinna (outer ear) External auditory canal
External and internal details of human ear
The outward portion of the ear projected upwards is called the ear auricle or pinna. Similarly,
the lower downward hanging part is called the earlobe. The pinna and earlobe run inward
towards the temporal lobe of the brain where it ends in a thin delicate membrane. The tube
Modern Concept Science - 9 357
that runs from the outer ear to the middle ear is called the auditory canal or ear canal. It is
filled with cerumen (ear wax) that is lubricated by an oily substance called sebum. The sebum
along with the ear wax checks in the presence of dirt and insects.
The final end of the outer ear which picks up the sound vibrations and transfers them to the
middle ear is called the ear drum or tymphanic membrane. It is so delicate that a feeble push
to the ear drum with a foreign material can puncture it.
The Middle Ear (posterior ear drum, air filled cavity, ear ossicles)
The middle ear lies between the outer ear and Malleus Incus
the inner ear in a small air-filled cavity. It is
connected to the throat directly. The middle Stapes
ear holds three small ear bones connected
together called the ear ossicles, namely the Auditory tube Oval
malleus, the incus and the stapes. The ear Middle ear window
ossicles connect to the conical part of the ear Eardrum
drum on one side and the depressed part of
the internal ear on the other. As the vibrations
are successively transferred from the tymphanic membrane to these ear ossicles (from malleus
to incus and incus to stapes), they amplify the vibrations to and fro like a lever.
The air-filled cavity that encloses the ear ossicles is called the tympanic cavity. A tube emerges
from the tymphanic cavity and connects the middle ear to the throat. This tube is called
Eustachian tube. The function of the Eustachian tube is to equalise the air pressure in the
middle ear with the atmospheric pressure in the external ear. This is necessary for better
hearing otherwise the vibrations will be suppressed and hearing would be weak. The throat
regularly forces air in and out of the Eustachian tube while Memory Tips
eating, swallowing or yawning. To facilitate this, one can
chew gums or chocolates while travelling to the areas of The middle ear has an Eustachian
higher altitudes viz. aeroplanes or hilly areas. Passengers in tube which helps to equalise the
an aeroplane are given chocolates or gums to force air in and air pressure in the middle ear with
out of the middle ear for equalizing the middle ear air the atmospheric pressure in the
pressure with the external air pressure. external ear.
FACT WITH REASON
Passengers in aeroplanes are suggested to chew gums or chocolates, why?
Passengers in aeroplanes are suggested to chew gums or chocolates because at higher altitudes air pres-
sure alters. Chewing action equalizes the air pressure in the external and middle ear. So, it helps in better
hearing.
Inner/Internal ear (cochlea, vestibule and semi-circular canals)
The inner ear is the special structure which is enclosed inside the rigid bones called the
bony labyrinth. It is responsible for converting the signals of sound, gravity and motion into
electrical signals. It consists of two main parts: the semi-circular canals and the cochlea fused
to a third part called the vestibule.
358 Sense Organs
a) The cochlea : It is a spiral fluid filled Vestibular labyrinth Canalith
hollow organ present inside the bony particles
labyrinth. The cochlea is attached to Utricle
the stapes of the middle ear at the part Semicircular
canals
called the oval window. It receives
sound vibrations from the stapes and
convert them into electrical signals
that are then sent to the brain. Eardrum
b) The semicircular canals : The Outer ear Inner ear
semicircular canals are three roughly
perpendicular hollow and oval cavities attached to the vestibule of the middle ear. They
help in equilibrium and posture maintenance.
Mechanism of hearing
At the oval window of the cochlea, the stapes vibrate with a piston like action (moving
inside and outside) passing the sound vibrations into the cochlear passage. The cochlear
passage is filled with a dense fluid called the perilymph which, in turn, passes the vibrations
ascending to the top of cochlea and then descending back. During the ascent and descent, the
perilymph transfers the vibrations to another liquid called endolymph. This finally activates
the specialized organ called the organ of corti. The organ of corti is a special structure present
in between endolymph and perilymph and has sound sensitive hair cells on them. When these
hair cells are bent due to vibrations, they fire and send electrical signals to the brain. Thus,
hearing is achieved.
Auricle Incus Scala Scala
vestibuli tympani
Malleus
External auditory canal wOivnadlow
Cochlea
Tympanic membrane Stapes Organ of Corti
(Ear drum)
Sound waves Round window
Basilar membrane
Outer ear Middle ear Inner ear
Mechanism of hearing
FACT WITH REASON
We cannot stabilize ourselves after spinning many times, why?
We cannot stabilize ourselves after spinning many times because our brain maintains equilibrium by
matching the information from eyes and liquid in semicircular canals. After we spin and stop, the fluid in
semicircular canals continues to spin, but the eyes suggest that we are at rest. Due to this confusion, we
tend to fall.
Modern Concept Science - 9 359
Care of the ears
i) Ears should never be pricked with any material that might puncture the ear drum. Even
with a cotton swab, you should be extremely cautious not to push it too hard inside.
ii) Each time after taking a bath, the ears should be cleaned and checked for water entry
iii) Ear wax should be removed regularly.
iv) Throat, nose and ear infections are interrelated. Therefore, they should all be checked.
v) Visiting to an ENT (ear, nose, throat) specialist each year might be a good idea.
Nose
The nose is the primary organ of smell and external Frontal sinus Superior turbinate
respiration. It is a protruding organ at the centre of Sphenoid sinus
the face that has two holes known as nasal passages
or nostrils. The nostrils are separated by a central
cartilage and upper nasal bone. The upper nasal
cavities contain sticky substance called mucus
which forms a thin lining known as mucous lining. Nasal vestibule
Eustachian tube
The mucus lining has sensitive olfactory cells
known as chemoreceptors. These cells have hair Internal details of nose with smell receptors
like structures that are sensitive to certain chemicals. When we breathe in air, the minute
chemical gases mixed with the air pass through the nasal passage and reach the upper mucous
lining. The chemicals then get mixed with the mucous and activate the hair like structures.
Depending on the nature of the chemical mixed, the hair like structures send various signals
to the brain where the sense of smell is realised.
FACT WITH REASON
We do not smell as well as food doesn’t taste good during common cold, why?
During common cold, our nasal epithelium with olfactory nerve cells is swollen and covered with mucus. It
stops being sensitive to chemicals. The smelling vapours can not be received by the chemoreceptors and we
do not smell. As the sense of smell is linked with the sense of taste, food doesn’t taste good.
Hairs inside the nasal cavity
The hairs present inside the nose prevent dust, dirt, insects and foreign materials from entering
the nose. This is necessary as it might lead to various problems like allergic reactions, common
cold, bronchitis etc.
Care of the nose
i) Nostrils should be cleaned daily with clean water.
ii) It should never be pricked or blown hard. Blowing the nose hard can cause significant
damage to both the ears and nose.
iii) We should visit an ENT specialist for the regular check-up of ear, nose and throat.
360 Sense Organs
Tongue
The tongue is a muscular organ present inside the Bitter
oral (mouth) cavity which helps in the taste of food, Taste buds
swallowing, chewing and speaking. It can be moved
or twisted with the help of eight muscles out of which Sour
four muscles are free and the rest four are attached to
the bone. The surface of the tongue contains rough Salty
hair-like projections called the papillae. Most types of Sweet
papillae bear chemical sensitive cells called the
chemoreceptors or gustatory receptors. These cells Various sites of tastes in tongue
allow us to realise the various tastes such as salty,
sour, sweet, bitter and umami (taste of ripe tomatoes,
cheese etc.). The taste buds are concentrated at
various regions of the tongue as shown in the figure.
Taste buds are those structures on the tongue which
contain taste receptor cells.
The sense of taste is realized at the taste buds. The taste Taste pore
bud has a taste pore on it through which the saliva Epithelium
mixed with various chemicals enters inside. These Sensory cells
chemicals activate the sensory gustatory cells which Supporting cells
are a type of chemoreceptors. The chemoreceptors
in return fire and send electrical signals through the Nerve
gustatory nerve to the brain. The brain analyses the
signal and thus we feel the taste. Structure of a taste bud
Care of the tongue
i) The tongue should be cleaned thoroughly with a soft cleaner. Applying force on the
tongue may damage the taste buds.
ii) Very hot and cold objects should be avoided as they may create numbness in the tongue.
Skin
The skin is the largest sense organ of
the human body that belongs to the
integumentary system. It forms the outer
protective coat all over the body except at
pores (opening of mouth, nose, anus etc.).
The surface of the skin has various pores
for sweat, hair and lipid production. Under
the skin, various sensitive cells called
mechanoreceptors are present that sense
touch, pain, pressure and heat.
Detailed structure of human skin
Modern Concept Science - 9 361
Care of the skin
i) We should take a bath daily with a gentle mild soap.
ii) Clothes worn should be comfortable and sweat absorbing.
iii) Infections and allergies should be treated early.
iv) The skin should be kept moist and be protected from the UV rays.
v) Skin of sensitive areas like the underarms and groins should be washed properly and
treated if itchy sensation is felt.
ANSWER WRITING SKILL
1. What is the function of cornea in the eye?
Ans: The function of cornea is to protect the internal parts of the eyes from germs, dust, dirt, etc. and to
refract the light rays onto the lens.
2. Define yellow spot and blind spot.
Ans: The region on the retina where image is formed is called yellow spot. It contains photoreceptor cells.
Similarly, the region on the retina where no image is formed is called blind spot. Optic nerve arises from
the blind spot.
3. Name the ear ossicles.
Ans: The ossicles present in the middle ear are malleus, incus and stapes.
4. What are taste buds?
Ans: Taste buds are those structures on the tongue which contain taste receptor cells.
5. How do we hear?
Ans: The sound waves travel through the ear drum to the middle ear ossicles. The ear ossicles amplify the
sound and transfer it to the oval window of the cochlea. The cochlear passage is filled with a dense
fluid called the perilymph which passes the vibrations ascending to the top of the cochlea and then
descending back. During the ascent and descent, the perilymph transfers the vibrations to another
liquid called endolymph. This finally activates the specialised organ called the organ of corti. The organ
of corti has sound sensitive hair cells on them. When these hair cells are bent due to vibrations, they fire
and send electrical signals to the brain. Thus, hearing is achieved.
6. How do we smell?
Ans: When we breathe in air, the minute chemical gases mixed with the air pass through the nasal passage
and reach the upper mucous lining. The chemicals then get mixed with the mucous and activate the hair
like structures. Depending on the nature of the chemical mixed, the hair like structures send various
signals to the brain where the sense of smell is realized.
7. Explain the mechanism of taste.
Ans: The sense of taste is realised at the taste buds. The taste bud has a taste pore on it through which the
saliva mixed with various chemicals enters inside. These chemicals activate the sensory gustatory
cells which are a type of chemoreceptors. The chemoreceptors, in return fire and send electrical signals
through the gustatory nerve to the brain. The brain analyses the signal and thus we feel the taste.
362 Sense Organs
8. Sketch a well labeled diagram of human eye. retina
Ans: The diagram is drawn alongside: Fovea
Iris
Pupil
Cornea
Lens Optic nerve
9. Which part of our ear helps us to ride bicycles and motor bikes with comfort? Discuss the mechanism.
Ans: The vestibular system of the internal ear helps in balancing or in static and dynamic equilibrium e.g. in
riding bicycles, bikes, etc. The semicircular canal contains a fluid called endolymph. These canals widen
in the region called the ampulla where sensitive hair cells are located. The hair cells are placed into a
gelatinous material called the cupula. When the head is moved, the flow of endolymph moves the cupula
and bends the hair cells. This eventually fires the hair cells and the motion is detected by our brain.
10. Observe the diagram alongside and answer the questions that follow:
a) Which part amplifies the sound? Ear Ossicles Semicircular
Ans: The ear ossicles of the middle ear amplify the sound. Malleus Incus Stapes canals
Vestibular
nerve
b) Which part is involved in balancing?
Ans: Semicircular canals of the internal ear are involved in balancing. Tympanic Tympanic cavity Cochlea Cochlear nerve
c)
What connects the incus and cochlea? membrane Eustachian tube
Pinna (outer ear) External auditory canal
Ans: The stapes connect the incus and cochlea.
STEPS EXERCISE
STEP 1
1. What is a receptor?
2. Write the functions of pupil and iris.
3. What is accommodation?
4. Which substance is the lens of the eye made up of?
5. Define rods and cones.
6. What is Eustachian tube?
7. Name the ear ossicles.
8. Name any two liquids present in the inner ear.
9. What is cochlea?
10. What are papillae of tongue?
Modern Concept Science - 9 363
STEP 2
11. What is the function of lens in the eye?
12. What do the ciliary muscles do?
13. How do some people fail to distinguish colours?
14. Why do we see optical illusion?
15. Name the tastes that our tongue can perceive.
16. Define dermis and epidermis.
17. What happens to your eyes when you suddenly exit out of a cinema hall? Why?
STEP 3
18. Write down the methods to care our:
a) Eye b) Ear c) Tongue
19. Explain the mechanism of vision.
20. Explain how we are able to smell.
21. Why can’t we stabilize ourselves after spinning many times?
22. Why does the music become clearer over a headphone after we yawn?
23. Why do we not see anything for sometime as soon as we enter the movie hall?
24. Food doesn’t taste good during common cold. Why?
25. Why are passengers in aeroplanes suggested to chew gums or chocolates?
STEP 4
26. Explain the mechanism of hearing.
27. How do we balance our body? Explain.
28. Describe the journey of light inside our eye.
29. Make a list of all sense organs and their functions.
30. Write a short note on sense organs.
364 Sense Organs
UNIT Estimated teaching periods Theory Practical
2 0
20
Evolution
Syllabus issued by CDC Jean B. de Lamarck
Introduction to evolution of organisms on the earth
Evidences of evolution
Theories of evolution (Lamark's theory and Darwin's theory)
Criticisms of theories of evolution (Lamark's theory and Darwin's theory)
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
discuss the evidences of organic evolution.
explain the theories of organic evolution.
Key terms and terminologies of the unit
1. Materialism: A belief in non-spiritual world of matter and its interaction is called materialism.
2. Hypothesis: A theory based on a few facts that is put to investigation to prove it is called hypothesis.
3. Evolution: The gradual development of life forms from simple to complex or early to present is called
evolution.
4. Organic evolution: The transformation of lower organisms into complex and higher organisms by modifying
themselves is called as organic or biological evolution.
5. Fossils: Fossils are the remains of ancient organisms or their imprints on the rocks that is visible
on careful digging.
6. Palaeontology: The branch of science that studies fossils is called palaeontology.
7. Homologous organs: The organs having similar structure and origin but different function in the real world are
called homologous organs.
8. Analogous organs: The organs that are similar in function but different in origin and structure are called
analogous organs.
9. Vestigial organs: The organs which had typical functions in the past but have become functionless or
rudimentary are known as vestigial organs.
10. Bridge organisms: The organisms having the characteristics of two or more groups of other organisms are
known as bridge organisms or connecting links.
11. Embryology: The branch of science that studies embryos of different organisms is called embryology.
12. Darwinism: The theory of organic evolution given by Charles Darwin is called Darwinism.
13. Lamarckism: The theory of organic evolution given by Jean-Baptiste Lamarck is called Lamarckism.
14. Variation: The difference in the structural and behavioural characteristics of organisms is called
variation.
15. Mutation: A sudden change from the parent type that is seen in an organism due to a defect in
genes is called mutation.
Modern Concept Science - 9 365
Introduction
The earth is a planet with varieties of life forms. There are simpler forms of life like the
unicellular organisms and complex ones like the mammals in the same biosphere. Were these
creatures same in the past as they are now? If not, what were they like in the distant past?
How did life originate in the first place? Well, there are various theories which deal with these
questions. In this chapter, we will discuss some of the convincing theories which explain how
the organisms evolved on this earth.
When life originated?
The oldest remains of organisms found on the earth were about 3.5 billion years ago buried in
rocks of South Africa and Australia. These rocks are called stromatolites. These rocks had
imprints of bacteria which are one of the oldest organisms found on the earth.
How life originated? Memory Tips
The best possible explanation of how life originated in the Evolution is defined simply as
earth can be put forward in two steps:
genetic change over time.
a) Chemical evolution
The process by which simpler compounds arranged themselves to form complex
compounds like DNA, RNA, proteins etc. is called chemical evolution. Due to chemical
evolution, complex substances like the proteins were organized to form the cell of the
first organism.
b) Organic Evolution Memory Tips
The first organism to get originated was a unicellular
organism (e.g. a bacteria). But what happened after All organisms on the earth were
that? Where were the monkeys, tigers, trees and formed from the some pre-existing
humans? The answer is: life gradually became complex organisms in the past.
from the simpler forms. Some unicellular organisms gradually and continuously
developed to convert into multicellular organisms. Likewise, invertebrates developed
into vertebrates and gymnosperms into angiosperms and so on. This transformation
of lower organisms into complex and higher organisms by modifying themselves is
termed as organic or biological evolution. The living organisms that we see around
today were not the same in the past. Biological evolution is currently the most scientific
and accepted theory in the community of scientists.
The central idea of biological evolution is that most of the species existing now shared
a common ancestor. It is in the same way that your father and uncle share the same
grandfather. Hence, it suggests that all the organisms living today were cousins at some
point in history. For example, all mammals, amphibians, reptiles and birds were fishes
living in the oceans millions of years ago. In a similar manner, it forms a tree like structure
to suggest which organism evolved from which form of ancestor. This structure is called
an evolution tree. A small segment of the tree is shown in the picture below.
The theory of biological evolution is not just a hypothesis. It is based upon the evidences
collected by various scientists from various branches of science. The study of evolution is
a comprehensive study of embryology, genetics, geology, palaeontology, biochemistry,
anatomy and other branches of science.
366 Evolution
Land plants
Birds
Insects
Reptiles Mammals
Amphibians
Arachnids
Fishes Mollusks
Brachiopod
Crustacean
Worms Seaweed
Bryozoans
Vertebrates
Echinoderms
Sponges Coelenterates
Protozoans Protophytes
Protists
Evidences of Evolution
Several branches of science reach to a similar conclusion that the organic evolution is real.
Evolution continuously changed organisms in the past and present and will gradually and
continuously transform organisms in the future too. But it is a very slow process that is not
observed directly. The change in organisms is so slight that we do not observe it in our lifetime
or several lifetimes. Some of the evidences which support evolution are:
a) Evidences from palaeontology : the study of fossils
Fossils are the remains of ancient organisms or their Study of fossils
imprints on the rocks that is visible on careful digging.
When organisms die and get buried in the soil without
much decomposition, they get crushed upon by the
sediments and harden over millions of years. During
this process, they leave a mark of their bodies on the
Modern Concept Science - 9 367
underlying rocks. The temperature and pressure are extreme during fossil formation.
In this way, fossils are formed. The branch of science that studies fossils is called
palaeontology.
Look at the given picture of the sedimentary rock layers. The fossils of organisms buried
in the past are depicted in different strata or layers. The fossils of organisms found at
the deeper strata lived on this earth earlier compared to the fossils of organisms buried
at the upper strata. This is because the sediments mostly deposited on one layer upon
the other and got pushed successively inside the earth. The actual fossils found on these
rocks had a certain trend as follows:
i) Different strata of rocks had different structures of fossils of the same creature.
ii) The deeper strata had primitive and less developed fossils of creatures and the
upper strata had developed and advanced fossils of creatures.
iii) Some of the fossils of organisms found deep inside the earth do not exist today.
From the points above, we can conclude that the organisms buried earlier in the deeper
part of the earth were primitive. In the course of millions of years, they transformed
gradually into advanced species. Some of them could not survive due to unfavorable
conditions and got extinct. One of such species was an organism called the Archaeopteryx
which looked both like a reptile and a bird. This proves the idea of organic evolution.
FACT WITH REASON
Humans have not evolved from chimpanzees. Give reason.
Humans have not evolved from chimpanzees or any other recent apes because study of fossil suggests that
human and chimpanzee evolve from a common ancestor that looked neither like a human nor a chimpanzee.
b) Evidences from comparative morphology and anatomy
Morphology is the study of external structures and forms while the anatomy is the
study of internal structures of organisms. The same organs of different species may
either be similar in structure and function or might differ. On the basis of the origin,
structure and function, these organs show some evidences in support of evolution.
i) Homologous organs
Look at the picture alongside carefully where
the flipper of a seal, wing of a bird, patagium of
a bat, forelimb of a horse and arm of a human is
shown. These organs are similar in structure. The
placement of bones, muscles and the distribution
of blood vessels are also in a similar manner. But
these organs have different functions in the real
world. For example; human arm is for grasping Human Cat Whale Bat
things but the horse limbs are for running. Homologous organs
The organs having similar structure and origin
368 Evolution
but different function in the real world are called homologous organs. The presence
of homologous organs suggests that the organisms having them belong to a certain
common ancestor. For instance: bat, cat, human, horse etc. have four limbs but a fish
does not. It implies that bat, cat, human and horse had a common ancestor in the past
as their limbs were formed from a similar origin. Thus, it proves that it is evolution that
modifies organs of organisms over time.
ii) Analogous organs
The organs that are similar in function but analogous
different in origin and structure are called
analogous organs. These organs differ in
external and internal features and origin
but can perform similar tasks. Let us take Insect
an example of three analogous organs:
wing of a bat, bird and an insect. All these
three organs are adapted to flying in
spite of having different structures. These
organs have been evolved from organs of Pterodactyl Bird Bat
different ancestors but are evolved for the Analogous organs
same cause i.e. flying. Bat has heavier body than that of an insect. That’s why its wings
are muscular to induce more force in flying. Analogous organs provide the evidence for
evolution by stressing that organs of different species can modify over time to perform
the desired function. Since it suggests in modification of a body part over time, it proves
the point of evolution.
FACT WITH REASON
Analogous organs suggest evolution. Explain.
Animals from different ancestor modify their organs for the common function. Such organs are analogous
organs. And modification is the root for evolution.
C) Evidences from the vestigial organs
The human body is the overall package of organs and systems having different functions.
In the past, as humans were in the verge of physical and mental development, they
got several organs modified due to environmental and genetic causes. Some of these
organs failed to perform a particular function but were still carried on to the successive
generations. The organs which had typical functions in the past but have become
functionless or rudimentary are known as vestigial organs. Humans have numerous
vestigial organs which are functionless. Some of them are illustrated in the table below:
Ear muscles
Nictitating
membrane
Body hair
Modern Concept Science - 9 369
Segmental Vermiform
muscles appendix
Vestigial organs of human body Coccyx
Vestigial Organs Functional in Function
Vermiform appendix Herbivores digesting cellulose
Ear muscles Animals locating the sound source
Nictitating membrane Frogs, crocodiles etc. moistening and protecting the eye
Coccyx bone Chimpanzee, horse etc. helping in moving and balancing
Wisdom teeth Human ancestors chewing cellulose
Breast nipples in men Mammalian females milk feeding
The presence of functionless vestigial organs in humans suggests that humans and
other organisms that have these fully functional organs had common ancestor in the
past.
c) Evidences from studying embryos Memory Tips
When a zygote grows, it turns into a structure Human beings have vermiform
called the embryo. The study of embryo is called appendix because our ancestors
embryology. Embryology tells us that we turn into might have lots of plant diet.
a typical structure on the basis of information stored
on our genes (specific groups of DNA). The point is:
if organisms have evolved from a common ancestor,
they have similar genes and thus their bodies
show some similarities to each other at least in the
embryonic stage of development. Certain species
like the fish, salamander, tortoise, chick, rabbit and
human resemble each other to some extent at some
stage in their embryonic development. As predicted
by the biogenetic theory or recapitulation theory, it
is believed that they had similar genes in the past i.e.
they have been evolved from a common ancestor. Embryos of vertebrates
This proves that evolution is real.
FACT WITH REASON
Humans and cows have same ancestors. Give reason.
Humans and cows have same ancestors because the study of development of embryo of both species suggests it.
370 Evolution
d) Evidences from connecting links or bridge organisms
Archaeopteryx Lung fish Platypus
Characteristics of Characteristics of Characteristics of
Reptiles Birds Fish Amphibians Reptiles and Mammals
birds
1. Toothed 1. Wings with 1. Paired fins 1. Lives both in 1. Egg laying 1. Hairy
beak feathers 2. Cold blooded 2. Mammary
2. Gills land and water
2. Bony tail 2. Four toes and has a glands
3. Wing claws 3. Scales in body 2. Has lungs beak 3. Diaphragm
There are various groups of organisms in Connecting link Between the group
biology divided on the basis of specific
characteristics. These organisms are grouped Archaeopteryx Reptiles and birds
under heads like the reptiles, amphibians,
mammals etc. A reptile differs from a Cycas Pteridophytes and
mammal and a mammal differs from a bird. gymnosperms
Some organisms, on the other hand, have the
characteristics of two or more groups and thus Euglena Animals and plants
resemble more than one group of organisms.
Duck billed platypus Reptiles, birds and
mammals
Protopterus (lung fishes) Bonny fishes and amphibia
Rickettsia Virus and bacteria
Virus Living and non-living
The organism having the characteristics of two or more groups of other organisms
is known as bridge organism or connecting link. Connecting links are the ancestors
of two definite groups of organisms which they resemble. Some of those species are
Archaeopteryx (fossilized), platypus (Australian) and lungfishes (fish with lungs). The
table above depicts their characteristics with their resembling groups. The existence of
connecting link is the evidence that some groups of organisms had common ancestor in
the past or were evolved from that group. This strongly supports the theory of evolution.
FACT WITH REASON
Why is platypus called connection between reptiles, birds and mammals?
Platypus is called connection between reptiles, birds and mammals as it lays eggs, has a beak, is cold-
blooded and has mammary glands and hair on their body.
e) Evidences from the distribution of organisms
This evidence is based on the aspects: geography, climate and origin.
Geography
Some animals are found to be isolated only in one part of the world, e.g. kangaroo
in Australia. When Australia was connected to the southern super continent millions
of years ago, mammals like the kangaroo were the first ones to evolve. Before other
species of mammals had evolved and moved to Australia, Australia got separated from
Modern Concept Science - 9 371
southern super continent. This resulted in the fact that Australia now has got more
than half of the population of kangaroo like creatures as other mammals were brought
later on artificially. If all mammals had evolved at the same time, Australia would have
shared all species of mammals in the distant past.
Climate and origin
Climate and origin both change the way organisms evolve. Organisms having the same
ancestor may also be different if they live in different climatic conditions. On the other
hand, those organisms that live in similar climatic conditions might also be different
from each other if they are of different origins. This fact tells us that organisms might
change over time due to environmental and genetic factors and hence suggests that
evolution occurred.
FACT WITH REASON
Why are humans living in different region of earth are slightly different from each other?
Even though we are same species, people living in different region of earth are different from each other it is
because of the adaptation to the surrounding land form, weather, availability of food etc.
Why is the theory of biological evolution supported by many people?
Theory of biological evolution is supported by many people because of evidences collected from various
fields such as embryology, palaeontology, anatomy etc.
Theories of organic evolution
In the text above, we discussed some evidences of evolution. The explanation of the process
of evolution is proposed by several biologists in the form of different theories of evolution.
Among them, three theories are prominent:
a) Lamarckism : Jean Baptiste de Lamarck’s theory of evolution
b) Darwinism : Charles Darwin’s theory of evolution Memory Tips
c) Mutation theory : Hugo De Vries’ theory Lamarckism is important because
a) Lamarckism it is the first theory that supports
Lamarckism is the overall idea proposed by French theory of evolution.
biologist Jean Baptiste de Lamarck (1744 A.D. – 1829 A.D.). He was the first person to propose
an evolutionary idea. Lamarck pointed out that organisms are exposed to different types of
environments due to which they realize certain needs to be fulfilled. These needs stimulate
the biological processes in them and hence they tend to satisfy these needs. In general,
Lamarckism as published in different books of Lamarck like the Philosophie Zoologique is based
on the following points:
i) Nature’s role in evolution
ii) Use and disuse of organs
iii) Inheritance of acquired characteristics
i) Nature’s role in evolution
Lamarck argued that organisms modify their structures and behaviour according to the
change in environment or nature. With the natural phenomena of climate, weather and
372 Evolution
habitat, organisms feel the ‘need’ to modify themselves according to the environment
they live in. For instance, yak living in cold regions realized the need to grow excessive
hair on its body, water creatures learnt to swim as they felt the need to efficiently move
in water.
ii) Use and disuse of organs
Lamarck put forward this idea
of use and disuse of organs
relating it to the organs’
use over time. It states that
organisms have an internal
feeling of need to adjust with
the environment. They, in
turn, change their behaviour
because of this adjustment. Elongation of neck of giraffe according to Lamarck
With the changed behaviour, the use of some organs of their bodies increases while other
organs remain unused. According to Lamarckism, if this happens for a considerable
period of time, the growth of these unused organs becomes affected. The organs that
are repeatedly used become more grown and developed in the successive generations
while the unused organs either become rudimentary or will disappear.
In a classical example of this idea, Lamarck took the long neck of the giraffe to explain
the effect of overuse of an organ. He says that, in the past, giraffes were short necked and
couldn’t get enough food because of the presence of tall trees. Then, in each successive
generation, the giraffe felt the need to get those leaves from the taller trees and so
started to stretch its neck. Now, the behaviour of giraffe has been changed and so the
information of a longer neck is transferred to the next generation making its successors
long-necked. It consequently resulted in the long necks of the present day giraffes.
iii) Inheritance of acquired characteristics Memory Tips
The behaviour of organisms is shaped by three
factors: genetic, environmental and acquired factors. A glimpse of Lamarck's theory:
Lamarck focused on the environmental and acquired Inheritance of acquired characters
factors and said that these two shape the genetic and organ usage causes evolution.
factors. He argued that those qualities or behaviours that we acquire in our lifetime will
pass on to our next generations. This also includes the information of use and disuse of
an organ. If you imply this to your daily life, it would be quite inconsistent. This is to say
that the information we obtain in our life such as reading, writing, speaking, dancing
etc. will pass on to our next generation. The modern advancement in the study of genes
of organisms disproves this theory and is no longer considered as a scientific theory.
Criticisms on Lamarckism
Lamarckism was based on extensive study of plants and animals. During the time of Lamarck,
which was even before Charles Darwin, there had been no research on genes and heredity that
he could apply. So, his theory was discredited and criticisms were raised as follows:
Modern Concept Science - 9 373
i) Acquired characteristics are generally not inherited
Unlike the predictions of Lamarck, the characteristics that we acquire in our lifetime
do not get transferred to our offspring. If the father is PhD in Physics, his son/daughter
won’t be born as a physicist. If you grow biceps and triceps in your lifetime, the same
won’t grow by birth in your offspring. Thus, Lamarckism fails on this part of the theory.
An experiment was conducted in mice by an evolutionary biologist August Weismann
in an attempt to test the theory of use and disuse of organs. In this experiment, the tail
of mice was cut in 21 successive generations. In the 22nd generation, the mice were born
with same length of tails in contrast to the Lamarck’s predictions. If Lamarck’s theory
had been correct, in each generation, the tail would have to be shorter and shorter as the
mouse didn’t use its tail throughout its life. This disproved the theory of use and disuse
of organs.
ii) Organisms do not always develop by growing
Lamarck had this idea of ever-growing concept of organisms from the past to the
present. But not all organisms continue to grow towards their successive generations.
Present day angiosperms are of smaller size as compared to their gymnosperms and
fern ancestors.
iii) The need and desire of a species do not lead to the formation of new structures
The idea of Lamarck was simple. If you live in an environment which is constantly
changing, you feel this need to grow new structures to adapt with the environment. But
in reality, desire and need are abstract terms and are not considered scientific enough
to grow new structures.
iv) The use and disuse of organs do not affect its size
Lamarck’s idea of use and disuse of organs was criticized on the basis of the fact that
when we use our organs repeatedly, the information is acquired in the life time and not
transferred to our genes. Therefore, it is unlikely that it passes on to the next generation.
For instance: we use our heart all the time, but it doesn’t grow a bit. Similarly, we see
with our eyes constantly but they don’t grow but instead their power decrease with age.
FACT WITH REASON
Why is the theory 'use and disuse of organ' unreliable?
Theory of use and disuse of organ is unreliable because continuous use of eyes does not make them better
but worst.
Darwinism
The overall concept proposed by English naturalist and geologist Charles Robert Darwin (1809
A.D. – 1882 A.D.) regarding how evolution occurs is known as Darwinism. Charles Darwin
proposed this theory on the basis of his five years’ exploration (a voyage in ocean islands in
a ship named as HMS Beagle, from 27 December 1831 to 2 October 1836). The distribution of
vegetation, wildlife and fossils that he discovered compelled him to do a detailed research. He
analysed the data for many years and wrote a book called ‘The Origin of Species’ in 1859 A.D.
His theory is often called Darwin’s theory of Natural Selection or simply Darwinism.
374 Evolution
FACT WITH REASON
Why is Darwinism more accepted than Lamarckism?
Darwinism is more accepted than Lamarckism because Darwin has collected lots of evidences to support
his theory.
The central idea of Darwin’s theory can be summarized in the following points:
a) Overproduction in biosphere
b) Struggle for existence
c) Variation and heredity
d) Natural selection of fit organisms
e) Origin of new species
a) Overproduction in the biosphere
All species in the biosphere have a tremendous capacity to reproduce and hence they
produce a large number of offspring. An organism produces more number of offspring
than needed to continue its generation. In other words, all organisms tend to reproduce
in larger numbers to ensure their existence in the biosphere. According to Darwinism,
overproduction is the driving force which causes the species to adapt with nature.
FACT WITH REASON
All of the species have enormous fertility but their population remains balanced in nature, why?
All of the species have enormous fertility but population remains balanced in nature because most of
offspring are killed by predators, diseases and disasters.
b) Struggle for existence
If all couples have offspring and all of them survive, then the earth would be populated
by trillions and trillions of species and would immediately exceed the carrying capacity.
But, the fact remains that not all species survive and not all couples reproduce. The ones
that survive have to struggle with the others and the nature for food, habitat and mate. For
instance, fish lays thousands of eggs at a time but only a few survive. The survived ones
have to depend on the same food, water and mating partners. Likewise, in a period of about
3.5 months, tiger gives birth to 3-6 cubs out of which only one or two survive. Due to this
struggle for existence, the population of a species in the biosphere more or less remains
constant. The death rate in the biosphere is higher because of the following reasons.
i) Intraspecific struggle: struggle with the species of the same kind
ii) Interspecific struggle: struggle with the other species
iii) Struggle with the nature: struggle with climate, weather, natural disasters etc.
c) Variation and heredity
The survived species of organisms wander in different places for food, habitat and
mating partner. Due to this, the organisms are influenced by environment in many
ways. Same species of organisms that are in different habitat may eat different foods due
to which they might vary over time. The difference in the structural and behavioural
Modern Concept Science - 9 375
characteristics of organisms is called variation. Variation occurs within the same species
too. The variations in offspring are transferred to the successive generations and
recorded in the genes.
FACT WITH REASON
Why do the individuals of same species look different?
Individual of same species look different because of variation caused due to environment or sexual reproduction.
d) Natural selection of fit organisms
Variations occur in every other organism in the world. These variations are of two types:
favourable and unfavourable. The favourable variations are those that are suited best
to the environment which they live in. Similarly, the unfavourable variations are those
that do not suit the best. Organisms having favourable variations flourish the most and
are more likely to reproduce at a healthier rate. These organisms, in turn, adapt the
best to nature and hence the nature selects them. On the other hand, organisms bearing
unfavourable variations won’t be able to cope with the environment. As a result, they
become extremely rare or extinct.
e) Origin of new species
Variations are ever-lasting. In each generation of species, it gives rise to new
characteristics, behaviours and abilities. These qualities and the changing environment
enable an organism to be continuously different from its former type. In the long run,
these organisms will be so different from each other that they won’t be able to breed
with each other. In this way, new species are born. The new species experience same
rule of the nature; according to Darwin and thus this process continuously goes on.
Criticisms on Darwinism
No theories and principles are universal. New advancement Memory Tips
in technology and understanding of science keep on
challenging the older theories. Darwin did best in A glimpse of Darwin's theory:
explaining how evolution works, but he couldn’t explain Variations and natural selection
how exactly new species are formed. Some of the criticisms causes evolution.
on Darwinism are:
i) Not scientifically proven
Darwin’s theory is not scientifically proven because it cannot be observed directly.
Therefore, it can only be called the most plausible theory available in the scientists’
community. Those theorists who don’t believe in organic evolution repeatedly challenge
Darwinism and ask for proof. Till today, Darwinism has also only defended on the
grounds that there are no other alternative theories that are as convincing as Darwinism.
ii) Variations are unlikely to create new species
According to Darwin’s theory, small variations cause changes by which new species
are emerged. But variations and small genetic changes added up to millions of years
are merely frequent accidents occurring in genes. The probability of errors in genes that
might produce new species is extremely unlikely. It is like saying that a new sentence is
formed by the haphazard typing of the keys of a keyboard by a small child.
376 Evolution
iii) Darwin’s theory can’t explain the existence of vestigial organs
According to Darwinism, parts of organisms remain only when they are suited best to the
nature. What about vestigial organs that don’t have a particular function? Why does the
nature still continue to keep them? No explanation is offered by Darwin’s theory to this.
iv) Unable to explain how actually variations arise
Variations were only understood clearly after Gregor Johann Mendel did experiments
on peas plants. Darwin had no idea of how variations could occur in each successive
generation.
v) Still got unresolved mysteries within
Darwinism sounds easier to listen to. But it has several parts which it couldn’t be
explained. For instance, it predicts that birds and mammals were evolved from reptiles.
But how could an egg laying organism give birth to a mammal? This has remained a
mystery. This is an example of mystery of the mysteries.
Certainly, every evolution theory has loopholes. It is because evolution is a science
understood by evidences and not experiment. It can’t be re-created in a laboratory or
seen directly. Despite criticisms, Darwinism is considered as the most scientific theory
because it is based on observations on varieties of species and is consistent with present
day study of genetics.
ANSWER WRITING SKILL
1. What are fossils?
Ans: Fossils are the remains of ancient organisms or their imprints on the rocks which become visible on
careful digging.
2. Name any two vestigial organs.
Ans: The two vestigial organs are: nictitating membrane and vermiform appendix.
3. Define Darwinism.
Ans: The overall concept proposed by English naturalist and geologist Charles Robert Darwin regarding the
evolutionary process is known as Darwinism.
4. What do you understand by struggle for existence?
Ans: Struggle for existence is the competition between living organisms for food, habitat, mating partner and
other needs.
5. How do fossils provide evidence for evolution?
Ans: The study of fossils indicates that different strata of rocks had different structures of fossils of the same
creature. The deeper strata had primitive and less developed fossils of creatures while the upper strata
had developed and advanced fossils of creatures. Similarly, some of the fossils of organisms found deep
inside the earth do not exist today. This suggests that the organisms in the past were evolved to be the
creatures of the present and hence it provides the evidence for evolution.
6. Make a list of four vestigial organs and their functions in other animals.
Ans: The list of vestigial organs is presented in the table below:
Vestigial Organs Functional in Function
Vermiform appendix Herbivores digesting cellulose
Modern Concept Science - 9 377
Ear muscles Animals locating the sound source
Nictitating membrane Frogs, crocodiles etc. moistening and protecting the eye
Wisdom teeth Human ancestors chewing cellulose
7. Write any two differences between homologous and analogous organs.
Ans: The two differences between homologous and analogous organs are tabulated below:
S.N. Homologous organs S.N. Analogous organs
1 Homologous are similar in structure. 1 Analogous organs are different in structure.
2 They are different in function. 2 They are similar in function.
8. Explain Lamarck’s idea of use and disuse of organs.
Ans: It states that organisms have an internal feeling of need to adjust with the environment. So, they change
their behaviour because of this adjustment. With the changed behaviour, the use of some organs of
their bodies is increased while other organs remain unused. If this happens for a considerable period of
time, the growth of these unused organs becomes affected. The organs that are repeatedly used become
more grown and developed in the successive generations while the unused organs either become
rudimentary or will disappear.
9. How does Darwin explain about the origin of new species?
Ans: The difference in the way of living in each generation of species gives rise to new characteristics,
behaviour and abilities. These qualities plus the changing environment enables an organism to be
continuously different from its former type. In the long run, these organisms will be so different from
each other that they won’t be able to breed with each other. In this way, new species are born. The new
species experience same rule of the nature; according to Darwin and thus this process continuously
goes on.
10. Look at the embryo of different organisms shown in the given figure. How does this figure support the
theory of organic evolution? Explain.
Ans: According to the theory of organic evolution, the organisms are gradually
modified from one form to another. If the organisms have evolved from
a common ancestor, their bodies show some similarity to each other at
least in the embryonic stage of development.
Ans: In the given figure, fish, chick, and human resemble each other to some
extent at some stage in their embryonic development. As predicted by the
biogenetic theory, it is believed that they had similar genes in the past.
Hence they have been evolved from a common ancestor. This proves that
evolution is real.
Embryos of vertebrates
STEPS EXERCISE
STEP 1
1. State the theory of spontaneous generation.
2. What is organic evolution?
3. What are fossils?
4. Write the meaning of homologous, analogous and vestigial organs.
378 Evolution
5. Name two vestigial organs.
6. What is embryology?
7. Define a connecting link.
8. What are acquired characteristics?
9. Write the concept of Lamarckism in a sentence.
10. What is natural selection?
11. Define mutation.
STEP 2
12. Write any two differences between homologous and analogous organs.
13. Name any four vestigial organs in the human body.
14. Name any two examples of connecting link.
15. Write any four sentences as criticisms for Lamarckism.
16. Explain overproduction of species.
17. State Hugo De Vries’ theory of mutation.
STEP 3
18. How does the evidence from homologous organs support evolution?
19. How does the study of connecting links support evolution?
20. Explain the concept of struggle for existence developed by Darwin.
21. How does the inheritance of characteristics occur according to Lamarckism?
22. What does Darwinism suggest about the origin of new species?
23. Classify these organs as homologous, analogous and vestigial:
a) Flipper of a seal and wing of a bird
b) Wisdom teeth and coccyx bone
c) The wing of a bat and the wing of an insect
d) The arm of a human and the forelimbs of a horse
e) Nictitating membrane and vermiform appendix
f) The fins of a fish and flippers of a whale
STEP 4
24. Explain the criticisms on Darwinism in detail.
25. Write down the criticisms on Lamarckism and explain in detail.
26. Explain the main idea of Darwinism regarding natural selection.
27. Explain the central idea of Lamarckism.
28. Neither Lamarckism nor Darwinism is a perfect theory in explaining evolution. Do you
agree with this statement? Why/why not?
Modern Concept Science - 9 379
UNIT Estimated teaching periods Theory Practical
8 2
21
Nature and Environment
Syllabus issued by CDC A. G. Tansley
Introduction to nature and environment
Factors influencing in the plants and animals
Abiotic factors and biotic factors
Ecosystem and its types
Food chain, food web and ecosystem services
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
explain the abiotic factors (air, light, temperature, soil and water) and biotic factors (plants and
animals) that affect ecosystem.
introduce ecosystem with examples.
explain the interrelationship between plants and animals (autotrophism and heterotrophism).
explain the basic needs (food, shelter and cloth) and human dependence on plants and animals.
describe ecosystem services and their types.
Key terms and terminologies of the unit
1. Biome: A biome is a combination of several ecosystems.
2. Ecosystem: Ecosystem is a small unit of an environment which consists of living and non-living
components that exchange matter and energy.
3. Ecology: The branch of biology which studies the ecosystem and the interrelationship between its
different components is known as ecology.
4. Abiotic components: The non-living components present in the ecosystem are known as the abiotic
components.
5. Biotic components: The living components present in the ecosystem are known as the biotic components.
6. Edaphic factors: Abiotic factors related to the physical and chemical composition of the soil are called
edaphic factors.
7. Producers: The organisms that produce food in the ecosystem are known as the producers.
8. Consumers: Consumers are the organisms which depend on producers for their food.
9. Primary consumers: The consumers that directly depend on plants for food are known as primary consumers.
10. Secondary consumers: The consumers that depend on the primary consumer for food are known as the
secondary consumers.
11. Tertiary consumers: The consumers that depend on the secondary consumer for food are known as the
tertiary consumers.
380 Nature and Environment
12. Decomposers: Decomposers are the microorganisms in the ecosystem that recycle the dead and decomposed
substances and convert into simpler forms.
13. Food Chain: The transfer of food from the lower to higher trophic levels in the ecosystem by the process of
eating and being eaten is known as food chain.
14. Food Web: An ecosystem is thus a web of several food chains that are linked together by the process of
eating and being eaten. This is called a food web.
15. Detritus: Detritus is a dead organic matter. It includes the bodies or fragments of dead organisms as
well as fecal material.
16. CBD: CBD means a Convention on Biological Diversity held on June 5, 1992 A.D. to conserve
biodiversity, use it sustainably and share its resource benefits fairly and equitably.
17. MA: MA means the Millennium Ecological Assessment published in 2005 A.D. It is an effort by
former UN Secretary General Kofi Annan to ascertain the human impact on the environment.
Introduction Environment
The Earth’s environment is home to millions of species: tiny to big, plants to Biosphere
animals and aquatic to terrestrial. The word ‘environment’ is derived from Biome
the French word ‘Environner’ meaning ‘to surround’. But the concept of
environment is not just limited to the surroundings. It includes all the living Ecosystem
and non-living components that exist on this earth either visible or invisible. Community
The largest unit of environment is called the biosphere. We all live in the part of
the earth called biosphere that sustains life.
In the earth, the lithosphere (land part), hydrosphere (water part) and small Population
part of atmosphere (air) make up the biosphere. The biosphere is divided into
smaller fragments of areas with similar climate and vegetation called the biome. Organism
For example, aquatic biome, terrestrial biome etc. A biome is the combination
of several systems. The unit of a biome is called an ecosystem. Ecosystems constitute similar
groups of organisms living in a common habitat called community. The community bears
many species of organism. Each species of organisms is known as the population.
What is an ecosystem?
The word ‘eco’ means environment and ‘system’ means the structure. Thus, ecosystem
represents a systematic structure of environment. In this structure, there is everything that
we have around: plants, animals, microbes, sunlight, air, soil etc. They share food and energy
with each other. The word ‘ecosystem’ was coined by British ecologist Arthur Tansley in 1935
A.D. After that several scientists like Raymond Lindeman, Eugene P. Odum have contributed
to the better understanding of the ecosystem. Odum is the first ecologist to write a book on
ecosystem.
Ecosystem is a small unit of an environment which consists of living and non-living components
that exchange matter and energy. The Convention on Biological Diversity (CBD) describes
ecosystem as “a dynamic complex of plant, animal and micro-organism communities and
their non-living environment interacting as a functional unit”. The ecosystem is automatically
Modern Concept Science - 9 381
regulated by natural forces. Some ecosystems are simple while others are complex. Most of
the ecosystems are natural while some are created by human. But the common feature in
every ecosystem is that they are made up of living and non-living components. The branch
of biology which studies the ecosystem and the interrelationship between its different
components is known as ecology.
FACT WITH REASON
Ecosystem is dynamic, why?
Ecosystem is dynamic because there is a continuous exchange of food and energy among its different com-
ponents.
Ecological components
The components that run and sustain an ecosystem are known as ecological components. There
are two kinds of ecological components. They are: biotic components and abiotic components.
Abiotic components
The non-living components present in the ecosystem are known as the abiotic components.
Look at the given picture of a grassland ecosystem. The soil, air, water, sunlight, minerals, etc.
are the abiotic components. These components provide a basis for the biotic components for
survival. Without abiotic components, the living components have no physical basis of life.
The abiotic factors are further classified into two groups:
Abiotic Climatic
Edaphic
Components Producers
of ecosystem
Biotic Consumers
Decomposers
Grassland food web
a) Climatic abiotic components
The non-living components of the ecosystem that are related to climate and weather
are known as the climatic abiotic components. For example, heat, rain, light, wind etc.
b) Edaphic abiotic components
The non-living components that are related to the characteristics of the soil are known
as the edaphic abiotic components. For example, pH of soil, minerals, land texture etc.
382 Nature and Environment
Some important abiotic factors
a) Sunlight and temperature
The visible light that we get from the sun are Oxygen out
heat energy and light energy. The heat energy Light energy
heats up the earth’s crust, evaporates water,
continues water cycle and keeps a habitable Carbonindioxide Water and
temperature on the earth. In absence of sunlight, Green cells where minerals
everyone will freeze to death. Different food is made
ecosystems require different variations of
temperature. Land organisms can bear certain Process of making food by green plants
change in temperature, but aquatic organisms
are very sensitive to it. Temperature directly
affects the reproduction, seeding, fertilisation,
seed dispersal, prey-predator relationship etc.
in an ecosystem. Balanced temperature helps
to regulate the several aspects of ecosystem.
Likewise, the light energy is the source of energy for photosynthesis. Light makes it
possible to form glucose and oxygen from carbon dioxide and water. Without light
energy, no food is produced without which ecosystems are impossible.
b) Air
The air is a mixture of various vital gases like carbon dioxide, oxygen, nitrogen etc.
which are important to the living organism in the ecosystems. Carbon dioxide is used
up by the plants during photosynthesis to prepare food. This food is transferred to the
animals successively. In the same way, oxygen makes respiration possible by which
energy is released in both plants and animals. Likewise, nitrogen is directly needed by
the plants. They take it through the soil. Nitrogen is a key element in the formation of
plant body made up of mostly proteins.
c) Water
Water constitutes about 70 percent to 90 percent of all living bodies. It helps to circulate
useful materials in the cell and to excrete wastes. It also forms a 2/3rd part of the earth
sustaining the life of millions of aquatic animals and plants directly. The vital processes
like the circulation, excretion, reproduction etc. cannot occur without water.
d) Soil
The soil is the region on the earth’s crust where food grows. The roots of the plants are
fixed to it from where they absorb nitrates, nitrites, water, minerals, etc. Soil is also the
habitat for all organisms on the earth. The acidity and basicity of the soil, its water and
air content and the texture determine the habitat of plants. Soil is home to most of the
animals like earthworms, scorpions, ants.
Modern Concept Science - 9 383
Biotic components
The living components present in the ecosystem Pond ecosystem
are known as the biotic components. In the given
figure, the leopard, zebra, hyena, trees, vultures
and even termites are the biotic components. They
include plants, animals and microbes present in the
ecosystem. The organisms in the picture are in the
state of eating and being eaten. For any ecosystem
to work, it needs energy and food. The availability
of food and energy should be unlimited. If this
process stops, the ecosystem crashes and everyone
dies. The biotic components in the ecosystem are
divided into three types. They are:
a) Producers
The organisms that produce food in the ecosystem are known as the producers.
Generally, the producers in the ecosystem are green plants, dead organic matter and
the phytoplankton. The sunlight helps in the process of photosynthesis. The producers
generate food and transfer to the consumers.
FACT WITH REASON
Plants are called producers, why?
Plants are called producers because they produce food using air, water and sunlight.
Plants are called transducers, why?
Plants are called transducers because they produce food by transforming solar energy into chemical energy
(starch) using sunlight, water and air in presence of chlorophyll in the leaves.
b) Consumers
Consumers are the organisms which depends on producers for their food. There are
many levels of consumers on the basis of hierarchy of food transfer:
i) Primary consumer: The consumers that directly depend on plants for food are
known as primary consumers. Herbivores fall under this category. Some of the
examples of primary consumers are goat, horse, grasshopper, zooplanktons, etc.
FACT WITH REASON
Goat is called primary consumer, why?
Goat is called primary consumer because it depends on plant for its food.
ii) Secondary consumer: The consumers that depend on the primary consumer for
food are known as the secondary consumers. Carnivores are mostly secondary
consumers. As they depend upon the primary consumers, they indirectly depend
on plants. For example, dog, cat, fox, fish, bird, snake, etc.
iii) Tertiary consumer: The consumers that depend on the secondary consumer
for food are known as the tertiary consumers. They are also referred to as top
carnivores. For example, lion, tiger, hyena, eagle, whale, crocodile, etc.
384 Nature and Environment
c) Decomposers
Decomposers are the microorganisms in the ecosystem that recycle the dead and
decomposed substances and convert into simpler forms. These simpler forms of
nutrients obtained can again be used up by the plants. Decomposers are the armies
of nature. They are at work every time something lies dead in nature. Examples of
decomposers are bacteria and fungi (mushroom, mucor etc.)
FACT WITH REASON
Mushrooms are called saprophyte, why?
Mushrooms are called saprophyte because they absorb nutrients directly from dead and decaying materials.
Modes of Nutrition
Food is obtained by various ways in the ecosystem. These ways are known as modes of
nutrition. There are generally two modes of nutrition: autotrophic and heterotrophic.
a) Autotrophic nutrition
The word ‘autotroph’ is derived from the Greek words “autos” meaning self and ‘trophe’
meaning nutrition. Hence, it is a type of obtaining food by preparing by themselves.
Green plants obtain their food from autotrophic nutrition. So, they are called autotrophs.
The autotrophs are the starters of the food transfer in an ecosystem. For example, green
plants like fruits, flowers, trees, euglena (animal).
b) Heterotrophic nutrition
‘Hetero’ refers to ‘other’ and ‘trophy’ means nourishment. Thus, the process in which
organisms depend upon other organisms for their food is termed as heterotrophic
nutrition. They are simply called consumers as they depend on others for food. All
animals (except euglena), some carnivorous green plants and non-green plants fall
under this category. Heterotrophs can be of the following types:
i) Saprophytic nutrition
The organisms that depend on dead and decayed matter for food are termed as
the saprophytes. This act of obtaining food is known as saprophytic nutrition.
Saprophytes obtain their food from decomposed matter. For example, bacteria,
mushroom, mould, yeast, etc.
ii) Parasitic nutrition
The type of nutrition in which an organism gets its food by sucking or living
on/in the body of host organism is known as parasitic nutrition. In this type of
nutrition, there is one way relationship. The host organism from which the food
is taken doesn’t benefit anything from it. For example, leech, cuscuta (plant) etc.
Modern Concept Science - 9 385
iii) Holozoic nutrition
The word ‘holo’ means whole and ‘zoikos’ refers to ‘of animals’. Therefore, those
organisms that feed on the whole body of organic food and digest it with chemicals
inside their body are known as holozoic organisms. In this process, solid or liquid
organic matter is ingested, chemicals digest them, nutrients are absorbed and
assimilated and finally undigested matter is excreted out. For example, human
being, lion, cow, etc. On the basis of eating habit, the holozoic organisms can be
categorised into the following types:
i) Herbivores: These are plant eating organisms. For example, deer,
grasshopper etc.
ii) Carnivores: These are flesh eating organisms. For example, lion, tiger, etc.
iii) Omnivores: These are organisms that feed on both plants and flesh. For
example, crow, human etc.
iv) Detritivores: These are organisms that feed on dead and decayed matter.
For example, earthworm, ant, etc.
How does an ecosystem work? Quaternary consumers
Tertiary consumers
The whole ecosystem is sustained on two main
factors: appropriate climate and transfer of Secondary consumers
food and energy. The appropriate climate is Primary consumers
provided by the abiotic factors. Similarly, the Producers
transfer of food is made possible by the help
of producers, consumers and decomposers. Trophic level
The producers produce food and pass on to
the consumers. The consumers pass the food
content to various levels. After these producers
and consumers are dead, the decomposers
return the food available in their bodies to the
soil, by converting the complex compounds to
simpler compounds. Each level of organism
through which the food is transferred is
called the trophic level. In the given figure,
an example of food transfer in various trophic
levels is shown.
i) The first and the lowest trophic level is occupied by the producers (green plants).
ii) The insects and bugs feed on green plants and are in the second trophic level. They are
primary consumers.
iii) The mouse, frog and a small bird feed on insects and are in the third trophic level. They
are called secondary consumers.
iv) The snakes feed on mice and frogs and are in the tertiary trophic level.
v) The eagle is in the Quaternary trophic level and feeds on snakes.
386 Nature and Environment
This transfer of food takes place by two main processes:
a) Food chain
The transfer of food from the lower to higher trophic levels in the Production
ecosystem by the process of eating and being eaten is known as food
chain. A simple food chain undergoes three processes: production, Green plants
consumption and recycling as shown in the figure alongside. The
food is produced in the producer level by the green plants. The Consumption
animals eat the plant products and other animals and thus the food Consumers
gets transferred from lower level to higher level of consumers. Finally,
after the producers and consumers are all dead, the decomposers Recycling
recycle the nutrients present in their bodies and return them to the
soil so that the plants can use them again. This food cycle continues Decomposers
so that there is continuous supply of food to every other organism on the earth. On the basis
of food producers, there are two kinds of food chains:
i) Grazing food chain
The food chain that starts with autotrophs is known as the grazing food chain. The food
sources are the green plants. They transfer the food to the consumers which ultimately
are recycled by the decomposers. A simple grazing food chain transfers food from the
lowest to highest trophic level i.e. producer to consumers to decomposers. Examples of
the grazing food chain are given below:
Grazing food Grass Grasshopper Frog Snake
chain
Producer Primary consumer Secondary consumer Tertiary consumer
Corn Rat Snake
Producer Primary consumer Secondary consumer
FACT WITH REASON
Number of producer is always more than consumer in ecosystem, why?
Number of producer is always more than consumer in ecosystem because producers are the source of food.
If by any chance producer is less in number, consumers will die of starvation.
Killing a snake because it is poisonous is a wrong thing to do, why?
Killing a snake is a bad thing to do because snake is secondary consumer of ecosystem. If it is killed number
of rats, frog, insects and so on will increase and disturb ecosystem.
ii) Detritus food chain
The food chain that starts with dead organic matter is known as the detritus food chain.
The main source of energy in this food chain is the decomposed organic matter. Unlike
grazing food chain, the food and energy in the detritus food chain doesn’t come from
the solar energy. An example of such a food chain is given below:
Detritus food Dead matter Flies Frog Snake Hawk
chain
Producer Primary Secondary Tertiary Quaternary
(Bacteria and Fungi) consumer consumer consumer consumer
Modern Concept Science - 9 387
b) Food web
A single organism has various choices of food in
the ecosystem. In the same way, that organism
can also be eaten by a variety of other organisms.
An ecosystem is thus a web of several food chains
that are linked together by the process of eating
and being eaten. This is called a food web. A food
web is the combination of several food chains that
are integrated together. There are several levels
of producers, primary consumers, secondary
consumers, tertiary consumers and so on in a food
web. A simple diagram of terrestrial food web
is shown in the given figure. In the diagram, it is
evident that the grasshopper is the food for hawk, Food web
weasel, shrew and spider. Similarly, the hawk has several choices of food: the grasshopper,
the spider, the shrew and the weasel. There is a probability that these organisms will flourish
because they have several food choices.
Types of ecosystem
Depending on the habitat the organisms live in, there are two types of ecosystems as shown
in the diagram: terrestrial and aquatic.
Ecosystem
Aquatic ecosystem Terrestrial ecosystem
Freshwater Marine water Grassland ecosystem
ecosystem ecosystem Forest ecosystem
Pond/Lake ecosystem
River ecosystem Desert ecosystem
Terrestrial ecosystem
The ecosystem in which organism exchange food and energy mostly on land is known as
the terrestrial ecosystem. It can either be natural or man-made. Natural ecosystems are more
diverse and broader than artificial ones. The natural ones are tundra, forest, desert or dry land,
grassland ecosystems etc. Likewise, the artificial ecosystems are crop field, garden, aquarium
ecosystems etc.
388 Nature and Environment
Aquatic ecosystem
The type of ecosystem in which exchange of food and energy takes place inside water bodies
is called the aquatic ecosystem. It is further classified into two types: fresh water and marine
ecosystems. The fresh water ecosystem includes the non-salty water bodies like the pond,
lake, river, spring, swampy areas etc. Similarly, the marine includes the salty water bodies like
the seas and oceans.
Different ecosystems have different animals and plants that share the abiotic world. For
instance, the aquatic ecosystem gets its food through aquatic plants and phytoplankton but
the terrestrial ecosystem obtains its food from variety of flowering and non-flowering plants.
Ecosystem type Sub-type Components Abiotic
Biotic
a. Grassland a. Grasses, herbs, shrubs etc. : producer a. Soil
b. Minerals
b. Deer, frogs, insects, rabbits etc. : c. Air
primary consumer d. Temperature
e. Humidity
c. Birds, foxes, jackals, wolves etc. : f. Sunlight
secondary consumers g. Water
h. Rocks
d. Tiger, lion, hyena, hawk, vultures,
eagles etc. : tertiary consumers
b. Desert a. Shrubs and herbs like cactus, a. Occasional
opuntia etc : producer rainfall
b. Insects, lizards, rodents etc. : b. Sand
primary consumers
c. Variable
c. Spider, scorpion, snake etc. : temperature
secondary consumers
d. Moisture
d. Hawk, fox etc. : tertiary consumers
e. Minerals
c. Pond a. Phytoplanktons like diatoms, a. Water
hydrilla, lemna, algae etc. : b. Minerals
producers c. Organic deposits
d. Inorganic salts
b. Zooplanktons, protozoa, larvae etc. : e. Temperature
primary consumer f. Soil
g. Air
c. Small fish, frogs, insects etc. : h. Sunlight
Secondary consumers
d. Big fishes, water snake, kingfisher,
heron, eagle etc. : tertiary consumer
a. Phytoplanktons like diatoms, algae, a. Water
sea weed, dinoflagellates etc. : b. Minerals
producers c. Organic deposits
b. Zooplanktons, protozoa, larvae, d. Inorganic salts
d. Sea and shrimp etc. : primary consumer e. Temperature
ocean
c. Lantern fish, sunfish, etc. : Secondary f. Soil
consumers g. Air
h. Sunlight
d. Squid, mackerel, smaller sharks etc. :
tertiary consumer
e. Large sharks : quaternary consumers
Interrelationship between plants and animals in ecosystem
Plants and animals are two important aspects of the biotic environment. They are
interdependent on each other in many ways on the abiotic environment for gaining basic
needs. Their interrelationship can be best described based on the following points:
Modern Concept Science - 9 389
a) Food
Both plants and animals get their food with the help of solar energy. The plants are the
reservoir of food for primary consumers. As the primary consumers feed on plants,
they assimilate the nutrients in their body forming skeleton and tissues. The secondary
consumers that feed on the primary consumer are indirectly feeding on the plants. The
same applies to the higher consumers too. After all these organisms die, the organic
complex compounds present on their bodies are converted into simpler ones which the
plants take through the roots. This process of decomposition is done by the decomposers:
the bacteria and the fungi. If the food continuously gets transferred from the producers
to consumers but not back to producers, most plants will not flourish much. If there had
been no consumers at all in the ecosystem, the environment would have been ruled by
large, deadly and insectivorous plants.
b) Oxygen and carbon dioxide
Plants use sunlight as energy and carbon dioxide and water as raw materials for making
food. The carbon dioxide without which plants cannot make food comes from the
animals through respiration. In turn, for respiration, plants produce oxygen. Also, the
proteins that animals need to grow are obtained from the plants.
c) Shelter and protection
Plants are home to millions of animals that live in the wild. Their timber, wood etc.
are the main materials for building houses for humans. Plants provide us shelter and
protection. Birds and arboreal animals spend most of their life on trees so that they can
keep their eggs and babies safe.
Dependency of human for basic needs
There are certain needs which humans have to fulfill to sustain in the environment. The basic
needs are: food, clothes and shelter. While other streams of science also consider education,
sex and health as the basic needs, we limit basic needs as minimum requirements needed
for human to survive. All these basic needs are fulfilled from plant extracts, animals and the
environment on which human depends. Without them, survival of human is impossible.
a) Food
Food is the most fundamental need of all living organisms. Without food, the cells do
not function and bodies do not develop and grow. Humans directly feed on vegetables,
fruits, salads, legumes, nuts, pulses, cereals like rice, wheat, maize, barley. Moreover,
as humans are omnivores they also depend on cooked meat obtained from animals.
Buff, mutton, chicken, egg, lamb, milk, butter, curd and ghee are some of the animal
products that we feed on.
b) Clothes
Clothes are not just our physical need; they’re also our social and cultural need. Clothes
help us to protect against harsh climate, hot sun, chilled winter, etc. Clothes make us
accepted in the society as decent beings. We generally wear two kinds of clothes: one
that is made from plant fibres and other from animals. The plant fibres used for clothing
are cotton, jute, hemp, flax etc. Similarly, wool, silk, fur, hide etc. are the animal products
that we use for cloth fibres.
390 Nature and Environment
FACT WITH REASON
What are planktons, nektons and benthos?
a) Planktons are weak swimming organisms that flow with natural water current and are unable to
swim against it. Some of the planktons are:
i) Phytoplankton: ‘phyto’ means plant. These are the plant planktons that live in water. For ex-
ample, diatoms, dinoflagellates, etc.
ii) Zooplankton: ‘zoo’ means animal. These are the animal planktons that live in water. For ex-
ample,-larvae of crab, fish, sea star, krill, etc.
b) Nektons are free and fast swimming organisms that spend most of their time in water. For example,
whales, fish, squid, etc.
c) Benthos are aquatic organisms that live in the sedimentary areas of sea or oceanic floor. They can be
attached or free-moving. For example, sea weed, star fish, brittle star, crabs, etc.
c) Shelter
We need shelter for protection against changing weather conditions, sleeping, resting,
pleasure and socializing. Humans build houses for this purpose. The common materials
to build them come from timber, wood, minerals, bricks, straw, etc. The main sources of
these materials are nature and the plants.
Ecosystem Services
The ecosystem services are the benefits and advantages, which we obtain from the ecosystem
and its components. The biodiversity is the part of the ecosystem. We get several advantages
like food, clothes, energy and shelter from biodiversity. Ecosystem acts as a bridge between
the environment and the human beings. According to the Convention on Biological Diversity
(CBD) and the Millennium Ecological Assessment (MA), we get four kinds of services from
ecosystem:
a) Provisioning services
The material products obtained from ecosystem are known as the provisioning services.
These services or benefits are:
i) Fresh water for drinking, bathing, cooking etc.
ii) Foods like vegetables, fruits and cereals to eat
iii) Fibres for materials and clothing e.g. cotton, wool, hemp, flax etc.
iv) Firewood and timber
v) Biochemicals and medicinal herbs for natural remedy of diseases
vi) Genetic resources for interbreeding and bioengineering.
b) Regulating services
These are the benefits obtained from the regulation of the ecosystem processes. These
benefits can be pointed out as:
i) Climate regulation and maintenance
ii) Disease control by reduction in mosquito population, cholera etc.
Modern Concept Science - 9 391
iii) Water purification with bio filters and reduction of ocean waste
iv) Water regulation in wetlands, forests, seas, oceans etc.
v) Pollination to fertilise the male and female parts of the plant
c) Cultural services
These are the benefits that we obtain from non-material and abstract values of the
ecosystem. These include:
i) Spiritual, aesthetic and religious values
ii) Recreation and eco-tourism that add to national income
iii) Inspirational and educational values for arts and architecture
iv) Cultural heritage promotion
d) Supporting services
These are the services that we obtain while receiving all other ecosystem services. These
services are as follows:
i) Soil formation by weathering of rocks
ii) Nutrient recycling through various biogeochemical cycles
iii) Primary production of food for all organism
ANSWER WRITING SKILL
1. What is ecology?
Ans: The branch of biology which studies the ecosystem and the interrelationship between its different
components is known ecology.
2. Write any two examples of decomposers.
Ans: The two examples of decomposers are: bacteria and fungi.
3. Define biotic and abiotic components.
Ans: The non-living components present in the ecosystem are known as abiotic components. Similarly, the
living components present in the ecosystem are known as the biotic components.
4. What is food chain?
Ans: The transfer of food from the lower to higher trophic levels in the ecosystem by the process of eating and
being eaten is known as food chain
5. Write any two differences between food chain and food web.
Ans: The two differences between food chain and food web is given below:
S.N. Food chain S.N. Food web
1 A food chain contains only a single 1 A food web can have more than one
organism in a tropic level. organism in a trophic level.
2 Food chain is a unit of food web. 2 Food web is the integrated form of
food chain.
392 Nature and Environment
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