Download Chapter-01

BIOLOGY
Are eyes enough to enjoy this natural splendour ...?
Didn't you notice the picture drawn by
Raju and its caption? What is your opinion
about the caption? You too might have got
the opportunity to enjoy such scenes from
nature. What have your blissful
experiences in such situations been?
—
Colourful scenes
—
The chirping of birds
—
The gentle pat of the breeze
—
The intoxicating fragrance of flowers
—
The honey-sweetness of mango
7
BIOLOGY
Though these are different experiences,
can't they be felt at the same time? Can’t
we hear, touch, smell and taste while
seeing?
Let us examine the parts of the eye. Make
a list of those you know.
Have you ever thought how this happens?
—
You know that we get information about
the changes in the environment through
the sense organs. What are the sense
organs we have? What is the function of
each? Prepare a note on it.
Don't you want to know how these parts
are arranged in the eye? Collect the eye of
an animal from the butcher shop. Identify
its morphological characteristics by
observing and feeling it. Prepare a note
based on the indicators.
.....................................................................
—
Pupil
—
Retina
.....................................................................
Eye
Eyes enable us to see things. How are they
protected? List the various means for the
protection of eyes.
—
Position of the eyes – sockets in the
skull
—
Tears
—
—
Lysozyme, the enzyme present in tears
destroys germs that enter the eyes to a
certain extent.
Indicators
—
Colour
—
Shape
—
Firmness
With the help of your teacher take a cross
section of the eye that has been collected
by you. Making use of the features you
observed and the following description
write a note on the structure of the eye.
Fill up the blanks in Figure 1.1.
Eye – The Window opening to Nature
The eye ball has three layers. The outermost layer is the sclera. It imparts firmness to the
eye ball. The transparent front portion of the sclera is the cornea. The anterior part of the
eye except the cornea is protected by a membrane called conjunctiva.
The middle layer, choroid contains many blood capillaries. The tissues in the eye receive
oxygen and nutrients from the blood that flows through these capillaries. In the anterior
part of the choroid behind the cornea there is a circular dark screen named iris. Pupil is
the aperture at the centre of the iris. The pupil contracts when the intensity of light increases
and dilates when it decreases. In this way it regulates the amount of light entering the
eye. This is facilitated by the muscles in the iris. The convex lens is placed behind the
pupil. It is connected to the ciliary muscles by ligaments.
8
BIOLOGY
Retina is the innermost layer of the eye where the image is formed. Cells that are
stimulated by light are seen here. The part of the retina with greatest vision is called the
yellow spot and that with no vision is called the blind spot. The optic nerve carrying
impulses to the brain starts from the blind spot.
Inside the eye there are two chambers. The chamber between the lens and the cornea is
called aqueous chamber. It is filled with a watery fluid, called aqueous humour. This fluid
supplies nutrients and oxygen to the cells of the cornea and the lens. The aqueous humour
formed from the blood is reabsorbed into the blood itself. The large chamber seen
between the lens and the retina is the vitreous chamber. It is filled with the jelly like vitreous
humour which helps to maintain the shape of the eyeball.
ciliary muscles
sclera
?
?
iris
?
?
?
?
vitreous chamber
yellow spot
ligaments
?
optic nerve
Fig 1.1 Structure of the eye
Couldn't you identify the parts you had seen in the cross section of the eye in the
figure? Now illustrate each part of the eye, their peculiarities and functions in an
interrelated manner in your Science diary. A model is given below.
Cornea
peculiarity
Transparent
anterior part
function
Allows light rays
to enter
Outermost
layer
9
BIOLOGY
You have understood the structure and function of the different parts of the eye. Of
them the peculiarities of the lens and the retina deserve special mention.
Lens and Vision
You have seen the position and peculiarities of the lens of the eye.
In order to understand how the lens helps to view near and distant objects, analyse
Fig.1.2 A, B and the following description and prepare notes.
ciliary muscles
ligaments
lens
Fig - 1.2.
A
B
Viewing distant objects
Viewing near objects
—
Ciliary muscles relax
—
Ciliary muscles contract
—
Ligaments contract
—
Ligaments relax
—
Curvature of lens decreases
—
Curvature of lens increases
—
Focal length increases
—
Focal length decreases
Formation of Image
How is the image formed in the retina? Analyse Figure 1.3. Note down the peculiarities
of the image.
image
object
Fig - 1.3. Formation of image in the retina.
Characteristics of the Image
—
Real
—
—
10
BIOLOGY
Is vision possible merely by the formation of the image in the retina? What happens in
the retina after the image is formed? To know this, the structure of the retina is to be
learnt first.
Read the description given below. Analyse it with the help of indicators.
On Exposure to Light
rod cell
cone cell
Fig - 1.4. Structure of the retina
There are two types of receptors in the retina which are stimulated by light rays. They are
rod cells and cone cells. Rod cells are stimulated by dim light and cone cells by intense
light. This is because of the peculiarities in their pigments.
Rod cells contain the pigment rhodopsin and cone cells contain the pigment photopsin.
Retinal formed from vitamin A and opsin the protein molecule are their basic components.
In the presence of light these pigments dissociate to form retinal and opsin. It is this
chemical change that generates nerve impulses. Retinal and opsin can again combine
together to form pigments. Retinal and opsin formed by the dissociation of rhodopsin
do not recombine in intense light. Hence in intense light rhodopsin becomes inactive.
Photopsin dissociates in intense light. Hence bright light is required to see the world of
colours. Vision in dim light is decreased by vitamin A deficiency. This condition is called
night blindness.
In man, there are three types of cone cells which are stimulated to the maximum by red,
green and yellow rays. Colour of the object seen by us depends upon the type of cone
cells stimulated by light rays from the object. If all the three types of cone cells are stimulated
at the same rate we see white colour and if none of these three is stimulated we see
black colour. Many other mammals such as dog and cat have only two types of cone
cells. They see the entire world as a blend of two colours. Some human beings may lack
one of these types of cones due to certain hereditary defects. As a result they are unable
to see certain colours. This is called colour blindness. Many animals including birds
11
BIOLOGY
have four or more types of cones. The world they see is more colourful than ours. In man
rod cells are few and cone cells are more in the yellow spot. Rod cells and cone cells are
absent in the blind spot. Animals like cat and owl have more rod cells than man. So they
have greater power of vision at night. Since the owl has no cone cells it cannot see in
intense light.
Indicators
« What are the changes taking place in the rod cells in dim light and intense light?
« How is vitamin A related to vision?
« How do rod cells differ from cone cells physiologically?
« What is the difference between night blindness and colour blindness?
« What is the reason for the absence of vision at the blind spot?
Let us try an activity to examine the power of vision in the blind spot?
Spotting the Blind spot
Hold the given figure one arm away in front of the eye. Shut the right eye and look at the
circle with the left eye. Slowly bring the figure towards the eye. At a particular position,
doesn't the square disappear? When it is brought closer to the eye it reappears. Repeat
the experiment with the left eye closed and viewing the square with the right eye. What
happens to the circle? This happens when the image of the object falls on the blind spot
in the retina.
How does the information received by the
sense organs reach the brain? Look at the
following facts related to this.
—
Brain, spinal cord, nerves and sensory
receptors constitute the nervous
system.
—
It is the nervous system that controls
and co-ordinates the functions of the
body.
Now analyse the following figures and
description based on indicators and
record your inferences.
—
The basic unit of the nervous system
is neuron.
.....................................................................
12
BIOLOGY
Neuron and Nerve
Neuron is the basic unit of nervous system and are seen more in the brain and the
spinal cord. There are millions of neurons in the human body. They are not capable of
division.
A neuron has three important parts- cyton or cell body, axon and dendron. Axon and
dendron are developed from the cyton. Dendrites are branches of the dendron.
Branches of the axon are called axonites. The axon in certain neurons is covered with
a sheath made up of the lipid, myelin. It has a glistening white colour. The functions of
the myelin sheath are to provide nutrients and protection to the nerve cells and to enhance
the transmission of nerve impulses by acting as an insulator. The swollen tips of the
axonites are called synaptic knobs.
dendrite
dendron
axon
myelin sheath
axonite
nucleus
synaptic knob
cyton
Fig - 1.5 Structure of a neuron
A group of nerve fibres or axons covered by connective tissue
constitutes a nerve. Nerves are of three types; sensory, motor
and mixed nerves. Sensory nerves are composed of nerve
fibres carrying impulses from the sense organs to the brain
Fig - 1.6. Nerve
and spinal cord. Nerves consisting of nerve fibres that transmit
messages from the brain and spinal cord to different organs
and muscles constitute motor nerves. Nerves composed of both sensory and motor
nerve fibres are mixed nerves.
axon
A cluster of cell bodies found in certain nerves which appears like a tiny globular swelling
is called nerve ganglion.
Indicators
—
Difference between axon and dendron
—
Three types of nerves
—
Ganglion
Nerves carry the messages related to the
responses that are formed as a result of
changes taking place in the surroundings
and in the body. Read the description
given below and prepare a note on how
information about the surroundings is
transmitted through the receptors.
.....................................................................
13
BIOLOGY
Stimulus, Receptors, Impulse, Response
Nerve cells or receptors that are capable of receiving stimuli from within the body and
external environment are located in sense organs and in other different organs. Receptors
are modified neurons. They are of different types. Rods and cones in the eye, sound
receptors in the ear, taste receptors on the tongue, olfactory receptors in the nose and
receptors on the skin are examples.
As in all cells, opposite electric charges exist on either side of the plasma membrane
of the receptor cells also. Changes in the internal and external environment of the body
cause variations in the electrical equilibrium existing on either side of the plasma
membrane. Such changes are known as stimuli. eg:- light, heat, cold etc. The flow of
electric charges resulting from variations in equilibrium are called impulses. When such
impulses reach the brain or the spinal cord through nerves corresponding changes take
place in the body. These changes are termed responses.
You have understood that when images are formed in the retina, the receptors located
there are stimulated. As a result impulses are formed. These impulses sometimes have
to pass through more than one neuron to reach the brain. How are neurons
interconnected? Complete the flow chart analysing the given description and
Figure 1.7.
Synapse
Synapse is the junction between two neurons. Impulses transmitted from one neuron to
another pass through the synapse. When impulses reach the synaptic knob
neurotransmitter (eg: acetyl choline) is secreted into the synaptic cleft. It transforms nerve
impulses to chemical impulses and transmits them on to the adjacent neuron.
dendrite
synaptic knob
dendron
synaptic cleft
axon
synapse
Fig - 1.7. Synapse between neurons
Dendrite
Cyton
Axon
ASpØ \yqtdmWns‚ sU≥ss{U‰v
14
BIOLOGY
Is the synapse seen only at the point where two neurons meet? Analyse Figure 1.8 and
form inferences.
.....................................................................................................................
synaptic cleft
muscle cell
synaptic knob
Fig - 1.8. Synapse between a nerve cell and a muscle cell
Hope you have understood how the information of the formation of the image in the
retina reaches the brain? Observe Figure 1.9 and write down the neural activities related
to this sequentially.
brain
optic nerve
image
centre of vision
optic nerve
Fig - 1.9. Impulses from the eye to the brain
—
Photoreceptors are stimulated
—
...................................................................
—
How is the experience of vision made
possible when impulses reach the brain?
To understand this, much more is to be
learnt about the brain which is the centre
of the nervous system. Analyse Figures
1.10 and 1.11 and descriptions based on
the indicators and prepare notes.
...................................................................
15
BIOLOGY
movement touch
cerebrum
thought
grey matter
white matter
thalamus
hypothalamus
vision
cerebellum
pituitary
gland
medulla
oblongata
spinal cord
Fig - 1.10. Structure of the brain
speech
taste
smell
hearing
maintenance
of equilibrium
Fig - 1.11. Parts of the brain that control various
activities in man
Brain – The Centre of Wonders
The brain is the most complex organ in the human body. It is the centre of all the peculiar
characteristics which make man unique. The brain is protected inside a hard case called
skull. The three layered covering of the brain is called meninges. The cerebrospinal fluid
(CSF) fills between the inner layers. The CSF which is formed from the blood is
reabsorbed into the blood. CSF helps to provide nutrients and oxygen to the tissues in
the brain, and also ensures the protection of the brain. CSF is also contained in the
cerebral ventricles, the cavities in the brain.
Cerebrum is the largest part of the brain. Cytons (cell bodies) of neurons are seen crowded
together in this part. The outer part of the cerebrum is grey coloured due to the absence
of myelin. This is called grey matter. The folds and grooves in the grey matter are
adaptations to accommodate more number of neurons. The interior of the cerebrum is
called white matter. Nerve fibres covered with white coloured myelin are seen in this
part.
The cerebrum is the centre of qualities like intelligence, memory, thought and imagination.
Senses like sight, hearing and taste are made possible by this part. Impulses from sense
organs reach specific centers of the cerebrum through nerves. Here impulses are
interpreted and integrated to give us precise sensations. The cerebrum controls all
voluntary movements.
Cerebellum, the part that lies below the cerebrum, is the second largest part of the brain.
Here too, grooves and folds are present. The cerebellum maintains the equilibrium of
the body by coordinating muscular activities.
Medulla oblongata is a stem-like part seen close to the cerebellum below the cerebrum.
Medulla oblongata controls involuntary actions like breathing, heartbeat, etc. It continues
as the spinal cord which extends through the vertebral column till its posterior end. In the
medulla oblongata and spinal cord, white matter is seen outside and grey matter inside.
The thalamus seen at the interior of the brain is the relay station of impulses to and from
the cerebrum. Impulses reaching the thalamus are analysed before being relayed. At
the base of the thalamus is the hypothalamus which plays an important role in maintaining
homeostasis.
16
BIOLOGY
Indicators
« How is the brain protected?
« What are the main parts of the brain?
« How do the cerebrum and medulla
oblongata differ in structure and
function?
« What is the role of the brain in
providing sensory experiences?
« Which part of the brain enabled Raju
to draw the scenery?
Don't you doubt why two eyes are needed
to see? One can see even with a single eye.
Try the given activity.
Remove the cap of a pen and hand it over
to your friend. Ask your friend to replace
the cap on the pen in your hand, with one
eye closed, standing at a distance of
atleast half a metre.
What happens? Find the reason and write
it down.
.....................................................................
Man has the power to focus both eyes at
the same time on objects in front of him.
This is binocular vision. As a result, a three
dimensional vision, giving a correct
understanding of the distance from the
object, thickness of the object etc. is
possible.
Behind Hearing
You have understood how vision is experienced. Is it in the same manner that hearing
also is effected? In order to understand the structure and function of the ear, analyse
the following figure and description with the help of indicators.
External ear
Internal ear
Ear drum or tympanum
Membrane at the innermost end
of the ear canal. Vibrate in
accordance with sound waves.
Middle Ear
semicircular canals
auditory
nerve
Cochlea
Part with
three fluidfilled sacs.
Auditory
receptors
are situated
in this part.
Pinna
Directs the
sound
waves
towards the
ear canal.
Ear canal
Directs the sound waves to the
ear drum. Small hairs and ear
wax seen in this area prevent
dust and pathogens.
incus
malleus
stapes
Ear ossicles
Direct the vibrations
that occur in the ear
drum towards the
oval window.
Eustachian tube
Oval window
Vibrates in
accordance with the
vibration of stapes.
Fig - 1.12. Structure of the ear
Connects middle
ear with the
pharynx. Regulates
the pressure in the
middle ear.
17
BIOLOGY
Cochlea and Hearing
The part of the inner ear appears like the shell of a snail is the cochlea. It has three
internal chambers. The middle chamber is filled with a fluid called endolymph and the
other two chambers with the fluid perilymph. Auditory receptors are seen in the Organ of
Corti located in the middle chamber. Vibration of the oval window causes vibration in
the perilymph. This is transmitted to the endolymph and the auditory receptors are
stimulated. Impulses thus formed reach the auditory centre of the brain through the auditory
nerve.
Indicators
—
Parts of the external ear, middle ear, and internal ear and the functions of each.
—
The way through which the vibrations of the tympanic membrane reach the interior
of cochlea.
—
The mode of formation of impulses.
—
Part to which impulses reach through the auditory nerve.
Discuss how the sensation of hearing is enabled. Based on your inferences fill up the
flow chart.
Sound wave
Pinna
Cochlea
Auditory centre of the brain
Is hearing the only function of the ear?
Look at the following picture.
18
Don’t turn round
and round my
child... you will feel
giddy!
BIOLOGY
Why do you feel giddy when you turn round and round?
Based on indicators analyse the given description, Figure 1.13 and the flow chart and
form inferences.
.....................................................................................................................
Ear and Equilibrium of the Body
The semicircular canals and vestibule that are parts of the inner ear help to maintain the
equilibrium of the body. These parts are filled with endolymph.
Semicircular canals – They are three in number, seen adjacent to the utricle. Their
swollen end is called ampulla. Receptors in the ampulla are stimulated in accordance
with the movement of the head.
Vestibule – It lies adjacent to the semicircular canals. It has two sacs – utricle and saccule.
Receptors in these are stimulated according to the movements of the head.
semicircular canals
ampulla
utricle
saccule
} vestibule
cochlea
Fig - 1.13. Parts of the inner ear
Movement of
the head and
other parts of
the body
Creates movements in
the endolymph of
semicircular canals and
vestibule
Cerebellum
Receptors are
stimulated
Impulse
Auditory
nerve
Coordinates muscular activities and
maintains equilibrium of the body
Indicators
« Where are the receptors related to equilibrium of body located?
« What are the changes brought about by body movements in the parts related to
equilibrium?
« Why is giddiness felt when you turn round and round?
19
BIOLOGY
Till now we have been discussing the structure and function of the eye and the ear, and
the control of the brain over their functions. How are the other senses experienced?
The Sense of Taste
The taste buds located on the tongue, cheek and throat enable us to detect taste. Primary
tastes such as sweetness, bitterness, sourness and saltiness can be detected by different
taste buds. Analyse Figure 1.14 and understand how taste is detected and complete
the flow chart.
bitterness
taste receptors
impulses to the
centre of taste in
the brain
sweetness
taste buds
sourness
saltiness
minute pore to taste bud
Fig - 1.14. Taste buds in the tongue
Food particles
dissolve in the
saliva
Taste receptors in
the taste buds are
stimulated
Nerve
The sense of
taste
To detect Smell
Olfactory receptors seen on the walls of the nasal cavity are stimulated by olfactory
particles which enter the nose through air. Analyse the illustration given below and
enlist the activities related to the formation of olfactory sensation sequentially.
olfactory receptor
Olfactory Nerve
Impulses reach the
olfactory centre of
the brain.
olfactory receptor
mucus
Fig - 1.15. Olfactory receptors
20
BIOLOGY
—
Olfactory particles enter the nose along with air.
—
Olfactory particles dissolve in the mucus and reach the olfactory receptors.
—
Olfactory receptors are stimulated and impulses are formed.
—
……………………..................................................................
—
Skin – the Largest Sense Organ
We recognise several stimuli through the skin. What are they? Analyse Figure 1.16 and
identify the receptors in the skin.
pain
heat
touch
cold
pressure
Fig - 1.16. Receptors in the skin
When receptors are stimulated impulses
are formed. Impulses reach the brain
through respective nerves and become
senses. The largest number of receptors is
seen in fingers and palm. You have now
understood the significant role of the sense
organs in recognizing changes in our
surroundings and responding to them
specifically. The sense organs may have
disorders or diseases due to many
reasons. Myopia (short sight),
hypermetropia (long sight) etc. are eye
disorders. You might have studied about
it. Find the causes and remedies for these
and complete the following table.
21
BIOLOGY
Condition
Cause
Remedy
Hypermetropia
Myopia
this. Since the eye lens becomes
opaque, the power of vision gradually
diminishes. Surgical replacement of
the lens is the remedial measure.
Note the indications given about other
disorders of the eye and the ear. Collect
more information about them and record
it in the Science diary.
—
—
Squint eye: The condition where both
eyes are not able to focus on the same
object. This defect can be rectified
through surgery.
Glaucoma: As reabsorption of aqueous
humour is hindered, pressure
increases in the eye. The optic nerve
and photoreceptors in the retina may
get damaged due to glaucoma. Early
treatment may prevent the onset of
blindness.
—
Presbyopia: This visual disorder is
usually seen in middle-aged persons.
This is due to the loss of elasticity of
the lens. In this condition nearby
objects are not clearly seen. Use of
convex lens is the remedial measure.
—
Cataract: Aged people are affected by
22
—
Deafness: This is the condition in
which hearing ability is lost. It can be
caused by several reasons.
Many habits of modern life adversely
affect the health of our sense organs. Find
such habits and their ill effects and
organize a seminar on the topic.
Not only man, all living beings have
sensory experiences. However, man can
transform mere sensations into blissful
experiences. While enjoying a scenery
what we experience is an integrated blend
of sight, hearing, touch, etc., a fresh
experience beyond absolute sensation.
This experience is rendered by the brain.
The painting by Raju is the outcome of
such an experience. It is the brain that
provides us the ability to enjoy nature.
BIOLOGY
Follow up Activities
1) An ordinary situation is given below. Analyse it and answer the questions.
—
Two friends meet after a long time and have a chat.
(a) What are the sensory functions performed here?
(b) What are the nervous activities that enabled them to recognize each other?
(c) How do the ear and the brain help in their communication? Analyse it.
(d) Write an example of another situation in which sense organs and brain function
in collaboration.
2) Enlist the nervous functions that take place in relation to the following situations
sequentially.
(a) Sweetness is experienced on chewing toffee.
(b) Enjoying the fragrance of flowers.
(c ) The circus performer does not fall while walking over a wire.
3) Correct the mistakes, if any, in the flow chart and redraw.
Stimulus
Dendrite
Synapse
Axonite
Cyton
Axon
Adjacent Neuron
Dendron
4) Complete the table
Sense organs
Receptors
Stimuli
Experience
Eye
Rod Cells
Cone cells
Light
Vision
Tastes like saltiness,
sourness, sweetness,
bitterness
Taste
Ear
Nose
Tongue
Skin
— Touch
— Pain
—
—
23