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1. IGCSE- BIOLOGY
TOPIC: COORDINATION AND
RESPONSE
BY MICHAEL SIR & eD
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2. Sense organs
• Sense organs are group of receptor
cells that are able to respond to
specific stimuli.
• The stimuli are light, sound, touch,
temperature and chemicals.
• The following are the five sense
organs in the human body:
the eyes, ears, nose, tongue, and skin.
• Your eye, for example, is a sense organ,
and contains receptor cells in the
retina. These receptor cells in the eye
are sensitive to light.
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3. Sense organs
• Skin = touch i.e. pain,
pressure, temperature
etc.
• Eyes = light
• Nose = smell
• Ears = noise
• Tongue = taste
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4. Sense organs- the structure of the eye
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5. Sense organs- the structure of the eye
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6. Sense organs- the structure of the eye
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• Each eye is set in a bony socket in
the skull, which protects the eye.
• The front of the eye is covered by
a thin, transparent membrane
called the conjunctiva, which
helps to protect the parts behind
it.
• Only the very front of the eye is
not surrounded by bone.
• The eye is filled with fluid, which
helps to keep it in shape.

7. Sense organs- the structure of the eye
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• The surface of the eye is always
kept moist by a fluid made in the
tear glands.
• This fluid contains an enzyme
called lysozyme, which can kill
bacteria.
• The fluid is washed across your
eye by your eyelids when you
blink.
• The eyelids, eyebrows and
eyelashes also help to stop dirt
from landing on the surface of
your eyes.

8. Parts of the eye their functions
• Retina: contains light receptors,
some sensitive to light of different
colours.
• Cornea: refracts light.
• Iris: controls how much light enters
pupil.
• Lens: focuses light and objects onto
retina.
• Optic nerve: carries impulses from
the retina to the brain.
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9. Parts of the eye their functions
• Pupil: Controls the amount of light
entering the eye
• Blind spot: this is the part of the
retina where optic nerves leaves
and where there are no receptor
cells.
• Fovea: this is the part of the retina
where light is focused if you look
directly at an object and this is the
part of the retina where cone cells
are tightly packed
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10. Retina
• The retina is at the back of
the eye. This is where the
receptor cells are.
• When light falls on a receptor
cell in the retina, the cell
sends an electrical impulse
along the optic nerve to the
brain.
• The brain uses the impulses
from each receptor cell to
build up an image.
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11. Retina
• Some of these receptor
cells are sensitive to light
of different colours,
enabling us to see
coloured images.
• There are no receptor
cells where the optic
nerve leaves the retina.
This part is called the
blind spot.
• If light falls on the blind
spot, no impulses will be
sent to the brain.
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12. Rods and cones
• We have two kinds of receptor cells
called photoreceptors in the retina.
They are the Cones and rods.
• Rods are sensitive to quite dim light
(night vision), but they do not
respond to colour.
• Cones are able to distinguish
between the different colours of light,
but they only function when the light
is quite bright.
• We have three different kinds of
cones, sensitive to red, green and
blue light.
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13. Rods and cones
• Rods therefore allow us to see in
dim light but only in black and
white, while cones give us colour
vision.
• The fovea contains almost entirely
cones, packed tightly together.
• When we look directly at an object,
we use our cones to produce a
sharp image, in colour.
• Rods are found further out on the
retina and are less tightly packed.
• Rods show us a less detailed image.
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14. Rods and cones
• The closer together the receptor
cells are, the clearer the image the
brain can produce.
• The part of the retina where the
receptor cells are packed most
closely together is called the fovea.
• This is the part of the retina where
light is focused when you look
straight at an object.
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15. Differences between cones and rods
Cones Rods
1. Cones gives us colour vision.
They able to distinguish between the different colours
of light. The colours are red, blue and green.
1. Rods allow us to see in dim light but only in black
and white.
2. Cones are packed tightly together in and around the
fovea of the retina to create a sharp and a detailed
image.
2. Rods are less tightly packed together around the
edge of the retina which creates a less detailed image.
3. Good for seeing in high light intensity. Good for seeing in low light intensity (night vision).
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16. Differences between rods and cones
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17. Pupil reflex or iris reflex
• The iris is a circular piece of tissue in front of
the eye and it is the coloured part of the eye.
• The gab between the Iris is called the pupil.
• Circular muscle and Radial muscles are
antagonistic muscles that are in the iris or
pupil.
• They work together to control an action and
have opposite effects. When one muscle
contracts, the other relaxes.
• These muscles works together to control the
amount of light entering the eyes during high
and low light intensity.
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18. Pupil reflex or iris reflex (High light intensity)
• When light intensity is
high,
• the circular muscle
contract, the radial
muscle relax
• this makes the pupil
smaller to prevent too
much light from
entering the eye.
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19. Pupil reflex or iris reflex (low light intensity)
• When light intensity
is low,
• the circular muscle
relax the radial
muscle contracts and,
• this makes the pupil
larger to allow more
light into the eye.
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20. Accommodation
• Accommodation: it is the change in the shape of the eye lens to focus on
objects at different distances from the eye.
• For the brain to see a clear image, there must be a clear image focused on
the retina.
• Light rays must be bent so that they focus exactly onto the retina.
• Bending light rays is called refraction.
• The shape of the lens is altered, to make it bend light rays by different
amounts.
• The thicker the lens, the more it bends the light rays. The thinner it is, the
less it bends them.
• The lens makes fine adjustments and refracts light rays..
• Most refraction of the light entering the eye is done by the cornea.
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21. How an image is focused onto the retina
• The figure shows how the
cornea and lens focus light
onto the retina.
• The image on the retina is
upside down.
• The brain interprets this so
that you see it the right
way up.
• Note that:
• The suspensory ligaments
and the ciliary muscle
works together during the
eye’s focus on near and
distant object.
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22. Focusing on near objects
• When focusing on near objects,
• the suspensory ligaments
become loose/slacked and the
ciliary muscle contract.
• this makes the lens fat, thick and
large.
• This happens because light rays
from near object diverges(move
away from point) so need much
refraction to focus properly.
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23. Focusing on distant objects
• When focusing on distant
objects,
• the suspensory ligaments
become tense and the ciliary
muscle relax.
• this makes the lens long and
thin.
• This happens because light
rays from distant object are
parallel so do not need much
refraction to focus properly.
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