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1. SCIENCE
FORM 2

2. CHAPTER 1
THE WORLD THROUGH OUR
SENSES

3. 1.1 Sensory Organs
Sensory organs, stimuli and senses
a) Sensory organs : organs that detect
stimuli.
b) Stimuli : Any changes that occur in the
surroundings which lead to respond.
c) Senses: detectors used to find out the
stimuli.

4. sense sensory organs stimuli detected
touch skin
touch, pressure,
heat, cold, pain
taste tongue taste
smell nose smell
sight eyes light
hearing ears sound

5. d) HOTS
Without sensory organs, we will not be
able to know what is happening in our
surroundings. So, we must take care of
our sensory organs from injuries.

6. Responses to stimuli
• response: a reaction
• receptor: a structure that recognize a stimulus in
the internal/ external environment of an organism.
• nerve impulses: electrical messages produced by
receptor.
• effector: part of the body that carry out responses.
Example: muscle and gland

7. Common pathway from the stimulus to
response in humans

9. 1.2 Sense of touch
The Skin - largest organ in the body.

11. The sensitivity of the skin
• Different parts have different degrees of
sensitivity.
• Depends on:
1.how close together the receptors are -
the higher the concentration of receptor,
the more sensitive the skin is
2.how deep the receptor are - the thicker
the epidermis, the deeper the receptors
lie in the skin, the less sensitive the skin
is.

12. Examples:
• The back of the body is less sensitive as
it has very few touch reseptor.
• The knees and elbows are less sensitive
as they have a thick epidermis.
• The lips and fingertips are very sensitive
to touch due to the large number of touch
receptor. They have thin epidermis too.

13. HOTS- APPLICATION
• Blind people use the high sensitivity
of their fingertips to read in Braille.
Raised dots are used to represent
letters and numbers.

14. • Doctors give
injections to their
patients at the
forearms or hips.
One of the reasons
for this is these
areas are less
sensitive to pain.

15. • The back of the
hands has a lot of
heat and cold
receptors
(temperature).
Mother test the
temperature of
their babies' milk
on the back of their
hands.

17. 1.3 Sense of smell

18. 1.Two holes, called nostrils, open into a
large hollow space called nasal cavity.
2.The nasal cavity is lined with mucus. Its
roof has many smell receptors/ sensory
cell.
3.Adaptation:
The smell receptors are always covered
with mucus. Smells must dissolve in the
mucus before they can be detected.

19. • Smell receptors are located at the roof of
the nasal cavity. This position allows the
receptors to have the maximum exposure
to the air current.

20. Detecting smell.
• Smells are actually chemicals present in
the air.
• Food, perfume .............air.
• When we breathe in, .........cavity.
• The ......cells.
• These ........smell.
• After smelling ......brain.
• Then we ....... smell it.

21. HOTS!!!!!
• When a person catches a cold, they
cannot detect smell well. The nose
produces too much mucus which prevents
chemicals from stimulating the smell
receptors.
• A dog's sense of smell is hundred times
more sensitive than human. Therefore,
dogs are being used to track criminals and
to sniff out drugs or explosives.

23. 1.4 Sense of Taste
• draw figure 1.8 pg 9

24. draw figure 1.9 pg 10

25. 1.Tongue is sensitive to chemicals in
food that give taste to food.
2.Can detect 4 tastes: _____, _____,
_____ and _______.
3.The tongue is lined with taste buds
which contain many taste receptor.

26. How tastes are detected?
• When we chew our ..................saliva.
• The dissolved.......................impulses.
• These are ................................tastes.
pg 10

27. The senses of taste and smell.
• Both senses are needed to detect the
flavour of food/ drink accurately.
• The nasal cavity and mouth are
connected. When we eat/ drink,
chemicals from food move up to nasal
cavity and stimulate the smell
receptors in the nose as well.

28. • When we have cold, chemicals in the
food cannot reach the smell receptors
because excessive mucus or blocked
nose. So, the food become less tasty.