All living organisms are sensitive to changes inside and outside their bodies. They respond through movement or movement of their parts. Simple organisms like amoeba and algae are sensitive to light intensity through eye spots, while plants like trifolium and mimosa respond to light and touch. Plants sense light, water, and gravity and respond through tropisms like phototropism and geotropism, which are controlled by the plant hormone auxin.

1. Sensitivity in living organisms

2. All living organisms are sensitive
to changes inside and outside
their bodies. Animals respond to
changes in their environment by
moving elsewhere. Plants
respond to changes in their
surroundings by moving their
parts.

3. amoeba
Chlamydomonas
euglena
In some simple, unicellular organisms such as amoeba,
Chlamydomonas, and euglena, the cytoplasm as a whole is
sensitive. The eye-spots in euglena and Chlamydomonas help
them to detect changes in light intensity, enabling them to
move away from bright light.

4. Trifolium plant
Some plants also respond to changes in
light intensity. For example, the leaves
of the trifolium plant open and close
according to the amount of light falling
on them.

5. mimosa plant
Some plants are sensitive
to touch. For example, the
leaves of the mimosa
plant (touch me not) close
when touched.
Plants do not have sense
organs, but they can sense
light, water and the pull of
gravity. They respond by
growing towards them.

6. stimulus
The external effect which
produces changes in the
Behaviour of an organism is
called a stimulus., the reaction
of an organism to a stimulus is
called a response. The ability
of an organism to respond to a
stimulus is called sensitivity.
For example, light is a
stimulus. The turning of the
shoot towards light is
response.

7. Sensitivity
and
movement
in plants
Plants are
sensitive to
light, water
and the pull of
gravity. We can
see the
response in
the form of
movements
called tropism.

8. The response or bending of the shoot
towards light is called phototropism. This
response is necessary because plants need
light to make food. The shoot grows towards
the light and the leaves turn their upper
surfaces towards it. The bending or growth of
the roots towards the soil is called
geotropism.
The responses of the root and shoot are
controlled by a chemical substance called
auxin. Auxin is made in the cells found at the
tip of the root and shoot. Auxin speeds up
stem growth and sows down root growth.

9. The activity of auxin is affected by
light. When light falls on a plant
from above, the stem grows
straight. When light comes from
one side, the auxin collects on the
opposite side. This causes the stem
to bend towards light.
If a plant is laid on its side, auxin
gathers in the lower half of the
stem and root. The growth of the
root cells slows down and the root
curves downwards.

10. Sensitivity in animals
Animals need to sense changes in
their surroundings to be able to
survive in their environment.
Animals can detect changes in
temperature, pressure, light intensity and sudden noises, etc. these
changes or stimuli can be felt by special organs which make up the
sensory system. Simple organisms can perceive only general stimuli
such as light or darkness, heat, cold, etc. more developed animals have
a highly specialized sensory system, by which they receive information
about their surroundings, they are able to react or respond quickly to
these stimuli.

11. Coordination All the systems of the body are closely
linked and dependent on each other. For example, food
consumed is digested in the digestive system, absorbed into
the blood and distributed throughout the body by the
circulatory system. These systems work together and their
activities are intricately related. When we run or do some
strenuous exercise, our body needs more food and oxygen to
produce more energy. Our rate of respiration increases
automatically and heartbeat becomes faster. In the same way,
at the sight and smell of food, our mouth begins to water. The
working together of all the organs and systems of the body is
called coordination. Coordination in the body is brought about
by two systems: the nervous system and the endocrine system.

12. The nervous system is made up of
nervous tissue and the endocrine
system is made up of a number of
glands which produce chemical
substances that circulate in the
bloodstream.
All the organs of the body work in
coordination with each other. This
means that they do their work at the
right time and at the right speed, to
serve the body as a whole. Without
coordination, the whole body would
just be a collection of organs.

13. If we observe carefully, we find that our body is performing many
functions at the same time. Food is being digested; the blood is
circulating the digested food and oxygen to all parts of the body; the
kidneys are removing the waste; the lungs are taking
in oxygen and removing carbon dioxide.
The brain plays a very important part in
the body’s coordination. It receives
messages from the muscles and sense
organs and it sends back messages in
return. This is how if controls all the different parts of the body so that
they can work together.

14. The brain is made up of millions of nerve
cells which are connected to each other.
Messages are carried along the nerve cells
in the form of an electric wave or impulse,
in one direction only.
A nerve cell has a cell body with long thin branches
extending from it. The shorter branches are called
dendrites. Dendrites take in messages from other nerve
cells. One long branch, called axon, carries messages out
of the nerve cell. The axon connects with other nerve
cells and passes messages to muscles, glands, or organs.
Bundles of axons form a nerve.

15. The base of the brain extends into the spinal
cord. The spinal cord is a bundle of nerve cells
which runs down into the backbone. Along the
way, nerves branch off from the spinal cord to
go to the arms, lungs, heart, liver, stomach, and
intestines. Lower down, nerves branch off to
the legs and feet. Sensory nerve cells carry
messages from the sense organs to the brain
and spinal cord. Motor nerve cells carry
messages from the brain and spinal cord to the
muscles and glands.

16. The endocrine system
Coordination is also brought about by chemical
substances called hormones which are produced
in a set of glands called the endocrine glands. The
endocrine glands release tiny amounts of hormones
which circulate with the blood.
Certain parts of the body called target organs
respond to the hormones they control different parts of the
body to bring about various responses such as fear,
happiness, blushing, etc. responses to hormones may last
for a few minutes or may go on for many years.

17. Sense organs Sense organs are especially developed parts of
the body which can detect stimuli such as light,
sound, taste, touch, or smell. Sense organs are
made up of sensory cells. A particular sensory cell
can respond to only one kind of stimulus. For
example, a sensory cell in the eye can only detect
light waves. It cannot respond to sound waves.
The body has five senses: hearing, smell, sight,
taste, and touch., all these are detected by sense
organs which send information to the brain. There
are special parts in the brain that sort out this
information. Then the brain tells us what we are
seeing, hearing, smelling, tasting, and feeling.

18. Ear The ear is sense organ. It is sensitive to the
stimulus of sound. It stretches deep into the
skull, and turns sound waves into impulses
that the brain can interpret.
The ear has three different parts. These are
the outer ear, the middle ear, and the inner
ear.
The outer ear consists of a flap of skin
which is shaped like a funnel. It leads to a
hollow tube called ear canal, which leads to
the eardrum. The eardrum is a thin
membrane which vibrates when sound
waves strike it.

19. The middle ear is like a hollow chamber which
is filled with air. It contains three bones called ossicles. When the eardrum
vibrates, it makes the ossicles vibrate too. These vibrations are transmitted to
the inner ear through an oval opening called the oval window. From the
lower side of the middle ear a narrow tube runs into the back of the throat or
pharynx. This tube is called the Eustachian tube. The Eustachian tube helps to
adjust the air pressure inside the middle ear to the atmospheric pressure
outside. The inner ear lies behind the oval window. The inner ear is made up
of the cochlea, vestibule and the semicircular canals. The cochlea has a tube
which is coiled up like a snail’s shell.

20. The tube is filled with a fluid. When the
oval window vibrates it makes waves in
the fluid. There are thousands of
sensitive hairs inside the cochlea which
are connected to sensory cells. When
the fluid passes over the hairs, they send
impulses to the nerves, which carry
messages about sound to the brain. The
nerve which connects the ear to the
brain is called the auditory nerve.

21. Balance of the body
The semicircular canals and the vestibule help to maintain the
balance of the body. They contain fluid and sensitive hairs. if the
head is tilted to one side, the fluid in these tubes moves and the
nerves from the sensitive hairs inform the brain about the
position of the body.

22. Eye The eye is like a tiny camera. It
sends a steady stream of pictures
to the brain. The outer eye is
covered by a transparent tissue
called the cornea. The cornea
allows light to enter the eyed.
The coloured part of the eye is a
band of muscle called the iris. In
the centre of the iris is a hole,
called the pupil. The pupil
controls the amount of light
entering the eye by becoming
narrower or wider.

23. Behind the pupil is a double convex
lens. The thickness of the lens is
controlled by muscles. The lens
helps to focus a clear image of the
object on a layer of light-sensitive
cells called the retina. The retina
contains special cells called rods and
cones which respond to light and
colour. Nerve fibers from the retina
cells carry messages to the brain.
The nerve which connects the eye to
the brain is called the optic nerve.

24. Skin The skin is the largest sensory organ in
the body. Besides protecting the body, it
also senses pain, temperature, and
pressure. Nerves in the skin produce
sensations when the skin touched. The
skin is made up of three lagers. The outer
layer is called epidermis. It contains three
layers of cells. The outermost layer of the
epidermis is hard. It protects the inner
layers. The next layer of the skin is called
the dermis. It contains nerves and hair
follicles. It also contains small tissues
called sweat glands. The sweat glands
produce sweat. Oil glands and blood
capillaries are also found in the dermis.