Notes

ne day Boojho was eagerly
waiting to meet his
grandparents who were coming
to the town after a year. He was in a real
hurry as he wanted to receive them at the
bus-stop. He ran fast and reached the
bus-stop in a few minutes. He was
breathing rapidly. His grandmother asked
him why he was breathing so fast. Boojho
told her that he came running all the way.
But the question got stuck in his mind.
He wondered why running makes a
person breathe faster. The answer to
Boojho’s question lies in understanding
why we breathe. Breathing is a part of
respiration. Let us learn about respiration.
10.1 WHY DO WE RESPIRE?
In Chapter 2 you learnt that all
organisms are made of small
microscopic units called cells. A cell is
the smallest structural and functional
unit of an organism. Each cell of an
organism performs certain functions
such as nutrition, transport, excretion
and reproduction. To perform these
functions, the cell needs energy. Even
when we are eating, sleeping or reading
we require energy. But, where does this
energy come from? Can you say why
your parents insist that you should eat
regularly? The food has stored energy,
which is released during respiration.
Therefore, all living organisms respire
to get energy from food. During
breathing, we breathe in air. You know
that air contains oxygen. We breathe out
air which is rich in carbon dioxide. The
air we breathe in is transported to all
parts of the body and ultimately to each
cell. In the cells, oxygen in the air helps
in the breakdown of food. The process
of breakdown of food in the cell with the
release of energy is called cellular
respiration. Cellular respiration takes
place in the cells of all organisms.
In the cell, the food (glucose) is broken
down into carbon dioxide and water using
oxygen. When breakdown of glucose
occurs with the use of oxygen it is called
aerobic respiration. Food can also be
broken down, without using oxygen. This
is called anaerobic respiration.
Breakdown of food releases energy.
You should know that there are some
organisms such as yeast that can survive
in the absence of air. They are called
anaerobes. They get energy through
anaerobic respiration. In the absence of
oxygen, glucose breaks down into alcohol
and carbon dioxide, as given below:
RESPIRATION IN ORGANISMS 109
Our muscle cells can also respire
anaerobically, but only for a short time,
when there is a temporary deficiency of
oxygen. During heavy exercise, fast
running (Fig. 10.1), cycling, walking for
many hours or heavy weight lifting, the
demand for energy is high. But the
supply of oxygen to produce the energy
is limited. Then anaerobic respiration
takes places in the muscle cells to fulfil
the demand of energy:
Fig. 10.1 During exercise, some muscles may
respire anaerobically
Have you ever wondered why you get
muscle cramps after heavy exercise? The
cramps occur when muscle cells respire
anaerobically. The partial breakdown of
glucose produces lactic acid. The
accumulation of lactic acid causes muscle
cramps. We get relief from cramps after a
hot water bath or a massage. Can you
guess why it is so? Hot water bath or
massage improves circulation of blood.
As a result, the supply of oxygen to the
muscle cells increases. The increase in
the supply of oxygen results in the
complete breakdown of lactic acid into
carbon dioxide and water.
10.2 BREATHING
Activity 10.1
Yeasts are single-celled organisms.
They respire anaerobically and during
this process yield alcohol. They are,
therefore, used to make wine and beer.
CAUTION
Do this activity under the supervision
of your teacher.
Close your nostrils and mouth
tightly and look at a watch. What did
you feel after some time? How long were
you able to keep both of them closed?
Note down the time for which you could
hold your breath (Fig. 10.2).
So, now you know that you cannot
survive for long without breathing.
Breathing means taking in air rich
in oxygen and giving out air rich in
carbon dioxide with the help of
respiratory organs. The taking in of air
rich in oxygen into the body is called
inhalation and giving out of air rich in
carbon dioxide is known as exhalation.
It is a continuous process which goes
110 SCIENCE
you like to find out your breathing rate?
Do you want to know whether it is
constant or it changes according to the
requirement of oxygen by the body? Let
us find out by doing the following
activity.
Activity 10.2
Generally we are not aware that we are
breathing. However, if you try you can
count your rate of breathing. Breathe
in and out normally. Find out how many
times you breathe in and breathe out
in a minute? Did you inhale the same
number of times as you exhaled? Now
count your breathing rate (number of
breaths/minute) after brisk walk and
after running. Record your breathing
rate as soon as you finish and also after
complete rest. Tabulate your findings
and compare your breathing rates under
different conditions with those of your
classmates.
From the above activity, you must
have realised that whenever a person
needs extra energy, he/she breathes
faster. As a result more oxygen is
Boojho noticed that when he
released his breath after holding
it for some time, he had to
breathe heavily. Can you tell
him why it was so?
Fig. 10.2 Holding breath
on all the time and throughout the life
of an organism.
The number of times a person
breathes in a minute is termed as the
breathing rate. During breathing
inhalation and exhalation take place
alternately. A breath means one
inhalation plus one exhalation. Would
Table 10.1 Changes in breathing rate under different conditions
Name of the classmate Breathing rate
Normal After a brisk After running At rest
walk for 10 fast 100 m
minutes
Self
RESPIRATION IN ORGANISMS 111
Fig. 10.3 Variation in the breathing rate during
different activities
supplied to our cells. It speeds up
the breakdown of food and more
energy is released. Does this explain
why do we feel hungry after a
physical activity?
When you feel drowsy, does your
breathing rate slow down? Does your
body receive sufficient oxygen?
Activity 10.3
Figure 10.3 shows the various activities
carried out by a person during a normal
day. Can you say in which activity, the
rate of breathing will be the slowest and
in which it will be the fastest? Assign
numbers to the pictures in the order of
increasing rate of breathing according
to your experience.
10.3 HOW DO WE BREATHE?
Let us now learn about the mechanism
of breathing. Normally we take in air
through our nostrils. When we inhale
air, it passes through our nostrils into
the nasal cavity. From the nasal cavity,
the air reaches our lungs through the
windpipe. Lungs are present in the
chest cavity (Fig. 10.4). This cavity is
surrounded by ribs on the sides. A large,
muscular sheet called diaphragm forms
the floor of the chest cavity (Fig. 10.4).
Breathing involves the movement of the
diaphragm and the rib cage.
During inhalation, ribs move up and
outwards and diaphragm moves down.
This movement increases space in our
chest cavity and air rushes into the
lungs. The lungs get filled with air.
During exhalation, ribs move down and
inwards, while diaphragm moves up to
its former position. This reduces the size
of the chest cavity and air is pushed out
of the lungs (Fig. 10.5). These
movements in our body can be felt
On an average, an adult human being
at rest breathes in and out 15–18
times in a minute. During heavy
exercise, the breathing rate can
increase upto 25 times per minute.
While we exercise, not only do we
breathe fast, we also take deep breaths
and thus inhale more oxygen.
Paheli wants to know why
we yawn when we are
sleepy or drowsy.
112 SCIENCE
easily. Take a deep breath. Keep your
palm on the abdomen, feel the
movement of abdomen. What do you
find?
After having learnt that during
breathing there are changes in the size
of the chest cavity, children got involved
in the chest expansion competition.
Fig 10.4 Human respiratory system
The air around us has various types of unwanted particles, such as smoke, dust,
pollens, etc. When we inhale, the particles get trapped in the hair present in our
nasal cavity. However, sometimes these particles may get past the hair in the
nasal cavity. This may irritate the lining of the cavity, as a result of which we
sneeze. Sneezing expels these foreign particles from the inhaled air and a dustfree, clean air enters our body.
TAKE CARE: When you sneeze, you should cover your nose so that the foreign
particles you expel are not inhaled by other persons.
Smoking damages lungs. Smoking is
also linked to cancer. It must be
avoided.
Nasal passage
Oral cavity Pharynx
Trachea
Lungs
Ribs
Diaphragm
Everyone was boasting that she/he
could expand it the maximum. How
about doing this activity in the class
with your classmates?
RESPIRATION IN ORGANISMS 113
Activity 10.4
Take a deep breath. Measure the size
of the chest with a measuring tape
(Fig. 10.6) and record your observations
in Table 10.2. Measure the size of the
chest again when expanded and
indicate which classmate shows the
maximum expansion of the chest.
We can understand the mechanism
of breathing by a simple model.
Activity 10.5
Take a wide plastic bottle. Remove the
bottom. Get a Y-shaped glass or plastic
tube. Make a hole in the lid so that the
tube may pass through it. To the forked
end of the tube fix two deflated balloons.
Introduce the tube into the bottle as
shown in Fig. 10.7. Now cap the bottle.
Seal it to make it airtight. To the open
base of the bottle tie a thin rubber or
plastic sheet using a large rubber band.
Fig. 10.5 Mechanism of breathing in human beings
Table 10.2: Effect of breathing on the chest size of some classmates
Name of the Size of the chest (cm)
classmate During inhalation During exhalation Difference in size
Ribs move
out
Diaphragm
moves down
Air is
drawn in Air forced out
Ribs move
back
Diaphragm
moves back
(a) Inhalation (b) Exhalation
114 SCIENCE
Fig. 10.6 Measuring chest size
the hole in the lid in such a way that it
dips in lime water. Now blow gently
through the straw a few times (Fig.
10.8). Is there a change in the
appearance of lime water? Can you
explain this change on the basis of what
you learnt in Chapter 6?
You are aware that air we inhale or
exhale is a mixture of gases. What do
we exhale? Do we exhale only carbon
dioxide or a mixture of gases along with
it? You must have also observed that if
you exhale on a mirror, a film of
moisture appears on its surface. From
where do these droplets come?
To understand the expansion of the
lungs, pull the rubber sheet from the
base downwards and watch the balloons.
Next, push the rubber/plastic sheet up
and observe the balloons. Did you see
any changes in the balloons?
What do the balloons in this model
represent? What does the rubber sheet
represent?
Now, you should be able to explain
the mechanism of breathing.
10.4 WHAT DO WE BREATHE OUT ?
Activity 10.6
Take a slender, clean test tube or a
glass/plastic bottle. Make a hole in its
lid and fix it on the bottle. Pour some
freshly prepared lime water in the
test-tube. Insert a plastic straw through
Plastic
bottle
Balloons
Ruber
sheet
Fig 10.7 Model to show
mechanism of breathing
Boojho wants to know how
much air a person can hold in
the lungs.
Breathe for Better Life
Regular traditional breathing exercise (pranayama) can increase the capacity of
lungs to take in more air. Thus more oxygen can be supplied to the body cells
resulting in release of more energy.
RESPIRATION IN ORGANISMS 115
Fig.10.9 Tracheal system
Tracheae
10.5 BREATHING IN OTHER ANIMALS
Animals such as elephants, lions, cows,
goats, frogs, lizards, snakes, birds, have
lungs in their chest cavities like the
human beings.
How do other organisms breathe? Do
they also have lungs like those of human
beings? Let us find out.
Cockroach: A cockroach has small
openings on the sides of its body. Other
insects also have similar openings.
Boojho wants to know if
cockroaches, snails, fish,
earthworms, ants and mosquitoes
also have lungs.
These openings are called spiracles
(Fig. 10.9). Insects have a network of air
tubes called tracheae for gas exchange.
Oxygen rich air rushes through spiracles
into the tracheal tubes, diffuses into the
body tissue, and reaches every cell of
the body. Similarly, carbon dioxide from
the cells goes into the tracheal tubes and
moves out through spiracles. These air
tubes or tracheae are found only in
insects and not in any other group of
animals.
Fig 10.8 Effect of exhaled air on lime water
The percentage of oxygen and carbon
dioxide in inhaled and exhaled air.
Inhaled air Exhaled air
Lungs
21% oxygen 16.4% oxygen
4.4% carbon
dioxide
0.04% carbon
dioxide
Straw
Lime water
Earthworm: Recall from Chapter 9 of
Class VI that earthworms breathe
through their skins. The skin of an
earthworm feels moist and slimy on
touching. Gases can easily pass through
them. Though frogs have a pair of lungs
like human beings, they can also
breathe through their skin, which is
moist and slippery.
116 SCIENCE
Gills
Fig. 10.10 Breathing organs in fish
Paheli wants to know whether
roots, which are underground
also take in oxygen? If so, how?
dioxide. In the cells oxygen is used to
break down glucose into carbon dioxide
and water as in other organisms. In
plants each part can independently take
in oxygen from the air and give out
carbon dioxide. You have already learnt
in Chapter 1 that the leaves of the plants
have tiny pores called stomata for
exchange of oxygen and carbon dioxide.
Soil particles
Root hair
Air space
Fig. 10.11 Roots absorb air from the soil
10.6 BREATHING UNDER WATER
Can we breathe and survive in water?
There are many organisms which live in
water. How do they breathe under water?
You have studied in Class VI that
gills in fish help them to use oxygen
dissolved in water. Gills are projections
of the skin. You may wonder how gills
help in breathing. Gills are well supplied
with blood vessels (Fig. 10.10) for
exchange of gases.
Boojho has seen in television
programmes that whales and
dolphins often come up to the
water surface. They even release
a fountain of water sometimes
while moving upwards. Why do
they do so?
10.7 DO PLANTS ALSO RESPIRE?
Like other living organisms, plants also
respire for their survival as you have
learnt in Class VI. They also take in
oxygen from the air and give out carbon
Like all other living cells of the
plants, the root cells also need
oxygen to generate energy. Roots take
up air from the air spaces present
between the soil particles (Fig. 10.11).
RESPIRATION IN ORGANISMS 117
Can you guess what would happen
if a potted plant is overwatered?
In this chapter you learnt
that respiration is a vital biological
What you have learnt
n Respiration is essential for survival of living organisms. It releases energy
from the food.
n The oxygen we inhale is used to breakdown glucose into carbon dioxide
and water. Energy is released in the process.
n The breakdown of glucose occurs in the cells of an organism (cellular
respiration).
n If the food is broken down with the use of oxygen, it is called aerobic
respiration. If the breakdown occurs without the use of oxygen, the
respiration is called anaerobic respiration.
n During heavy exercise when the supply of oxygen to our muscle cells is
insufficient, food breakdown is by anaerobic respiration.
n Breathing is a part of the process of respiration during which an
organism takes in the oxygen-rich air and gives out air rich in carbon
dioxide. The respiratory organs for the exchange of gases vary in different
organisms.
n During inhalation, our lungs expand and then come back to the original
state as the air moves out during exhalation.
n Increased physical activity enhances the rate of breathing.
n In animals like cow, buffalo, dog and cat the respiratory organs and the
process of breathing are similar to those in humans.
n In earthworm, the exchange of gases occurs through the moist skin. In
fishes it takes place through gills and in insects through the tracheae.
n In a plant the roots take in air present in the soil. Leaves have tiny pores
called stomata through which they exchange gases. The breakdown of
glucose in the plant cells is similar to that in other living beings.
process. All living organisms need
to respire to get the energy needed
for their survival.