Page 94 - NCERT Science Class 10 English Medium

cells, or away from them and out into the air. The direction of diffusion
depends upon the environmental conditions and the requirements of
the plant. At night, when there is no photosynthesis occurring, CO
2
elimination is the major exchange activity going on. During the day,
CO generated during respiration is used up for photosynthesis, hence
2
there is no CO release. Instead, oxygen release is the major event at
2
this time.
Animals have evolved different organs for the uptake of oxygen
from the environment and for getting rid of the carbon dioxide
produced. Terrestrial animals can breathe the oxygen in the
atmosphere, but animals that live in water need to use the oxygen
dissolved in water.

Activity
Activity 5.6
Activity 5.6
Activity 5.65.6
5.6
Activity
n Observe fish in an aquarium. They open and close their mouths
and the gill-slits (or the operculum which covers the gill-slits)
behind their eyes also open and close. Are the timings of the
opening and closing of the mouth and gill-slits coordinated in some
manner?
n Count the number of times the fish opens and closes its mouth in
a minute.
n Compare this to the number of times you breathe in and out in a
minute.

Since the amount of dissolved oxygen is fairly low compared
to the amount of oxygen in the air, the rate of breathing in aquatic
organisms is much faster than that seen in terrestrial organisms.
Fishes take in water through their mouths and force it past the More to Know!
gills where the dissolved oxygen is taken up by blood.
Using tobacco directly or
Terrestrial organisms use the oxygen in the atmosphere for
any product of tobacco in
respiration. This oxygen is absorbed by different organs in the form of cigar, cigarettes,
different animals. All these organs have a structure that increases bidis, hookah, gutkha, etc.,
the surface area which is in contact with the oxygen-rich is harmful. Use of tobacco
atmosphere. Since the exchange of oxygen and carbon dioxide most commonly affects the
has to take place across this surface, this surface is very fine tongue, lungs, heart and
and delicate. In order to protect this surface, it is usually placed liver. Smokeless tobacco is
within the body, so there have to be passages that will take air also a major risk factor for
to this area. In addition, there is a mechanism for moving the air heart attacks, strokes,
in and out of this area where the oxygen is absorbed. pulmonary diseases and
several forms of cancers.
In human beings (Fig. 5.9), air is taken into the body through
There is a high incidence of
the nostrils. The air passing through the nostrils is filtered by
oral cancer in India due to
fine hairs that line the passage. The passage is also lined with
the chewing of tobacco in
mucus which helps in this process. From here, the air passes
the form of gutkha. Stay
through the throat and into the lungs. Rings of cartilage are
healthy; just say NO to
present in the throat. These ensure that the air-passage does tobacco and its products!
not collapse.

Life Processes 89

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