Page 85 - NCERT Science Class 10 English Medium

class, even if we are just asleep, this maintenance job has to go on.
The processes which together perform this maintenance job are
life processes.
Since these maintenance processes are needed to prevent damage
and break-down, energy is needed for them. This energy comes from
outside the body of the individual organism. So there must be a process
to transfer a source of energy from outside the body of the organism,
which we call food, to the inside, a process we commonly call nutrition.
If the body size of the organisms is to grow, additional raw material will
also be needed from outside. Since life on earth depends on carbon-
based molecules, most of these food sources are also carbon-based.
Depending on the complexity of these carbon sources, different
organisms can then use different kinds of nutritional processes.
The outside sources of energy could be quite varied, since the
environment is not under the control of the individual organism. These
sources of energy, therefore, need to be broken down or built up in the
body, and must be finally converted to a uniform source of energy that
can be used for the various molecular movements needed for
maintaining living structures, as well as to the kind of molecules the
body needs to grow. For this, a series of chemical reactions in the
body are necessary. Oxidising-reducing reactions are some of the most
common chemical means to break-down molecules. For this, many
organisms use oxygen sourced from outside the body. The process
of acquiring oxygen from outside the body, and to use it in the process
of break-down of food sources for cellular needs, is what we call
respiration.
In the case of a single-celled organism, no specific organs for taking
in food, exchange of gases or removal of wastes may be needed because
the entire surface of the organism is in contact with the environment.
But what happens when the body size of the organism increases and
the body design becomes more complex? In multi-cellular organisms,
all the cells may not be in direct contact with the surrounding
environment. Thus, simple diffusion will not meet the requirements of
all the cells.
We have seen previously how, in multi-cellular organisms, various
body parts have specialised in the functions they perform. We are familiar
with the idea of these specialised tissues, and with their organisation in
the body of the organism. It is therefore not surprising that the uptake
of food and of oxygen will also be the function of specialised tissues.
However, this poses a problem, since the food and oxygen are now taken
up at one place in the body of the organisms, while all parts of the body
need them. This situation creates a need for a transportation system for
carrying food and oxygen from one place to another in the body.
When chemical reactions use the carbon source and the oxygen for
energy generation, they create by-products that are not only useless
for the cells of the body, but could even be harmful. These waste by-
products are therefore needed to be removed from the body and discarded
outside by a process called excretion. Again, if the basic rules for body

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