NCERT Textbook: Life Processes | Science & Technology for UPSC CSE PDF Download

 Page 1


Life Processes
5 CHAPTER
H
ow do we tell the difference between what is alive and what is not
alive? If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.
Movements over very small scales will be invisible to the naked eye –
movements of molecules, for example. Is this invisible molecular
movement necessary for life? If we ask this question to professional
biologists, they will say yes. In fact, viruses do not show any molecular
movement in them (until they infect some cell), and that is partly why
there is a controversy about whether they are truly alive or not.
Why are molecular movements needed for life? We have seen in earlier
classes that living organisms are well-organised structures; they can
have tissues, tissues have cells, cells have smaller components in them,
and so on. Because of the effects of the environment, this organised,
ordered nature of living structures is very likely to keep breaking down
over time. If order breaks down, the organism will no longer be alive. So
living creatures must keep repairing and maintaining their structures.
Since all these structures are made up of molecules, they must move
molecules around all the time.
What are the maintenance processes in living organisms?
Let us explore.
5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHAT ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES?
The maintenance functions of living organisms must go on even when
they are not doing anything particular. Even when we are just sitting in
2024-25
Page 2


Life Processes
5 CHAPTER
H
ow do we tell the difference between what is alive and what is not
alive? If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.
Movements over very small scales will be invisible to the naked eye –
movements of molecules, for example. Is this invisible molecular
movement necessary for life? If we ask this question to professional
biologists, they will say yes. In fact, viruses do not show any molecular
movement in them (until they infect some cell), and that is partly why
there is a controversy about whether they are truly alive or not.
Why are molecular movements needed for life? We have seen in earlier
classes that living organisms are well-organised structures; they can
have tissues, tissues have cells, cells have smaller components in them,
and so on. Because of the effects of the environment, this organised,
ordered nature of living structures is very likely to keep breaking down
over time. If order breaks down, the organism will no longer be alive. So
living creatures must keep repairing and maintaining their structures.
Since all these structures are made up of molecules, they must move
molecules around all the time.
What are the maintenance processes in living organisms?
Let us explore.
5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHAT ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES?
The maintenance functions of living organisms must go on even when
they are not doing anything particular. Even when we are just sitting in
2024-25
Science
80
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
2024-25
Page 3


Life Processes
5 CHAPTER
H
ow do we tell the difference between what is alive and what is not
alive? If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.
Movements over very small scales will be invisible to the naked eye –
movements of molecules, for example. Is this invisible molecular
movement necessary for life? If we ask this question to professional
biologists, they will say yes. In fact, viruses do not show any molecular
movement in them (until they infect some cell), and that is partly why
there is a controversy about whether they are truly alive or not.
Why are molecular movements needed for life? We have seen in earlier
classes that living organisms are well-organised structures; they can
have tissues, tissues have cells, cells have smaller components in them,
and so on. Because of the effects of the environment, this organised,
ordered nature of living structures is very likely to keep breaking down
over time. If order breaks down, the organism will no longer be alive. So
living creatures must keep repairing and maintaining their structures.
Since all these structures are made up of molecules, they must move
molecules around all the time.
What are the maintenance processes in living organisms?
Let us explore.
5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHAT ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES?
The maintenance functions of living organisms must go on even when
they are not doing anything particular. Even when we are just sitting in
2024-25
Science
80
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
2024-25
Life Processes 81
design in multi-cellular organisms are followed, a specialised tissue for
excretion will be developed, which means that the transportation system
will need to transport waste away from cells to this excretory tissue.
Let us consider these various processes, so essential to maintain
life, one by one.
QUESTIONS
?
1. Why is diffusion insufficient to meet the oxygen requirements of multi-
cellular organisms like humans?
2. What criteria do we use to decide whether something is alive?
3. What are outside raw materials used for by an organism?
4. What processes would you consider essential for maintaining life?
5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION
When we walk or ride a bicycle, we are using up energy. Even when we
are not doing any apparent activity, energy is needed to maintain a
state of order in our body. We also need materials from outside in order
to grow, develop, synthesise protein and other substances needed in
the body. This source of energy and materials is the food we eat.
How do living things get their food?
The general requirement for energy and materials is common in all
organisms, but it is fulfilled in different ways. Some organisms use simple
food material obtained from inorganic sources in the form of carbon
dioxide and water. These organisms, the autotrophs, include green
plants and some bacteria. Other organisms utilise complex substances.
These complex substances have to be broken down into simpler ones
before they can be used for the upkeep and growth of the body. To
achieve this, organisms use bio-catalysts called enzymes. Thus, the
heterotrophs survival depends directly or indirectly on autotrophs.
Heterotrophic organisms include animals and fungi.
5.2.1 Autotrophic Nutrition
Carbon and energy requirements of the autotrophic organism are
fulfilled by photosynthesis. It is the process by which autotrophs take
in substances from the outside and convert them into stored forms of
energy. This material is taken in the form of carbon dioxide and water
which is converted into carbohydrates in the presence of sunlight and
chlorophyll. Carbohydrates are utilised for providing energy to the plant.
We will study how this takes place in the next section. The carbohydrates
which are not used immediately are stored in the form of starch, which
serves as the internal energy reserve to be used as and when required
by the plant. A somewhat similar situation is seen in us where some of
the energy derived from the food we eat is stored in our body in the form
of glycogen.
2024-25
Page 4


Life Processes
5 CHAPTER
H
ow do we tell the difference between what is alive and what is not
alive? If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.
Movements over very small scales will be invisible to the naked eye –
movements of molecules, for example. Is this invisible molecular
movement necessary for life? If we ask this question to professional
biologists, they will say yes. In fact, viruses do not show any molecular
movement in them (until they infect some cell), and that is partly why
there is a controversy about whether they are truly alive or not.
Why are molecular movements needed for life? We have seen in earlier
classes that living organisms are well-organised structures; they can
have tissues, tissues have cells, cells have smaller components in them,
and so on. Because of the effects of the environment, this organised,
ordered nature of living structures is very likely to keep breaking down
over time. If order breaks down, the organism will no longer be alive. So
living creatures must keep repairing and maintaining their structures.
Since all these structures are made up of molecules, they must move
molecules around all the time.
What are the maintenance processes in living organisms?
Let us explore.
5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHAT ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES?
The maintenance functions of living organisms must go on even when
they are not doing anything particular. Even when we are just sitting in
2024-25
Science
80
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
2024-25
Life Processes 81
design in multi-cellular organisms are followed, a specialised tissue for
excretion will be developed, which means that the transportation system
will need to transport waste away from cells to this excretory tissue.
Let us consider these various processes, so essential to maintain
life, one by one.
QUESTIONS
?
1. Why is diffusion insufficient to meet the oxygen requirements of multi-
cellular organisms like humans?
2. What criteria do we use to decide whether something is alive?
3. What are outside raw materials used for by an organism?
4. What processes would you consider essential for maintaining life?
5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION
When we walk or ride a bicycle, we are using up energy. Even when we
are not doing any apparent activity, energy is needed to maintain a
state of order in our body. We also need materials from outside in order
to grow, develop, synthesise protein and other substances needed in
the body. This source of energy and materials is the food we eat.
How do living things get their food?
The general requirement for energy and materials is common in all
organisms, but it is fulfilled in different ways. Some organisms use simple
food material obtained from inorganic sources in the form of carbon
dioxide and water. These organisms, the autotrophs, include green
plants and some bacteria. Other organisms utilise complex substances.
These complex substances have to be broken down into simpler ones
before they can be used for the upkeep and growth of the body. To
achieve this, organisms use bio-catalysts called enzymes. Thus, the
heterotrophs survival depends directly or indirectly on autotrophs.
Heterotrophic organisms include animals and fungi.
5.2.1 Autotrophic Nutrition
Carbon and energy requirements of the autotrophic organism are
fulfilled by photosynthesis. It is the process by which autotrophs take
in substances from the outside and convert them into stored forms of
energy. This material is taken in the form of carbon dioxide and water
which is converted into carbohydrates in the presence of sunlight and
chlorophyll. Carbohydrates are utilised for providing energy to the plant.
We will study how this takes place in the next section. The carbohydrates
which are not used immediately are stored in the form of starch, which
serves as the internal energy reserve to be used as and when required
by the plant. A somewhat similar situation is seen in us where some of
the energy derived from the food we eat is stored in our body in the form
of glycogen.
2024-25
Science
82
Let us now see what actually happens during the process of
photosynthesis. The following events occur during this process –
Figure 5.2 Figure 5.2 Figure 5.2 Figure 5.2 Figure 5.2
Variegated leaf (a) before
and (b) after starch test
Figure 5.1 Figure 5.1 Figure 5.1 Figure 5.1 Figure 5.1
Cross-section of a leaf
Activity 5.1 Activity 5.1 Activity 5.1 Activity 5.1 Activity 5.1
n Take a potted plant with variegated leaves – for example, money plant
or crotons.
n Keep the plant in a dark room for three days so that all the starch
gets used up.
n Now keep the plant in sunlight for about six hours.
n Pluck a leaf from the plant. Mark the green areas in it and trace them
on a sheet of paper.
n Dip the leaf in boiling water for a few minutes.
n After this, immerse it in a beaker containing alcohol.
n Carefully place the above beaker in a water-bath and heat till the
alcohol begins to boil.
n What happens to the colour of the leaf? What is the colour of the
solution?
n Now dip the leaf in a dilute solution of iodine for a few minutes.
n Take out the leaf and rinse off the iodine solution.
n Observe the colour of the leaf and compare this with the tracing of
the leaf done in the beginning (Fig. 5.2).
n What can you conclude about the presence of starch in various areas
of the leaf?
(i) Absorption of light energy by
chlorophyll.
(ii) Conversion of light energy to chemical
energy and splitting of water molecules
into hydrogen and oxygen.
(iii) Reduction of carbon dioxide to
carbohydrates.
These steps need not take place one after
the other immediately. For example, desert
plants take up carbon dioxide at night and
prepare an intermediate which is acted upon
by the energy absorbed by the chlorophyll
during the day.
Let us see how each of the components of
the above reaction are necessary for
photosynthesis.
If you carefully observe a cross-section of a
leaf under the microscope (shown in Fig. 5.1),
you will notice that some cells contain green
dots. These green dots are cell organelles called
chloroplasts which contain chlorophyll. Let us
do an activity which demonstrates that
chlorophyll is essential for photosynthesis.
2024-25
Page 5


Life Processes
5 CHAPTER
H
ow do we tell the difference between what is alive and what is not
alive? If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.
Movements over very small scales will be invisible to the naked eye –
movements of molecules, for example. Is this invisible molecular
movement necessary for life? If we ask this question to professional
biologists, they will say yes. In fact, viruses do not show any molecular
movement in them (until they infect some cell), and that is partly why
there is a controversy about whether they are truly alive or not.
Why are molecular movements needed for life? We have seen in earlier
classes that living organisms are well-organised structures; they can
have tissues, tissues have cells, cells have smaller components in them,
and so on. Because of the effects of the environment, this organised,
ordered nature of living structures is very likely to keep breaking down
over time. If order breaks down, the organism will no longer be alive. So
living creatures must keep repairing and maintaining their structures.
Since all these structures are made up of molecules, they must move
molecules around all the time.
What are the maintenance processes in living organisms?
Let us explore.
5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHA 5.1 WHAT ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES? T ARE LIFE PROCESSES?
The maintenance functions of living organisms must go on even when
they are not doing anything particular. Even when we are just sitting in
2024-25
Science
80
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
2024-25
Life Processes 81
design in multi-cellular organisms are followed, a specialised tissue for
excretion will be developed, which means that the transportation system
will need to transport waste away from cells to this excretory tissue.
Let us consider these various processes, so essential to maintain
life, one by one.
QUESTIONS
?
1. Why is diffusion insufficient to meet the oxygen requirements of multi-
cellular organisms like humans?
2. What criteria do we use to decide whether something is alive?
3. What are outside raw materials used for by an organism?
4. What processes would you consider essential for maintaining life?
5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION 5.2 NUTRITION
When we walk or ride a bicycle, we are using up energy. Even when we
are not doing any apparent activity, energy is needed to maintain a
state of order in our body. We also need materials from outside in order
to grow, develop, synthesise protein and other substances needed in
the body. This source of energy and materials is the food we eat.
How do living things get their food?
The general requirement for energy and materials is common in all
organisms, but it is fulfilled in different ways. Some organisms use simple
food material obtained from inorganic sources in the form of carbon
dioxide and water. These organisms, the autotrophs, include green
plants and some bacteria. Other organisms utilise complex substances.
These complex substances have to be broken down into simpler ones
before they can be used for the upkeep and growth of the body. To
achieve this, organisms use bio-catalysts called enzymes. Thus, the
heterotrophs survival depends directly or indirectly on autotrophs.
Heterotrophic organisms include animals and fungi.
5.2.1 Autotrophic Nutrition
Carbon and energy requirements of the autotrophic organism are
fulfilled by photosynthesis. It is the process by which autotrophs take
in substances from the outside and convert them into stored forms of
energy. This material is taken in the form of carbon dioxide and water
which is converted into carbohydrates in the presence of sunlight and
chlorophyll. Carbohydrates are utilised for providing energy to the plant.
We will study how this takes place in the next section. The carbohydrates
which are not used immediately are stored in the form of starch, which
serves as the internal energy reserve to be used as and when required
by the plant. A somewhat similar situation is seen in us where some of
the energy derived from the food we eat is stored in our body in the form
of glycogen.
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Let us now see what actually happens during the process of
photosynthesis. The following events occur during this process –
Figure 5.2 Figure 5.2 Figure 5.2 Figure 5.2 Figure 5.2
Variegated leaf (a) before
and (b) after starch test
Figure 5.1 Figure 5.1 Figure 5.1 Figure 5.1 Figure 5.1
Cross-section of a leaf
Activity 5.1 Activity 5.1 Activity 5.1 Activity 5.1 Activity 5.1
n Take a potted plant with variegated leaves – for example, money plant
or crotons.
n Keep the plant in a dark room for three days so that all the starch
gets used up.
n Now keep the plant in sunlight for about six hours.
n Pluck a leaf from the plant. Mark the green areas in it and trace them
on a sheet of paper.
n Dip the leaf in boiling water for a few minutes.
n After this, immerse it in a beaker containing alcohol.
n Carefully place the above beaker in a water-bath and heat till the
alcohol begins to boil.
n What happens to the colour of the leaf? What is the colour of the
solution?
n Now dip the leaf in a dilute solution of iodine for a few minutes.
n Take out the leaf and rinse off the iodine solution.
n Observe the colour of the leaf and compare this with the tracing of
the leaf done in the beginning (Fig. 5.2).
n What can you conclude about the presence of starch in various areas
of the leaf?
(i) Absorption of light energy by
chlorophyll.
(ii) Conversion of light energy to chemical
energy and splitting of water molecules
into hydrogen and oxygen.
(iii) Reduction of carbon dioxide to
carbohydrates.
These steps need not take place one after
the other immediately. For example, desert
plants take up carbon dioxide at night and
prepare an intermediate which is acted upon
by the energy absorbed by the chlorophyll
during the day.
Let us see how each of the components of
the above reaction are necessary for
photosynthesis.
If you carefully observe a cross-section of a
leaf under the microscope (shown in Fig. 5.1),
you will notice that some cells contain green
dots. These green dots are cell organelles called
chloroplasts which contain chlorophyll. Let us
do an activity which demonstrates that
chlorophyll is essential for photosynthesis.
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Now, let us study how the plant
obtains carbon dioxide. In Class IX,
we had talked about stomata (Fig. 5.3)
which are tiny pores present on the
surface of the leaves. Massive amounts
of gaseous exchange takes place in the
leaves through these pores for the
purpose of photosynthesis. But it is
important to note here that exchange
of gases occurs across the surface of
stems, roots and leaves as well. Since
large amounts of water can also be lost
through these stomata, the plant
closes these pores when it does not
need carbon dioxide for photosynthesis. The opening and closing of the
pore is a function of the guard cells. The guard cells swell when water
flows into them, causing the stomatal pore to open. Similarly the pore
closes if the guard cells shrink.
n Take two healthy potted plants
which are nearly the same size.
n Keep them in a dark room for
three days.
n Now place each plant on
separate glass plates. Place a
watch-glass containing potassium
hydroxide by the side of one of
the plants. The potassium
hydroxide is used to absorb
carbon dioxide.
n Cover both plants with separate
bell-jars as shown in Fig. 5.4.
n Use vaseline to seal the bottom
of the jars to the glass plates so
that the set-up is air-tight.
n Keep the plants in sunlight for
about two hours.
n Pluck a leaf from each plant and check for the presence of starch as in the above activity.
n Do both the leaves show the presence of the same amount of starch?
n What can you conclude from this activity?
Figure 5.3 Figure 5.3 Figure 5.3 Figure 5.3 Figure 5.3 (a) Open and (b) closed stomatal pore
Activity 5.2 Activity 5.2 Activity 5.2 Activity 5.2 Activity 5.2
Based on the two activities performed above, can we design an
experiment to demonstrate that sunlight is essential for photosynthesis?
So far, we have talked about how autotrophs meet their energy
requirements. But they also need other raw materials for building their
body. Water used in photosynthesis is taken up from the soil by the
roots in terrestrial plants. Other materials like nitrogen, phosphorus,
iron and magnesium are taken up from the soil. Nitrogen is an essential
element used in the synthesis of proteins and other compounds. This is
Figure 5.4 Experimental set-up (a) with potassium
hydroxide (b) without potassium hydroxide
(a) (b)
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