NCERT Textbook: Respiration in Plants | Biology Class 11 - NEET PDF Download

 Page 1


RESPIRATION IN PLANTS 153
All of us breathe to live, but why is breathing so essential to life? What
happens when we breathe? Also, do all living organisms, including plants
and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
be it absorption, transport, movement, reproduction or even breathing.
Where does all this energy come from? We know we eat food for energy –
but how is this energy taken from food?  How is this energy utilised? Do
all foods give the same amount of energy? Do plants ‘eat’? Where do plants
get their energy from? And micro-organisms – for their energy
requirements, do they eat ‘food’?
You may wonder at the several questions raised above – they may
seem to be very disconnected.  But in reality, the process of breathing is
very much connected to the process of release of energy from food.  Let us
try and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation of
some macromolecules that we call ‘food’. Only green plants and
cyanobacteria can prepare their own food; by the process of photosynthesis
they trap light energy and convert it into chemical energy that is stored in
the bonds of carbohydrates like glucose, sucrose and starch.  We must
remember that in green plants too, not all cells, tissues and organs
photosynthesise; only cells containing chloroplasts, that are most often
located in the superficial layers, carry out photosynthesis.  Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation. Hence, food has to be translocated to all non-
green parts. Animals are heterotrophic, i.e., they obtain food from plants
RESPIRATION IN PLANTS
CHAPTER  12
12.1 Do Plants
Breathe?
12.2 Glycolysis
12.3 Fermentation
12.4 Aerobic
Respiration
12.5 The
Respiratory
Balance Sheet
12.6 Amphibolic
Pathway
12.7 Respiratory
Quotient
2024-25
Page 2


RESPIRATION IN PLANTS 153
All of us breathe to live, but why is breathing so essential to life? What
happens when we breathe? Also, do all living organisms, including plants
and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
be it absorption, transport, movement, reproduction or even breathing.
Where does all this energy come from? We know we eat food for energy –
but how is this energy taken from food?  How is this energy utilised? Do
all foods give the same amount of energy? Do plants ‘eat’? Where do plants
get their energy from? And micro-organisms – for their energy
requirements, do they eat ‘food’?
You may wonder at the several questions raised above – they may
seem to be very disconnected.  But in reality, the process of breathing is
very much connected to the process of release of energy from food.  Let us
try and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation of
some macromolecules that we call ‘food’. Only green plants and
cyanobacteria can prepare their own food; by the process of photosynthesis
they trap light energy and convert it into chemical energy that is stored in
the bonds of carbohydrates like glucose, sucrose and starch.  We must
remember that in green plants too, not all cells, tissues and organs
photosynthesise; only cells containing chloroplasts, that are most often
located in the superficial layers, carry out photosynthesis.  Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation. Hence, food has to be translocated to all non-
green parts. Animals are heterotrophic, i.e., they obtain food from plants
RESPIRATION IN PLANTS
CHAPTER  12
12.1 Do Plants
Breathe?
12.2 Glycolysis
12.3 Fermentation
12.4 Aerobic
Respiration
12.5 The
Respiratory
Balance Sheet
12.6 Amphibolic
Pathway
12.7 Respiratory
Quotient
2024-25
154 BIOLOGY
directly (herbivores) or indirectly (carnivores). Saprophytes like fungi are
dependent on dead and decaying matter.  What is important to recognise
is that ultimately all the food that is respired for life processes comes from
photosynthesis. This chapter deals with cellular respiration or the
mechanism of breakdown of food materials within the cell to release
energy, and the trapping of this energy for synthesis of ATP.
Photosynthesis, of course, takes place within the chloroplasts (in the
eukaryotes), whereas the breakdown of complex molecules to yield energy
takes place in the cytoplasm and in the mitochondria  (also only in
eukaryotes).  The breaking of the C-C bonds of complex compounds
through oxidation within the cells, leading to release of considerable
amount of energy is called respiration.  The compounds that are oxidised
during this process are known as respiratory substrates. Usually
carbohydrates are oxidised to release energy, but proteins, fats and even
organic acids can be used as respiratory substances in some plants, under
certain conditions.  During oxidation within a cell, all the energy contained
in respiratory substrates is not released free into the cell, or in a single
step. It is released in a series of slow step-wise reactions controlled by
enzymes, and it is trapped as chemical energy in the form of ATP.  Hence,
it is important to understand that the energy released by oxidation in
respiration is not (or rather cannot be) used directly but is used to
synthesise ATP, which is broken down whenever (and wherever) energy
needs to be utilised. Hence, ATP acts as the energy currency of the cell.
This energy trapped in ATP is utilised in various energy-requiring
processes of the organisms, and the carbon skeleton produced during
respiration is used as precursors for biosynthesis of other molecules in
the cell.
12.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct.  Yes, plants require
O
2
 for respiration to occur and they also give out CO
2
.  Hence, plants have
systems in place that ensure the availability of O
2
.  Plants, unlike animals,
have no specialised organs for gaseous exchange but they have stomata
and lenticels for this purpose. There are several reasons why plants can
get along without respiratory organs. First, each plant part takes care of
its own gas-exchange needs.  There is very little transport of gases from
one plant part to another.  Second, plants do not present great demands
for gas exchange.  Roots, stems and leaves respire at rates far lower than
animals do. Only during photosynthesis are large volumes of gases
exchanged and, each leaf is well adapted to take care of its own needs
during these periods. When cells photosynthesise, availability of O
2
 is not
a problem in these cells since O
2
 is released within the cell.  Third, the
2024-25
Page 3


RESPIRATION IN PLANTS 153
All of us breathe to live, but why is breathing so essential to life? What
happens when we breathe? Also, do all living organisms, including plants
and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
be it absorption, transport, movement, reproduction or even breathing.
Where does all this energy come from? We know we eat food for energy –
but how is this energy taken from food?  How is this energy utilised? Do
all foods give the same amount of energy? Do plants ‘eat’? Where do plants
get their energy from? And micro-organisms – for their energy
requirements, do they eat ‘food’?
You may wonder at the several questions raised above – they may
seem to be very disconnected.  But in reality, the process of breathing is
very much connected to the process of release of energy from food.  Let us
try and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation of
some macromolecules that we call ‘food’. Only green plants and
cyanobacteria can prepare their own food; by the process of photosynthesis
they trap light energy and convert it into chemical energy that is stored in
the bonds of carbohydrates like glucose, sucrose and starch.  We must
remember that in green plants too, not all cells, tissues and organs
photosynthesise; only cells containing chloroplasts, that are most often
located in the superficial layers, carry out photosynthesis.  Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation. Hence, food has to be translocated to all non-
green parts. Animals are heterotrophic, i.e., they obtain food from plants
RESPIRATION IN PLANTS
CHAPTER  12
12.1 Do Plants
Breathe?
12.2 Glycolysis
12.3 Fermentation
12.4 Aerobic
Respiration
12.5 The
Respiratory
Balance Sheet
12.6 Amphibolic
Pathway
12.7 Respiratory
Quotient
2024-25
154 BIOLOGY
directly (herbivores) or indirectly (carnivores). Saprophytes like fungi are
dependent on dead and decaying matter.  What is important to recognise
is that ultimately all the food that is respired for life processes comes from
photosynthesis. This chapter deals with cellular respiration or the
mechanism of breakdown of food materials within the cell to release
energy, and the trapping of this energy for synthesis of ATP.
Photosynthesis, of course, takes place within the chloroplasts (in the
eukaryotes), whereas the breakdown of complex molecules to yield energy
takes place in the cytoplasm and in the mitochondria  (also only in
eukaryotes).  The breaking of the C-C bonds of complex compounds
through oxidation within the cells, leading to release of considerable
amount of energy is called respiration.  The compounds that are oxidised
during this process are known as respiratory substrates. Usually
carbohydrates are oxidised to release energy, but proteins, fats and even
organic acids can be used as respiratory substances in some plants, under
certain conditions.  During oxidation within a cell, all the energy contained
in respiratory substrates is not released free into the cell, or in a single
step. It is released in a series of slow step-wise reactions controlled by
enzymes, and it is trapped as chemical energy in the form of ATP.  Hence,
it is important to understand that the energy released by oxidation in
respiration is not (or rather cannot be) used directly but is used to
synthesise ATP, which is broken down whenever (and wherever) energy
needs to be utilised. Hence, ATP acts as the energy currency of the cell.
This energy trapped in ATP is utilised in various energy-requiring
processes of the organisms, and the carbon skeleton produced during
respiration is used as precursors for biosynthesis of other molecules in
the cell.
12.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct.  Yes, plants require
O
2
 for respiration to occur and they also give out CO
2
.  Hence, plants have
systems in place that ensure the availability of O
2
.  Plants, unlike animals,
have no specialised organs for gaseous exchange but they have stomata
and lenticels for this purpose. There are several reasons why plants can
get along without respiratory organs. First, each plant part takes care of
its own gas-exchange needs.  There is very little transport of gases from
one plant part to another.  Second, plants do not present great demands
for gas exchange.  Roots, stems and leaves respire at rates far lower than
animals do. Only during photosynthesis are large volumes of gases
exchanged and, each leaf is well adapted to take care of its own needs
during these periods. When cells photosynthesise, availability of O
2
 is not
a problem in these cells since O
2
 is released within the cell.  Third, the
2024-25
RESPIRATION IN PLANTS 155
distance that gases must diffuse even in large, bulky plants is not great.
Each living cell in a plant is located quite close to the surface of the plant.
‘This is true for leaves’, you may ask, ‘but what about thick, woody stems
and roots?’ In stems, the ‘living’ cells are organised in thin layers inside
and beneath the bark. They also have openings called lenticels. The cells
in the interior are dead and provide only mechanical support. Thus, most
cells of a plant have at least a part of their surface in contact with air. This
is also facilitated by the loose packing of parenchyma cells in leaves, stems
and roots, which provide an interconnected network of air spaces.
The complete combustion of glucose, which produces CO
2
 and H
2
O
as end products, yields energy most of which is given out as heat.
C H O O CO H O Energy
6 12 6 2 2 2
6 6 6 + ? ? ? + +
If this energy is to be useful to the cell, it should be able to utilise it to
synthesise other molecules that the cell requires.  The strategy that the
plant cell uses is to catabolise the glucose molecule in such a way that
not all the liberated energy goes out as heat.  The key is to oxidise glucose
not in one step but in several small steps enabling some steps to be just
large enough such that the energy released can be coupled to ATP
synthesis.  How this is done is, essentially, the story of respiration.
During the process of respiration, oxygen is utilised, and carbon
dioxide, water and energy are released as products. The combustion
reaction requires oxygen. But some cells live where oxygen may or may
not be available. Can you think of such situations (and organisms) where
O
2
 is not available?   There are sufficient reasons to believe that the first
cells on this planet lived in an atmosphere that lacked oxygen.  Even
among present-day living organisms, we know of several that are adapted
to anaerobic conditions.  Some of these organisms are facultative
anaerobes, while in others the requirement for anaerobic condition is
obligate.  In any case, all living organisms retain the enzymatic machinery
to partially oxidise glucose without the help of oxygen.  This breakdown
of glucose to pyruvic acid is called glycolysis.
12.2 GLYCOLYSIS
The term glycolysis has originated from the Greek words, glycos for sugar,
and lysis for splitting.  The scheme of glycolysis was given by Gustav
Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the
EMP pathway. In anaerobic organisms, it is the only process in respiration.
Glycolysis occurs in the cytoplasm of the cell and is present in all living
organisms.  In this process, glucose undergoes partial oxidation to form
two molecules of pyruvic acid. In plants, this glucose is derived from
sucrose, which is the end product of photosynthesis, or from storage
2024-25
Page 4


RESPIRATION IN PLANTS 153
All of us breathe to live, but why is breathing so essential to life? What
happens when we breathe? Also, do all living organisms, including plants
and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
be it absorption, transport, movement, reproduction or even breathing.
Where does all this energy come from? We know we eat food for energy –
but how is this energy taken from food?  How is this energy utilised? Do
all foods give the same amount of energy? Do plants ‘eat’? Where do plants
get their energy from? And micro-organisms – for their energy
requirements, do they eat ‘food’?
You may wonder at the several questions raised above – they may
seem to be very disconnected.  But in reality, the process of breathing is
very much connected to the process of release of energy from food.  Let us
try and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation of
some macromolecules that we call ‘food’. Only green plants and
cyanobacteria can prepare their own food; by the process of photosynthesis
they trap light energy and convert it into chemical energy that is stored in
the bonds of carbohydrates like glucose, sucrose and starch.  We must
remember that in green plants too, not all cells, tissues and organs
photosynthesise; only cells containing chloroplasts, that are most often
located in the superficial layers, carry out photosynthesis.  Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation. Hence, food has to be translocated to all non-
green parts. Animals are heterotrophic, i.e., they obtain food from plants
RESPIRATION IN PLANTS
CHAPTER  12
12.1 Do Plants
Breathe?
12.2 Glycolysis
12.3 Fermentation
12.4 Aerobic
Respiration
12.5 The
Respiratory
Balance Sheet
12.6 Amphibolic
Pathway
12.7 Respiratory
Quotient
2024-25
154 BIOLOGY
directly (herbivores) or indirectly (carnivores). Saprophytes like fungi are
dependent on dead and decaying matter.  What is important to recognise
is that ultimately all the food that is respired for life processes comes from
photosynthesis. This chapter deals with cellular respiration or the
mechanism of breakdown of food materials within the cell to release
energy, and the trapping of this energy for synthesis of ATP.
Photosynthesis, of course, takes place within the chloroplasts (in the
eukaryotes), whereas the breakdown of complex molecules to yield energy
takes place in the cytoplasm and in the mitochondria  (also only in
eukaryotes).  The breaking of the C-C bonds of complex compounds
through oxidation within the cells, leading to release of considerable
amount of energy is called respiration.  The compounds that are oxidised
during this process are known as respiratory substrates. Usually
carbohydrates are oxidised to release energy, but proteins, fats and even
organic acids can be used as respiratory substances in some plants, under
certain conditions.  During oxidation within a cell, all the energy contained
in respiratory substrates is not released free into the cell, or in a single
step. It is released in a series of slow step-wise reactions controlled by
enzymes, and it is trapped as chemical energy in the form of ATP.  Hence,
it is important to understand that the energy released by oxidation in
respiration is not (or rather cannot be) used directly but is used to
synthesise ATP, which is broken down whenever (and wherever) energy
needs to be utilised. Hence, ATP acts as the energy currency of the cell.
This energy trapped in ATP is utilised in various energy-requiring
processes of the organisms, and the carbon skeleton produced during
respiration is used as precursors for biosynthesis of other molecules in
the cell.
12.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct.  Yes, plants require
O
2
 for respiration to occur and they also give out CO
2
.  Hence, plants have
systems in place that ensure the availability of O
2
.  Plants, unlike animals,
have no specialised organs for gaseous exchange but they have stomata
and lenticels for this purpose. There are several reasons why plants can
get along without respiratory organs. First, each plant part takes care of
its own gas-exchange needs.  There is very little transport of gases from
one plant part to another.  Second, plants do not present great demands
for gas exchange.  Roots, stems and leaves respire at rates far lower than
animals do. Only during photosynthesis are large volumes of gases
exchanged and, each leaf is well adapted to take care of its own needs
during these periods. When cells photosynthesise, availability of O
2
 is not
a problem in these cells since O
2
 is released within the cell.  Third, the
2024-25
RESPIRATION IN PLANTS 155
distance that gases must diffuse even in large, bulky plants is not great.
Each living cell in a plant is located quite close to the surface of the plant.
‘This is true for leaves’, you may ask, ‘but what about thick, woody stems
and roots?’ In stems, the ‘living’ cells are organised in thin layers inside
and beneath the bark. They also have openings called lenticels. The cells
in the interior are dead and provide only mechanical support. Thus, most
cells of a plant have at least a part of their surface in contact with air. This
is also facilitated by the loose packing of parenchyma cells in leaves, stems
and roots, which provide an interconnected network of air spaces.
The complete combustion of glucose, which produces CO
2
 and H
2
O
as end products, yields energy most of which is given out as heat.
C H O O CO H O Energy
6 12 6 2 2 2
6 6 6 + ? ? ? + +
If this energy is to be useful to the cell, it should be able to utilise it to
synthesise other molecules that the cell requires.  The strategy that the
plant cell uses is to catabolise the glucose molecule in such a way that
not all the liberated energy goes out as heat.  The key is to oxidise glucose
not in one step but in several small steps enabling some steps to be just
large enough such that the energy released can be coupled to ATP
synthesis.  How this is done is, essentially, the story of respiration.
During the process of respiration, oxygen is utilised, and carbon
dioxide, water and energy are released as products. The combustion
reaction requires oxygen. But some cells live where oxygen may or may
not be available. Can you think of such situations (and organisms) where
O
2
 is not available?   There are sufficient reasons to believe that the first
cells on this planet lived in an atmosphere that lacked oxygen.  Even
among present-day living organisms, we know of several that are adapted
to anaerobic conditions.  Some of these organisms are facultative
anaerobes, while in others the requirement for anaerobic condition is
obligate.  In any case, all living organisms retain the enzymatic machinery
to partially oxidise glucose without the help of oxygen.  This breakdown
of glucose to pyruvic acid is called glycolysis.
12.2 GLYCOLYSIS
The term glycolysis has originated from the Greek words, glycos for sugar,
and lysis for splitting.  The scheme of glycolysis was given by Gustav
Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the
EMP pathway. In anaerobic organisms, it is the only process in respiration.
Glycolysis occurs in the cytoplasm of the cell and is present in all living
organisms.  In this process, glucose undergoes partial oxidation to form
two molecules of pyruvic acid. In plants, this glucose is derived from
sucrose, which is the end product of photosynthesis, or from storage
2024-25
156 BIOLOGY
carbohydrates. Sucrose is converted into glucose
and fructose by the enzyme, invertase, and these
two monosaccharides readily enter the glycolytic
pathway. Glucose and fructose are
phosphorylated to give rise to glucose-6-
phosphate by the activity of the enzyme
hexokinase. This phosphorylated form of
glucose then isomerises to produce fructose-6-
phosphate. Subsequent steps of metabolism of
glucose and fructose are same.  The various
steps of glycolysis are depicted in Figure 12.1.
In glycolysis, a chain of ten reactions, under the
control of different enzymes, takes place to
produce pyruvate from glucose. While studying
the steps of glycolysis, please note the steps at
which utilisation or synthesis of ATP or (in this
case) NADH + H
+ 
take place.
ATP is utilised at two steps: first in the
conversion of glucose into glucose 6-phosphate
and second in the conversion of fructose
6-phosphate to fructose 1, 6-bisphosphate.
The fructose 1, 6-bisphosphate is split
into dihydroxyacetone phosphate and
3-phosphoglyceraldehyde (PGAL). We find
that there is one step where NADH + H
+ 
is
formed from NAD
+
; this is when
3-phosphoglyceraldehyde (PGAL) is converted
to 1, 3-bisphosphoglycerate (BPGA).  Two
redox-equivalents are removed (in the form of
two hydrogen atoms) from PGAL and transferred
to a molecule of NAD
+
. PGAL is oxidised and
with inorganic phosphate to get converted into
BPGA. The conversion of BPGA to
3-phosphoglyceric acid (PGA), is also an energy
yielding process; this energy is trapped by the
formation of ATP.  Another ATP is synthesised
during the conversion of PEP to pyruvic acid.
Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP
molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this
pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule?
Pyruvic acid is then the key product of
glycolysis. What is the metabolic fate of
pyruvate?  This depends on the cellular need.
Glucose
(6C)
Glucose-6-phosphate
(6C)
Fructose-6-phosphate
(6C)
Fructose1, 6-bisphosphate
(6C)
Triose phosphate
(glyceraldehyde-3-phosphate)
(3C)
Triose phosphate
(Dihydroxy acetone
phosphate)
(3C)
2 × Triose bisphosphate
(1,3 bisphosphoglyceric acid)
(3C)
2 × Triose phosphate
(3-phosphoglyceric acid)
(3C)
2 × 2-phosphoglycerate
2 × phosphoenolpyruvate
2 × Pyruvic acid
(3C)
ADP
ATP
ADP
ATP
ADP
ATP
ADP
NADH+H
+
NAD
+
H
2
O
ATP
Figure 12.1   Steps of glycolysis
2024-25
Page 5


RESPIRATION IN PLANTS 153
All of us breathe to live, but why is breathing so essential to life? What
happens when we breathe? Also, do all living organisms, including plants
and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
be it absorption, transport, movement, reproduction or even breathing.
Where does all this energy come from? We know we eat food for energy –
but how is this energy taken from food?  How is this energy utilised? Do
all foods give the same amount of energy? Do plants ‘eat’? Where do plants
get their energy from? And micro-organisms – for their energy
requirements, do they eat ‘food’?
You may wonder at the several questions raised above – they may
seem to be very disconnected.  But in reality, the process of breathing is
very much connected to the process of release of energy from food.  Let us
try and understand how this happens.
All the energy required for ‘life’ processes is obtained by oxidation of
some macromolecules that we call ‘food’. Only green plants and
cyanobacteria can prepare their own food; by the process of photosynthesis
they trap light energy and convert it into chemical energy that is stored in
the bonds of carbohydrates like glucose, sucrose and starch.  We must
remember that in green plants too, not all cells, tissues and organs
photosynthesise; only cells containing chloroplasts, that are most often
located in the superficial layers, carry out photosynthesis.  Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation. Hence, food has to be translocated to all non-
green parts. Animals are heterotrophic, i.e., they obtain food from plants
RESPIRATION IN PLANTS
CHAPTER  12
12.1 Do Plants
Breathe?
12.2 Glycolysis
12.3 Fermentation
12.4 Aerobic
Respiration
12.5 The
Respiratory
Balance Sheet
12.6 Amphibolic
Pathway
12.7 Respiratory
Quotient
2024-25
154 BIOLOGY
directly (herbivores) or indirectly (carnivores). Saprophytes like fungi are
dependent on dead and decaying matter.  What is important to recognise
is that ultimately all the food that is respired for life processes comes from
photosynthesis. This chapter deals with cellular respiration or the
mechanism of breakdown of food materials within the cell to release
energy, and the trapping of this energy for synthesis of ATP.
Photosynthesis, of course, takes place within the chloroplasts (in the
eukaryotes), whereas the breakdown of complex molecules to yield energy
takes place in the cytoplasm and in the mitochondria  (also only in
eukaryotes).  The breaking of the C-C bonds of complex compounds
through oxidation within the cells, leading to release of considerable
amount of energy is called respiration.  The compounds that are oxidised
during this process are known as respiratory substrates. Usually
carbohydrates are oxidised to release energy, but proteins, fats and even
organic acids can be used as respiratory substances in some plants, under
certain conditions.  During oxidation within a cell, all the energy contained
in respiratory substrates is not released free into the cell, or in a single
step. It is released in a series of slow step-wise reactions controlled by
enzymes, and it is trapped as chemical energy in the form of ATP.  Hence,
it is important to understand that the energy released by oxidation in
respiration is not (or rather cannot be) used directly but is used to
synthesise ATP, which is broken down whenever (and wherever) energy
needs to be utilised. Hence, ATP acts as the energy currency of the cell.
This energy trapped in ATP is utilised in various energy-requiring
processes of the organisms, and the carbon skeleton produced during
respiration is used as precursors for biosynthesis of other molecules in
the cell.
12.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct.  Yes, plants require
O
2
 for respiration to occur and they also give out CO
2
.  Hence, plants have
systems in place that ensure the availability of O
2
.  Plants, unlike animals,
have no specialised organs for gaseous exchange but they have stomata
and lenticels for this purpose. There are several reasons why plants can
get along without respiratory organs. First, each plant part takes care of
its own gas-exchange needs.  There is very little transport of gases from
one plant part to another.  Second, plants do not present great demands
for gas exchange.  Roots, stems and leaves respire at rates far lower than
animals do. Only during photosynthesis are large volumes of gases
exchanged and, each leaf is well adapted to take care of its own needs
during these periods. When cells photosynthesise, availability of O
2
 is not
a problem in these cells since O
2
 is released within the cell.  Third, the
2024-25
RESPIRATION IN PLANTS 155
distance that gases must diffuse even in large, bulky plants is not great.
Each living cell in a plant is located quite close to the surface of the plant.
‘This is true for leaves’, you may ask, ‘but what about thick, woody stems
and roots?’ In stems, the ‘living’ cells are organised in thin layers inside
and beneath the bark. They also have openings called lenticels. The cells
in the interior are dead and provide only mechanical support. Thus, most
cells of a plant have at least a part of their surface in contact with air. This
is also facilitated by the loose packing of parenchyma cells in leaves, stems
and roots, which provide an interconnected network of air spaces.
The complete combustion of glucose, which produces CO
2
 and H
2
O
as end products, yields energy most of which is given out as heat.
C H O O CO H O Energy
6 12 6 2 2 2
6 6 6 + ? ? ? + +
If this energy is to be useful to the cell, it should be able to utilise it to
synthesise other molecules that the cell requires.  The strategy that the
plant cell uses is to catabolise the glucose molecule in such a way that
not all the liberated energy goes out as heat.  The key is to oxidise glucose
not in one step but in several small steps enabling some steps to be just
large enough such that the energy released can be coupled to ATP
synthesis.  How this is done is, essentially, the story of respiration.
During the process of respiration, oxygen is utilised, and carbon
dioxide, water and energy are released as products. The combustion
reaction requires oxygen. But some cells live where oxygen may or may
not be available. Can you think of such situations (and organisms) where
O
2
 is not available?   There are sufficient reasons to believe that the first
cells on this planet lived in an atmosphere that lacked oxygen.  Even
among present-day living organisms, we know of several that are adapted
to anaerobic conditions.  Some of these organisms are facultative
anaerobes, while in others the requirement for anaerobic condition is
obligate.  In any case, all living organisms retain the enzymatic machinery
to partially oxidise glucose without the help of oxygen.  This breakdown
of glucose to pyruvic acid is called glycolysis.
12.2 GLYCOLYSIS
The term glycolysis has originated from the Greek words, glycos for sugar,
and lysis for splitting.  The scheme of glycolysis was given by Gustav
Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the
EMP pathway. In anaerobic organisms, it is the only process in respiration.
Glycolysis occurs in the cytoplasm of the cell and is present in all living
organisms.  In this process, glucose undergoes partial oxidation to form
two molecules of pyruvic acid. In plants, this glucose is derived from
sucrose, which is the end product of photosynthesis, or from storage
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156 BIOLOGY
carbohydrates. Sucrose is converted into glucose
and fructose by the enzyme, invertase, and these
two monosaccharides readily enter the glycolytic
pathway. Glucose and fructose are
phosphorylated to give rise to glucose-6-
phosphate by the activity of the enzyme
hexokinase. This phosphorylated form of
glucose then isomerises to produce fructose-6-
phosphate. Subsequent steps of metabolism of
glucose and fructose are same.  The various
steps of glycolysis are depicted in Figure 12.1.
In glycolysis, a chain of ten reactions, under the
control of different enzymes, takes place to
produce pyruvate from glucose. While studying
the steps of glycolysis, please note the steps at
which utilisation or synthesis of ATP or (in this
case) NADH + H
+ 
take place.
ATP is utilised at two steps: first in the
conversion of glucose into glucose 6-phosphate
and second in the conversion of fructose
6-phosphate to fructose 1, 6-bisphosphate.
The fructose 1, 6-bisphosphate is split
into dihydroxyacetone phosphate and
3-phosphoglyceraldehyde (PGAL). We find
that there is one step where NADH + H
+ 
is
formed from NAD
+
; this is when
3-phosphoglyceraldehyde (PGAL) is converted
to 1, 3-bisphosphoglycerate (BPGA).  Two
redox-equivalents are removed (in the form of
two hydrogen atoms) from PGAL and transferred
to a molecule of NAD
+
. PGAL is oxidised and
with inorganic phosphate to get converted into
BPGA. The conversion of BPGA to
3-phosphoglyceric acid (PGA), is also an energy
yielding process; this energy is trapped by the
formation of ATP.  Another ATP is synthesised
during the conversion of PEP to pyruvic acid.
Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP Can you then calculate how many ATP
molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this molecules are directly synthesised in this
pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule? pathway from one glucose molecule?
Pyruvic acid is then the key product of
glycolysis. What is the metabolic fate of
pyruvate?  This depends on the cellular need.
Glucose
(6C)
Glucose-6-phosphate
(6C)
Fructose-6-phosphate
(6C)
Fructose1, 6-bisphosphate
(6C)
Triose phosphate
(glyceraldehyde-3-phosphate)
(3C)
Triose phosphate
(Dihydroxy acetone
phosphate)
(3C)
2 × Triose bisphosphate
(1,3 bisphosphoglyceric acid)
(3C)
2 × Triose phosphate
(3-phosphoglyceric acid)
(3C)
2 × 2-phosphoglycerate
2 × phosphoenolpyruvate
2 × Pyruvic acid
(3C)
ADP
ATP
ADP
ATP
ADP
ATP
ADP
NADH+H
+
NAD
+
H
2
O
ATP
Figure 12.1   Steps of glycolysis
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RESPIRATION IN PLANTS 157
There are three major ways in which different cells handle pyruvic acid
produced by glycolysis.  These are lactic acid fermentation, alcoholic
fermentation and aerobic respiration. Fermentation takes place under
anaerobic conditions in many prokaryotes and unicellular eukaryotes.
For the complete oxidation of glucose to CO
2
 and H
2
O, however, organisms
adopt Krebs’ cycle which is also called as aerobic respiration. This requires
O
2
 supply.
12.3 FERMENTATION
In fermentation, say by yeast, the incomplete oxidation of glucose is
achieved under anaerobic conditions by sets of reactions where pyruvic
acid is converted to CO
2
 and ethanol. The enzymes, pyruvic acid
decarboxylase and alcohol dehydrogenase catalyse these reactions. Other
organisms like some bacteria produce lactic acid from pyruvic acid.  The
steps involved are shown in Figure 12.2. In animal cells also, like muscles
during exercise, when oxygen is inadequate for cellular respiration pyruvic
acid is reduced to lactic acid by lactate dehydrogenase.  The reducing
agent is NADH+H
+ 
 which is reoxidised to NAD
+
in both the processes.
In both lactic acid and alcohol
fermentation not much energy is released; less
than seven per cent of the energy in glucose is
released and not all of it is trapped as high
energy bonds of ATP.  Also, the processes are
hazardous – either acid or alcohol is produced.
What is the net ATPs that is synthesised
(calculate how many ATP are synthesised and
deduct the number of ATP utilised during
glycolysis) when one molecule of glucose is
fermented to alcohol or lactic acid?  Yeasts
poison themselves to death when the
concentration of alcohol reaches about 13 per
cent. What then would be What then would be What then would be What then would be What then would be the the the the the maximum maximum maximum maximum maximum
concentration of alcohol in beverages that concentration of alcohol in beverages that concentration of alcohol in beverages that concentration of alcohol in beverages that concentration of alcohol in beverages that
are naturally fermented? are naturally fermented? are naturally fermented? are naturally fermented? are naturally fermented? How do you think
alcoholic beverages of alcohol content greater
than this concentration are obtained?
What then is the process by which
organisms can carry out complete oxidation of
glucose and extract the energy stored to
Figure 12.2 Major pathways of anaerobic
respiration
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