226 BIOLOGY
C HAPTER 14
RESPIRATION IN PLANTS
14.1 Do Plants All of us breathe to live, but why is breathing so essential to life? What
Breathe? happens when we breathe? Also, do all living organisms, including plants
14.2 Glycolysis and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
14.3 Fermentation be it absorption, transport, movement, reproduction or even breathing.
14.4 Aerobic Where does all this energy come from? We know we eat food for energy –
Respiration 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
14.5 The Respiratory
get their energy from? And micro-organisms – for their energy
Balance Sheet
requirements, do they eat ‘food’?
14.6 Amphibolic You may wonder at the several questions raised above – they may
Pathway 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
14.7 Respiratory
try and understand how this happens.
Quotient
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
2020-21
, RESPIRATION IN PLANTS 227
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.
14.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct. Yes, plants require
O2 for respiration to occur and they also give out CO2. Hence, plants have
systems in place that ensure the availability of O2. 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 O2 is not
a problem in these cells since O2 is released within the cell. Third, the
2020-21
C HAPTER 14
RESPIRATION IN PLANTS
14.1 Do Plants All of us breathe to live, but why is breathing so essential to life? What
Breathe? happens when we breathe? Also, do all living organisms, including plants
14.2 Glycolysis and microbes, breathe? If so, how?
All living organisms need energy for carrying out daily life activities,
14.3 Fermentation be it absorption, transport, movement, reproduction or even breathing.
14.4 Aerobic Where does all this energy come from? We know we eat food for energy –
Respiration 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
14.5 The Respiratory
get their energy from? And micro-organisms – for their energy
Balance Sheet
requirements, do they eat ‘food’?
14.6 Amphibolic You may wonder at the several questions raised above – they may
Pathway 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
14.7 Respiratory
try and understand how this happens.
Quotient
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
2020-21
, RESPIRATION IN PLANTS 227
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.
14.1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct. Yes, plants require
O2 for respiration to occur and they also give out CO2. Hence, plants have
systems in place that ensure the availability of O2. 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 O2 is not
a problem in these cells since O2 is released within the cell. Third, the
2020-21
