226 BIOLOGY
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
R
ESPIRATION IN
P
LANTS
C
HAPTER
14
14.1 Do Plants
Breathe?
14.2 Glycolysis
14.3 Fermentation
14.4 Aerobic
Respiration
14.5 The Respiratory
Balance Sheet
14.6 Amphibolic
Pathway
14.7 Respiratory
Quotient
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
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
2020-21
228 BIOLOGY
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.
14.2 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
2020-21
RESPIRATION IN PLANTS
229
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 14.1. 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
molecules are directly synthesised in this
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 14.1 Steps of glycolysis
2020-21
230 BIOLOGY
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.
14.3 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 14.2. 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 the
maximum concentration of alcohol in
beverages that 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 14.2 Major pathways of anaerobic
respiration
2020-21
RESPIRATION IN PLANTS
231
synthesise a larger number of ATP molecules needed for cellular
metabolism? In eukaryotes these steps take place within the mitochondria
and this requires O
2
.
Aerobic respiration is the process that leads to a
complete oxidation of organic substances in the presence of oxygen, and
releases CO
2
, water and a large amount of energy present in the substrate.
This type of respiration is most common in higher organisms. We will
look at these processes in the next section.
14.4 AEROBIC RESPIRATION
For aerobic respiration to take place within the mitochondria, the final
product of glycolysis, pyruvate is transported from the cytoplasm into
the mitochondria. The crucial events in aerobic respiration are:
The complete oxidation of pyruvate by the stepwise removal of all
the hydrogen atoms, leaving three molecules of CO
2
.
The passing on of the electrons removed as part of the hydrogen
atoms to molecular O
2
with simultaneous synthesis of ATP.
What is interesting to note is that the first process takes place in the
matrix of the mitochondria while the second process is located on the
inner membrane of the mitochondria.
Pyruvate, which is formed by the glycolytic catabolism of carbohydrates
in the cytosol, after it enters mitochondrial matrix undergoes oxidative
decarboxylation by a complex set of reactions catalysed by pyruvic
dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require
the participation of several coenzymes, including NAD
+
and Coenzyme A.
Pyruvic acid CoA NAD
Mg
Pyruvate dehydrogenase
+ +
+
+2
+ + +
+
Acetyl CoA CO NADH H
2
During this process, two molecules of NADH are produced from the
metabolism of two molecules of pyruvic acid (produced from one glucose
molecule during glycolysis).
The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle,
more commonly called as Krebs’ cycle after the scientist Hans Krebs who
first elucidated it.
14.4.1 Tricarboxylic Acid Cycle
The TCA cycle starts with the condensation of acetyl group with oxaloacetic
acid (OAA) and water to yield citric acid (Figure 14.3). The reaction is
catalysed by the enzyme citrate synthase and a molecule of CoA is released.
Citrate is then isomerised to isocitrate. It is followed by two successive
steps of decarboxylation, leading to the formation of α-ketoglutaric acid
2020-21
232 BIOLOGY
Figure 14.3 The Citric acid cycle
Pyruvate
(3C)
Acetyl coenzyme A
(2C)
Citric acid
(6C)
Oxaloacetic acid
(4C)
CO
2
NAD
+
NADH+H
+
NADH+H
+
NAD
+
NAD
+
CO
2
CITRIC ACID CYCLE
α-ketoglutaric acid
(5C)
NADH+H
+
GDP
GTP
Succinic acid
(4C)
Malic acid
(4C)
FADH
2
FAD
+
Pyruvic acid NAD FAD H O ADP Pi
Mitochondrial Matrix
+ + + + +
+ +
4 2
2
+ +
+
3 4 4
2
CO NADH H
and then succinyl-CoA. In the remaining steps
of citric acid cycle, succinyl-CoA is oxidised
to OAA allowing the cycle to continue. During
the conversion of succinyl-CoA to succinic
acid a molecule of GTP is synthesised. This is
a substrate level phosphorylation. In a
coupled reaction GTP is converted to GDP with
the simultaneous synthesis of ATP from ADP.
Also there are three points in the cycle where
NAD
+
is reduced to NADH + H
+
and one point
where FAD
+
is reduced to FADH
2
. The
continued oxidation of acetyl CoA via the TCA
cycle requires the continued replenishment of
oxaloacetic acid, the first member of the cycle.
In addition it also requires regeneration of
NAD
+
and FAD
+
from NADH and FADH
2
respectively. The summary equation for this
phase of respiration may be written as follows:
+ +
2
FADH ATP
CoA
NAD
+
NADH+H
+
CO
2
We have till now seen that glucose has been br
oken down to release
CO
2
and eight molecules of NADH + H
+
; two of FADH
2
have been
synthesised besides just two molecules of ATP in TCA cycle. You may be
wondering why we have been discussing respiration at all – neither O
2
has come into the picture nor the promised large number of ATP has yet
been synthesised. Also what is the role of the NADH + H
+
and FADH
2
that
is synthesised?
Let us now understand the role of O
2
in respiration and
how ATP is synthesised.
14.4.2 Electron Transport System (ETS) and Oxidative
Phosphorylation
The following steps in the respiratory process are to release and utilise
the energy stored in NADH+H
+
and FADH
2.
This is accomplished when
they are oxidised through the electron transport system and the electrons
are passed on to O
2
resulting in the formation of H
2
O. The metabolic
pathway through which the electron passes from one carrier to another,
is called the electron transport system
(ETS) (Figure 14.4) and it is
present in the inner mitochondrial membrane. Electrons from NADH
2020-21