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226

BIOLOGY

C HAPTER 14
RESPIRATION IN PLANTS
14
...
2 Glycolysis
14
...
4 Aerobic
Respiration
14
...
6 Amphibolic
Pathway
14
...

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
...
Let us
try and understand how this happens
...
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
...
Hence, even
in green plants all other organs, tissues and cells that are non-green,
need food for oxidation
...
Animals are heterotrophic, i
...
, they obtain food from plants

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directly (herbivores) or indirectly (carnivores)
...
What is important to recognise
is that ultimately all the food that is respired for life processes comes from
photosynthesis
...

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 compounds that are oxidised
during this process are known as respiratory substrates
...
During oxidation within a cell, all the energy contained
in respiratory substrates is not released free into the cell, or in a single
step
...
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
...

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
...
1 DO PLANTS BREATHE?
Well, the answer to this question is not quite so direct
...
Hence, plants have
systems in place that ensure the availability of O2
...
There are several reasons why plants can
get along without respiratory organs
...
There is very little transport of gases from
one plant part to another
...
Roots, stems and leaves respire at rates far lower than
animals do
...
When cells photosynthesise, availability of O2 is not
a problem in these cells since O2 is released within the cell
...

Each living cell in a plant is located quite close to the surface of the plant
...
They also have openings called lenticels
...
Thus, most
cells of a plant have at least a part of their surface in contact with air
...

The complete combustion of glucose, which produces CO2 and H2O
as end products, yields energy most of which is given out as heat
...
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
...
How this is done is, essentially, the story of respiration
...
The combustion
reaction requires oxygen
...
Can you think of such situations (and organisms) where
O2 is not available? There are sufficient reasons to believe that the first
cells on this planet lived in an atmosphere that lacked oxygen
...
Some of these organisms are facultative
anaerobes, while in others the requirement for anaerobic condition is
obligate
...
This breakdown
of glucose to pyruvic acid is called glycolysis
...
2 GLYCOLYSIS
The term glycolysis has originated from the Greek words, glycos for sugar,
and lysis for splitting
...
Parnas, and is often referred to as the
EMP pathway
...

Glycolysis occurs in the cytoplasm of the cell and is present in all living
organisms
...
In plants, this glucose is derived from
sucrose, which is the end product of photosynthesis, or from storage

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carbohydrates
...
Glucose and fructose are
phosphorylated to give rise to glucose-6phosphate by the activity of the enzyme
hexokinase
...
Subsequent steps of metabolism of
glucose and fructose are same
...
1
...
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
...

The fructose 1, 6-bisphosphate is split
into dihydroxyacetone phosphate and
3-phosphoglyceraldehyde (PGAL)
...
Two
redox-equivalents are removed (in the form of
two hydrogen atoms) from PGAL and transferred
to a molecule of NAD+
...
The conversion of BPGA to
3-phosphoglyceric acid (PGA), is also an energy
yielding process; this energy is trapped by the
formation of ATP
...

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
...


Glucose
(6C)

ATP
ADP
Glucose-6-phosphate
(6C)
Fructose-6-phosphate
(6C)
ATP
ADP
Fructose1, 6-bisphosphate
(6C)
Triose phosphate
(glyceraldehyde-3-phosphate)
(3C)
NAD+

Triose phosphate
(Dihydroxy acetone
phosphate)
(3C)

NADH+H +
2 × Triose bisphosphate
(1,3 bisphosphoglyceric acid)
(3C)
ADP
ATP
2 × Triose phosphate
(3-phosphoglyceric acid)
(3C)

2 × 2-phosphoglycerate

H2O
2 × phosphoenolpyruvate
ADP
ATP

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2 × Pyruvic acid
(3C)

Figure 14
...
These are lactic acid fermentation, alcoholic
fermentation and aerobic respiration
...

For the complete oxidation of glucose to CO2 and H2O, however, organisms
adopt Krebs’ cycle which is also called as aerobic respiration
...


14
...
The enzymes, pyruvic acid
decarboxylase and alcohol dehydrogenase catalyse these reactions
...
The
steps involved are shown in Figure 14
...
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
...

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
...
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
...
2 Major pathways of anaerobic
organisms can carry out complete oxidation
respiration
of glucose and extract the energy stored to

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231

synthesise a larger number of ATP molecules needed for cellular
metabolism? In eukaryotes these steps take place within the mitochondria
and this requires O2
...

This type of respiration is most common in higher organisms
...


14
...
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 CO2
...

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
...
The reactions catalysed by pyruvic dehydrogenase require
the participation of several coenzymes, including NAD+ and Coenzyme A
...

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
...
4
...
3)
...

Citrate is then isomerised to isocitrate
...
3

GDP
GTP

The Citric acid cycle

and then succinyl-CoA
...
During
the conversion of succinyl-CoA to succinic
acid a molecule of GTP is synthesised
...
In a
coupled reaction GTP is converted to GDP with
the simultaneous synthesis of ATP from ADP
...
The
continued oxidation of acetyl CoA via the TCA
cycle requires the continued replenishment of
oxaloacetic acid, the first member of the cycle
...
The summary equation for this
phase of respiration may be written as follows:

Mitochondrial
Matrix → 3CO + 4 NADH + 4 H+
Pyruvic acid + 4NAD+ + FAD+ + 2H2 O + ADP + Pi 
 
2
+ FADH2 + ATP

We have till now seen that glucose has been broken down to release
CO2 and eight molecules of NADH + H+; two of FADH2 have been
synthesised besides just two molecules of ATP in TCA cycle
...
Also what is the role of the NADH + H+ and FADH2 that
is synthesised? Let us now understand the role of O2 in respiration and
how ATP is synthesised
...
4
...
This is accomplished when
they are oxidised through the electron transport system and the electrons
are passed on to O2 resulting in the formation of H2O
...
4) and it is
present in the inner mitochondrial membrane
...
Ubiquinone
also receives reducing equivalents via
FADH 2 (complex II) that is generated
during oxidation of succinate in the citric
acid cycle
...

Cytochrome c is a small protein attached
to the outer surface of the inner
membrane and acts as a mobile carrier
for transfer of electrons between complex
III and IV
...

When the electrons pass from one
carrier to another via complex I to IV in
the electron transport chain, they are
coupled to ATP synthase (complex V) for
the production of ATP from ADP and
inorganic phosphate
...
Oxidation of
one molecule of NADH gives rise to 3
molecules of ATP, while that of one
molecule of FADH2 produces 2 molecules
of ATP
...
4 Electron Transport System (ETS)
presence of oxygen, the role of oxygen is
limited to the terminal stage of the
process
...
Oxygen acts as the final
hydrogen acceptor
...
It is for this reason that the process is called
oxidative phosphorylation
...
As mentioned earlier, the energy released during the electron

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BIOLOGY

Figure 14
...
This
complex consists of two major components, F1
and F0 (Figure 14
...
The F1 headpiece is a
peripheral membrane protein complex and
contains the site for synthesis of ATP from ADP
and inorganic phosphate
...
The passage of protons through the
channel is coupled to the catalytic site of the F1
component for the production of ATP
...


14
...

These calculations can be made only on certain assumptions that:

•

There is a sequential, orderly pathway functioning, with one
substrate forming the next and with glycolysis, TCA cycle and ETS
pathway following one after another
...


•

None of the intermediates in the pathway are utilised to synthesise
any other compound
...


But this kind of assumptions are not really valid in a living system; all
pathways work simultaneously and do not take place one after another;
substrates enter the pathways and are withdrawn from it as and when
necessary; ATP is utilised as and when needed; enzymatic rates are
controlled by multiple means
...
Hence, there can be a net gain of 38 ATP molecules
during aerobic respiration of one molecule of glucose
...


•

In fermentation there is a net gain of only two molecules of ATP for
each molecule of glucose degraded to pyruvic acid whereas many
more molecules of ATP are generated under aerobic conditions
...


14
...
All carbohydrates are
usually first converted into glucose before they are used for respiration
...
See Figure
14
...
Fats would need to be broken down into glycerol and fatty acids
first
...
Glycerol would enter the pathway
after being converted to PGAL
...

Since respiration involves breakdown of substrates, the respiratory
process has traditionally been considered a catabolic process and the
respiratory pathway as a catabolic pathway
...
What is important to recognise is that it is these very
compounds that would be withdrawn from the respiratory pathway for the
synthesis of the said substrates
...
But when the organism needs to synthesise fatty acids, acetyl
CoA would be withdrawn from the respiratory pathway for it
...
Similarly, during breakdown and synthesis of
protein too, respiratory intermediates form the link
...
Because the respiratory pathway is involved in both anabolism
and catabolism, it would hence be better to consider the respiratory pathway
as an amphibolic pathway rather than as a catabolic one
...
6 Interrelationship among metabolic pathways showing respiration
mediated breakdown of different organic molecules to CO2 and H20

14
...
As you know, during
aerobic respiration, O2 is consumed and CO2 is released
...

RQ =

volume of CO2 evolved
volume of O2 consumed

The respiratory quotient depends upon the type of respiratory
substrate used during respiration
...
0
6O2

When fats are used in respiration, the RQ is less than 1
...
7
145O2

When proteins are respiratory substrates the ratio would be about
0
...

What is important to recognise is that in living organisms respiratory
substrates are often more than one; pure proteins or fats are never used
as respiratory substrates
...

Stomata and lenticels allow gaseous exchange by diffusion
...

The breaking of C-C bonds of complex organic molecules by oxidation cells
leading to the release of a lot of energy is called cellular respiration
...
Fats and proteins can also be broken down to
yield energy
...

Each glucose molecule is broken through a series of enzyme catalysed reactions
into two molecules of pyruvic acid
...
The fate of the
pyruvate depends on the availability of oxygen and the organism
...

Fermentation takes place under anaerobic conditions in many prokaryotes,
unicellular eukaryotes and in germinating seeds
...
Pyruvic acid is transported into the
mitochondria where it is converted into acetyl CoA with the release of CO2
...
NADH + H+ and FADH2 are generated in the Krebs’
cycle
...
This is accomplished through a

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system of electron carriers called electron transport system (ETS) located on the
inner membrane of the mitochondria
...
This is called
oxidative phosphorylation
...

The respiratory pathway is an amphibolic pathway as it involves both anabolism
and catabolism
...


EXERCISES
1
...
What are respiratory substrates? Name the most common respiratory substrate
...
Give the schematic representation of glycolysis?
4
...
Give the schematic representation of an overall view of Krebs’ cycle
...
Explain ETS
...
Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle
8
...
Discuss “The respiratory pathway is an amphibolic pathway
...
Define RQ
...
What is oxidative phosphorylation?
12

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