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

ESPIRATION IN

LANTS

HAPTER

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

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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

for respiration to occur and they also give out CO

. Hence, plants have

systems in place that ensure the availability of O

. 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

is not

a problem in these cells since O

is released within the cell. Third, the

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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

and H

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

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

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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

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

ATP

Figure 14.1 Steps of glycolysis

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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

and H

O, however, organisms

adopt Krebs’ cycle which is also called as aerobic respiration. This requires

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

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

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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 O

Aerobic respiration is the process that leads to a

complete oxidation of organic substances in the presence of oxygen, and

releases CO

, 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

• The passing on of the electrons removed as part of the hydrogen

atoms to molecular O

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

Pyruvate dehydrogenase

+ +  →   

 + + +

Acetyl CoA CO NADH H

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

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Figure 14.3 The Citric acid cycle

Pyruvate

(3C)

Acetyl coenzyme A

(2C)

Citric acid

(6C)

Oxaloacetic acid

(4C)

NAD

NADH+H

NAD

CITRIC ACID CYCLE

α-ketoglutaric acid

(5C)

NADH+H

GDP

GTP

Succinic acid

(4C)

Malic acid

(4C)

FADH

FAD

Pyruvic acid NAD FAD H O ADP Pi

Mitochondrial Matrix

+ + + + +  → 

+ +

4 2

   + +

3 4 4

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

. 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

respectively. The summary equation for this

phase of respiration may be written as follows:

+ +

FADH ATP

CoA

NAD

NADH+H

We have till now seen that glucose has been br

oken down to release

and eight molecules of NADH + H

; two of FADH

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

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

that

is synthesised?

Let us now understand the role of O

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

This is accomplished when

they are oxidised through the electron transport system and the electrons

are passed on to O

resulting in the formation of H

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

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233

produced in the mitochondrial matrix

during citric acid cycle are oxidised by an

NADH dehydrogenase (complex I), and

electrons are then transferred to

ubiquinone located

within the inner membrane. Ubiquinone

also receives reducing equivalents via

FADH

(complex II) that is generated

during oxidation of succinate in the citric

acid cycle. The reduced ubiquinone

(ubiquinol) is then oxidised with the

transfer of electrons to cytochrome c via

cytochrome bc

complex (complex III).

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. Complex IV refers to

cytochrome c oxidase complex containing

cytochromes a and a

, and two copper

centres.

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. The number of ATP

molecules synthesised depends on the

nature of the electron donor. Oxidation of

one molecule of NADH gives rise to 3

molecules of ATP, while that of one

molecule of FADH

produces 2 molecules

of ATP. Although the aerobic process of

respiration takes place only in the

presence of oxygen, the role of oxygen is

limited to the terminal stage of the

process. Yet, the presence of oxygen is vital, since it drives the whole

process by removing hydrogen from the system. Oxygen acts as the final

hydrogen acceptor. Unlike photophosphorylation wher

e it is the light

energy that is utilised for the production of proton gradient required for

phosphorylation, in respiration it is the energy of oxidation-reduction

utilised for the same process. It is for this reason that the process is called

oxidative phosphorylation.

You have already studied about the mechanism of membrane-linked

ATP synthesis as explained by chemiosmotic hypothesis in the earlier

chapter. As mentioned earlier, the energy released during the electron

Figure 14.4 Electron Transport System (ETS)

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234 BIOLOGY

transport system is utilised in synthesising ATP

with the help of ATP synthase (complex V). This

complex consists of two major components, F

and F

(Figure 14.5). The F

headpiece is a

peripheral membrane protein complex and

contains the site for synthesis of ATP from ADP

and inorganic phosphate. F

is an integral

membrane protein complex that forms the

channel through which protons cross the inner

membrane. The passage of protons through the

channel is coupled to the catalytic site of the F

component for the production of ATP. For each

ATP produced, 2H

passes through F

from the

intermembrane space to the matrix down the

electrochemical proton gradient.

14.5 THE RESPIRATORY BALANCE SHEET

It is possible to make calculations of the net gain of ATP for every glucose

molecule oxidised; but in reality this can remain only a theoretical exercise.

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.

• The NADH synthesised in glycolysis is transferred into the

mitochondria and undergoes oxidative phosphorylation.

• None of the intermediates in the pathway are utilised to synthesise

any other compound.

• Only glucose is being respired – no other alternative substrates are

entering in the pathway at any of the intermediary stages.

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. Yet, it is useful to do this exercise to

appreciate the beauty and efficiency of the living system in extraction

and storing energy. Hence, there can be a net gain of 38 ATP molecules

during aerobic respiration of one molecule of glucose.

Figure 14.5 Diagramatic presentation of ATP

synthesis in mitochondria

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Now let us compare fermentation and aerobic respiration:

• Fermentation accounts for only a partial breakdown of glucose

whereas in aerobic respiration it is completely degraded to CO

and

• 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.

• NADH is oxidised to NAD

rather slowly in fermentation, however

the reaction is very vigorous in case of aerobic respiration.

14.6 AMPHIBOLIC PATHWAY

Glucose is the favoured substrate for respiration. All carbohydrates are

usually first converted into glucose before they are used for respiration.

Other substrates can also be respired, as has been mentioned earlier, but

then they do not enter the respiratory pathway at the first step. See Figure

14.6 to see the points of entry of different substrates in the respiratory

pathway. Fats would need to be broken down into glycerol and fatty acids

first. If fatty acids were to be respired they would first be degraded to

acetyl CoA and enter the pathway. Glycerol would enter the pathway

after being converted to PGAL. The proteins would be degraded by

proteases and the individual amino acids (after deamination) depending

on their structure would enter the pathway at some stage within the Krebs’

cycle or even as pyruvate or acetyl CoA.

Since respiration involves breakdown of substrates, the respiratory

process has traditionally been considered a catabolic process and the

respiratory pathway as a catabolic pathway. But is this understanding

correct? We have discussed above,

at which points in the respiratory

pathway different substrates would enter if they were to be respired and

used to derive energy. 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. Hence, fatty acids would be broken down

to acetyl CoA before entering the respiratory pathway when it is used as a

substrate. But when the organism needs to synthesise fatty acids, acetyl

CoA would be withdrawn from the respiratory pathway for it. Hence, the

respiratory pathway comes into the picture both during breakdown and

synthesis of fatty acids. Similarly, during breakdown and synthesis of

protein too, respiratory intermediates form the link. Breaking down

processes within the living organism is catabolism, and synthesis is

anabolism. 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.

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14.7 RESPIRATORY QUOTIENT

Let us now look at another aspect of respiration. As you know, during

aerobic respiration, O

is consumed and CO

is released. The ratio of the

volume of CO

evolved to the volume of O

consumed in respiration is

called the respiratory quotient (RQ) or respiratory ratio.

volume of CO evolved

volume of O consumed

The respiratory quotient depends upon the type of respiratory

substrate used during respiration.

When carbohydrates are used as substrate and are completely

oxidised, the RQ will be 1, because equal amounts of CO

and O

are

evolved and consumed, respectively, as shown in the equation below :

Figure 14.6 Interrelationship among metabolic pathways showing respiration

mediated breakdown of different organic molecules to CO

and H

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C H O O CO H O Energy

6 12 6 2 2 2

6 6 6+  → + +

= =

1 0

When fats are used in respiration, the RQ is less than 1. Calculations

for a fatty acid, tripalmitin, if used as a substrate is shown:

2 145 102 98

51 98 6 2 2 2

( )C H O O CO H O energy+  → + +

Tripalmitin

= =

102

145

0 7

When proteins are respiratory substrates the ratio would be about

0.9.

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.

SUMMARY

Plants unlike animals have no special systems for breathing or gaseous exchange.

Stomata and lenticels allow gaseous exchange by diffusion. Almost all living cells

in a plant have their surfaces exposed to air.

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. Glucose is the

favoured substrate for respiration. Fats and proteins can also be broken down to

yield energy. The initial stage of cellular respiration takes place in the cytoplasm.

Each glucose molecule is broken through a series of enzyme catalysed reactions

into two molecules of pyruvic acid. This process is called glycolysis. The fate of the

pyruvate depends on the availability of oxygen and the organism. Under anaerobic

conditions either lactic acid fermentation or alcohol fermentation occurs.

Fermentation takes place under anaerobic conditions in many prokaryotes,

unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobic

respiration occurs in the presence of oxygen. Pyruvic acid is transported into the

mitochondria where it is converted into acetyl CoA with the release of CO

. Acetyl

CoA then enters the tricarboxylic acid pathway or Krebs’ cycle operating in the

matrix of the mitochondria. NADH + H

and FADH

are generated in the Krebs’

cycle. The energy in these molecules as well as that in the NADH

+ H

synthesised

during glycolysis are used to synthesise ATP. 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. The electrons, as they move through the

system, release enough energy that are trapped to synthesise ATP. This is called

oxidative phosphorylation. In this process O

is the ultimate acceptor of electrons

and it gets reduced to water.

The respiratory pathway is an amphibolic pathway as it involves both anabolism

and catabolism. The respiratory quotient depends upon the type of respiratory

substance used during respiration.

EXERCISES

1. Differentiate between

(a) Respiration and Combustion

(b) Glycolysis and Krebs’ cycle

2. What are respiratory substrates? Name the most common respiratory substrate.

3. Give the schematic representation of glycolysis?

4. What are the main steps in aerobic respiration? Where does it take place?

5. Give the schematic representation of an overall view of Krebs’ cycle.

6. Explain ETS.

7. Distinguish between the following:

(a) Aerobic respiration and Anaerobic respiration

(b) Glycolysis and Fermentation

8. What are the assumptions made during the calculation of net gain of ATP?

9. Discuss “The respiratory pathway is an amphibolic pathway.”

10. Define RQ. What is its value for fats?

11. What is oxidative phosphorylation?

12. What is the significance of step-wise release of energy in respiration?

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