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CHAPTER 14 : RESPIRATION IN PLANTS
RESPIRATION IN PLANTS
Chapter Outline:
• Prerequisites
• Learning Objectives
• Do Plants Breathe?
• Glycolysis
• Fermentation
• Aerobic Respiration
• The Respiratory Balance Sheet
• Amphibolic Pathway
• Respiratory Quotient
• Summary
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RESPIRATION IN PLANTS
Prerequisites
The term respiration was first used by animal physiologists to describe breathing movements of
animals, but was subsequently extended to include the chemical reactions
by which complex organic
-ve
substances like carbohydrates, fats and proteins are broken down to release CO2, water and energy.
In plants the problem of definition is slightly different because
1. Breathing movements are not performed. The gaseous exchange is often marked by photosynthesis in the day time. Oxygen need not be utilised.
2. CO2 may not be released in some cases. For these reasons plant physiologists use the term
‘respiration’ for the process of oxidation of foods in living cells.
3. All living organisms need energy for carrying out daily life activities like movement, transport,
absorption, reproduction and even breathing. The process of breathing is connected to the release
of energy from food.
4. All the energy required for life processes is obtained by oxidation of (some macromolecules) food.
The mechanism of break down of food materials with in the cell to release energy and trapping
this energy for synthesis of ATP is called cellular respiration.
5. The breaking of c-c bonds of complex compounds through oxidation within the cells leading to
release of energy is called respiration. The substances that are oxidised during the process are
known as respiratory substrates.
6. The respiratory substrate can be carbohydrate, protein or fat.During oxidation within the cell, all
the energy in respiratory substrate is not released into the cell in a single step.
7. It is released in a series of step-wise reactions controlled by enzymes and is trapped as chemical
energy in the form of ATP. Hence ATP acts as energy currency of the cell.
8. This energy trapped in ATP is utilised in various energy requiring processes of organisms. And the
carbon skeleton produced during respiration is used as precursors for the biosynthesis of other
molecules in the cell.
9. Respiration is an oxidative process like combustion, but differs from it in releasing the energy.
Learning Objectives
1. To identify the difference between breathing respiration and combustion.
2. To understand the concept of respiration.
3. To gain knowledge about the mechanism of respiration.
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RESPIRATION IN PLANTS
Do Plants Breathe
Plants require O2 for respiration and give out CO2. Plants have no specialised organs for gaseous
exchange,but they have stomata and lenticels.
CO2
Hibiscuss
Stomata
Lenticels
Plants can get along without respiratory organs because,
1. Each plant takes care of its own gas exchange needs there is very little transport of gases from
one plant part to another.
O2
O2
O2
O2
Plant
O2
O2
Plant Tissue
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RESPIRATION IN PLANTS
2. Plants do not present great demands for exchange roots, stems and leaves respire at rates
lower than animals. Only during photosynthesis large volumes of gases exchanged and each leaf is
adapted to take care of its own needs.
CO2
Leaves
O2
Stem
H 2O
Root
Plant Parts
Photosynthesis
3. The distance that gases must diffuse even in large, bulky plants is not great. The complete
combustion of glucose, which produces CO2 and H2O as end products yields energy most of which is
given out as heat.
This energy is useful to utilise it to synthesis other molecules that the cell requires. Glucose is oxidsed
such that the energy released can be coupled to ATP synthesis.
Oxygen(O2)
Carbon dioxide(CO2)
Oxygen
Water (H2O)
Glucose(C6H12O6)
Glucose
Reaction (2)
Reaction (1)
C6H12O6 + 6O2 6CO2 + 6H2O + Energy
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.
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RESPIRATION IN PLANTS
Kinds of Respiration
Cellular respiration may be divided into 2 categories depending upon the availability of atmospheric
oxygen. They are aerobic and anaerobic respiration.
Cellular Respiration
Aerobic Respiration
Anaerobic Respiration
Glycolysis
ATP
Glucose
(6C)
ADP
Glucose - 6 - Phosphate
(6C)
Fructose - 6 - phosphate
(6C)
ATP
2 x pyruvic acid
(3C)
ATP
ADP
ADP
Fructose1. - 6 - bisphosphate
(6C)
2 x Phosphoenolpyruvate
H 2O
2 x 2 Phosphoglycerate
Triose phosphate
(glyceraldehyde - 3 -phosphate)
(3C)
Triose phosphate
(Dihydroxy acetone phosphate)
(3C)
NAD+
NADH+H+
2 x Triose bisphosphate
(1.3 bisphosphoglyceric acid)
(3C)
2 x Triose phosphate
(3- Phosphoglyceric acid)
(3C)
ATP
ADP
Glycolysis
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RESPIRATION IN PLANTS
1. Aerobic Respiration
The reaction is an exothermic reaction releasing energy. The aerobic respiration is represented by the
following equation.
Respiration takes place in the presence of oxygen. With the help of enzymes and oxygen respiratory
substrates are completely oxidised to CO2 and water.
Carbon
Water (H2O)
dioxide(CO2)
Oxygen(O2)
Glucose(C6H12O6)
Reaction of Aerobic respiration
C6H12O6 + 6O2 6CO2+ 6H2O + Energy
2. Anaerobic respiration
Respiration which takes place in the absence of oxygen is termed as anaerobic respiration. The
respiratory substrates are incompletely oxidised to ethyl alcohol and CO2.
A small amount of energy is released. Anaerobic respiration is represented by the following equation.
Absence of oxygen
Ethyl Alcohol
CO2
C6H12O6 2C2H5OH + 2CO2+ 56 k.cal
Example: yeast, bacteria.
Yeast
Bacteria
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Anaerobic Respiration
Certain anaerobic bacteria (Bacillus botulinus) cannot tolerate oxygen. If they are exposed to aerobic
conditions, they will die. Such anaerobic organisms are called obligate anaerobes.
Certain organisms (yeasts) continue to carry out anaerobic respirations even under aerobic
conditions.Such organisms are called facultative anaerobes.
Facultative Anarobes
Obligate anaerobes
Bacteria (Bacillus botulinus)
Yeast
The anaerobic organisms lack complete system (Kreb’s cycle, ETS) of biological oxidation. Hence
small amount of energy in the form of ATP is released out.
Kinds of Respiration
Differences between aerobic and anaerobic respiration
Aerobic Respiration
Anaerobic Respiration
1.It takes place in the presence of oxygen.
1.It takes place in the absence of oxygen.
2.Carbohydrates are completely oxidised
and broken down to CO2 and H2O.
3. Large amount of energy is released.
4. Aerobic respiration is carried out by all
organisms.
2.Carbohydrates are oxidised partially with
no or little release of CO2.
3. A small amount of energy is released.
4. Anaerobic respiration is carried out by
certain micro organisms and storage organs.
5. ATP output is 36 molecules.
5. ATP output is 2 molecules.
6. It is an efficient process.
7. It occurs in 4 steps: a) Glycolysis
b) Oxidative phosphorylation of pyruvate
c) Krebs cycle
d) ETS
8. Glycolysis occurs in cytosol while
remaining 3 steps in mitochondria.
6. It is not an efficient process.
7. It occurs in 2 steps:
a) Glycolysis
b) Fermentation
9. The process is not toxic to cells.
9.The process is toxic to cells.
10. The end products are CO2 and water.
10. End products are ethyl alcohol and CO2.
8. Both the steps occur in cytoplasm.
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RESPIRATION IN PLANTS
Mechanism of Anaerobic Repiration
The process of anaerobic respiration occurs in 2 steps.
Mechanism of Anaerobic Respiration
Glycolysis
Fermentation
Glycolysis
Glykys - glucose(sweet), lysis = splitting. The enzymatic degradation of glucose into 2 molecules
of pyruvic acid. Glycolysis operates in the cytoplasm and is common to both aerobic and anaerobic
respiration.
cytoplasm
Glucose
Pyruvic acid
Mitochondria
The biochemical reactions were revealed by Embden, Meyerhoff and Paranas. In honour of them,
glycolysis is called EMP pathway. In any type of respiration glycolysis must be carried out first.
Embden
Paranas
Meyerhoff
In any type of respiration glycolysis must be carried out first. Hence james called glycolysis as core
respiration.
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RESPIRATION IN PLANTS
Phosphorylation
Isomerisation
Phosphorylation
Cleavage
Reactions of glycolysis are as follows
Isomerisation
Oxidation and phosphorylation
Isomerisation (Intra molecular shift)
Dehydration
1. Phosphorylation:- Glucose is phosphorylated in the presence of ATP and enzyme hexokinase.
This will yield glucose-6-phosphate and ADP.
Hexo kinase
Glucose 6 Phosphate
Glucose
Phosphorylation
Hexokinase
Glucose + ATP Glucose-6-phosphate
+ ADP
Mg2+
2) Isomerisation :- Glucose-6-phosphate is isomerised to fructose-6-phosphate by the action of
enzyme phosphohexose isomerase.
Phosphohexose
isomerase
Glucose - 6 - phosphate
Fructose - 6 - phosphate
Isomerisation
Glucose-6-phosphate
Phosphohexose
isomerase
fructose-6-phosphate.
3) Phosphorylation:- Fructose-6-phosphate is phosphorylated by ATP molecule in the presence of
an enzyme phospho fructokinase, so fructose 1,6 biphosphate and ADP are formed.
Fructose-v
1, 6 bisphosphate
Fructose6-phosphate
Phosphorylation
Frusctose-6-phosphate + ATP
Phospho fructokinase
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Fructose1,6 bisphosphate + ADP
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4) Cleavage: The hexose sugar, fructose 1,6-bisphosphate with the help of the enzyme aldolase,splits
into 2 molecules of 3 carbon substances (trioses) – glyceraldehyde-3-phosphate,dihydroxyacetone
phosphate.
Dihydroxyacetone
phosphate
Glyceraldehyde
-3-phosphate
Fructose 1,6-bisphosphate
Cleavage
Fructose-1,6-bisphosphate
Aldolase
Glyceraldehyde-3- phosphate +
Dihydroxyacetone phosphate [DHAP]
5) Isomerisation:- The two trioses are isomeric compounds of each other. Dihydroxyacetone
phosphate is not a suitable substance for biological oxidation. Hence dihydroxy acetone phosphate is
isomerised into glyceraldehyde-3-phosphate in the presence of isomerase. phosphate in the presence
of isomerase. Thus 2 molecules of glyceraldehyde-3-phosphate are formed. These molecules are
ready for biological oxidation.
Isomerase
glyceraldehyde
-3phosphate
Dihydroxyacetone
phosphate
Isomerisation
Dihydroxyacetone phosphate
Isomerase
Glyceraldehyde-3-phosphate
6. Oxidation & Phosphorylation: Glyceraldehyde-3-phosphate is oxidised and phosphorylated
into 1,3-bisphosphoglyceric acid. The enzyme glyceraldehyde-3-phosphate NAD oxido reductase
catalyses the reaction.
H
Oxidation & Phosphorylation
The enzyme removes 2 hydrogen atoms from glyceraldehyde-3-phosphate. NAD (co enzyme)
accepts two hydrogen atoms and becomes reduced to NADH.
During oxidation of glyceraldehyde-3-phosphate energy is released inorganic phosphate group
absorbs energy and becomes energy rich.
Glyceraldehyde-3-phosphate accepts this energy rich phosphate group and forms 1,3 – bis
phosphoglyceric acid.
NAD+
NADH+H+
Glyceraldehyde-3-phosphate + H3PO4 1,3-bisphosphoglyceric acid.
oxido reductase
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Dephosphorylation:- 1,3- Bisphosphoglyceric acid loses energy rich phosphate group in the prence
of an enzyme phosphotransferase and forms 3-phosphoglyceric acid. ADP accepts the energy rich
phosphate group to synthesise ATP.
The synthesis of ATP during conversion of substrate molecules is called substrate level phosphorylation.
Phospotransferase
1,3-biphosphoglyceric
acid
3-biphosphoglyceric
acid
Dephosphorylation
1,3-Bisphosphoglyceric acid + ADP
Phosphotransferase
3- phosphoglyceric acid + ATP
7) Isomerisation (Intra molecular shift):- In the presence of an enzyme phosphoglyceromutase,
the phosphat group attached to 3rd carbon atom of phosphoglyceric acid is shifted to the 2nd carbon
atom. (intra molecular shift), resulting in the formation of 2-phosphoglyceric acid.
3-Phosphoglyceric acid
2-Phosphoglyceric acid
Isomerisation
3-phosphoglyceric acid
Phosphoglyceromutase
2- phosphoglyceric acid
8) Dehydration : The enzyme enolase removes a molecule of water from 2-phosphoglyceric acid
and forms phospho enol pyruic acid.
2-Phosphoglyceric
acid
Phosphoenol pyruvic acid
Water (H2O)
Dehydration
2-phosphoglyceric acid
Enolase
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phosphoenol pyruvic acid + H2O
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Dephosphorylation : [PEP] The enzyme pyruvic kinase removes the energy rich phosphate group
from phosphoenol pyruvic acid and forms pyruvic acid. ADP accepts the energy rich phosphate group
to sythesise ATP. This is the second step of substrate level phosphyorylation.
Pyruvic acid
Phosphoenol
pyruvic acid
Dephosphorylation
Phosphoenolpyruic acid + ADP
Pyruvic kinase
Pyruvic acid + ATP
In glycolysis a glucose molecule splits into 2 trioses. Both the trioses are converted into 2 molecules
of pyruvic acid. During this conversion two molecules of NADH and four molecules of ATP are formed
2 molecules of ATP are utilized for phosphorylation of glucose into fructose 1,6 bisphosphate. Hence
the net gain of ATP is 2 molecules. The overall equation of glycolysis is represented as
C6H12O6 + 2NAD+ + 2ADP + 2Pi 2C3H4O3 + 2ATP + 2NADH + 2H++2H2O
(Pyruvic acid)
(Glucose)
Glycolysis
ATP
Glucose
(6C)
ADP
Glucose - 6 - Phosphate
(6C)
Fructose - 6 - phosphate
(6C)
ATP
2 x pyruvic acid
(3C)
ATP
ADP
ADP
Fructose1. - 6 - bisphosphate
(6C)
2 x Phosphoenolpyruvate
H 2O
Triose phosphate
(glyceraldehyde - 3 -phosphate)
(3C)
Triose phosphate
(Dihydroxy acetone phosphate)
(3C)
NAD+
NADH+H+
2 x Triose bisphosphate
(1.3 bisphosphoglyceric acid)
(3C)
2 x 2 Phosphoglycerate
2 x Triose phosphate
(3- Phosphoglyceric acid)
(3C)
ATP
ADP
Chat of Glycolysis
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Fermentation
The anaerobic respiration comes to an end with the formation of pyruvic acid. Because pyruic acid
does not undergo biological oxidation. But undergoes decarboxylation to form the end product. The
end product may be ethyl alcohol, lactic acid or butyric acid. Based on the nature of end product of
fermentation different types.
Glucose
Glyceraldehyde
3 - Phosphate
3 - Phosphoglyceric
acid
Lactic acid
NADH+H+
NAD+
NADH+H+
NAD+
Pyruvic
acid
NADH+H+
NAD+
Phosphoenol
Pyruvic acid
Ethanol + CO2
Fermentation
Alcoholic Fermentation
The conversion of pyruvic acid into ethyl alcohol
and CO2 is called alcoholic fermentation. Yeast
cells carry out alcoholic fermentation.
Yeast
Glucose
Lactic acid
Glyceraldehyde
3 - Phosphate
NADH+H+
NAD+
NAD+
3 - Phosphoglyceric
acid
NADH+H+
Pyruvic
acid
NADH+H+
NAD+
Ethanol + CO2
Phosphoenol
Pyruvic acid
Aerobic fermentation
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It includes 2 reactions
a) Decarboxylation
b) Reduction
a) Decarboxylation: Pyruvic acid undergoes decarboxylation in the presence of an enzyme pyruvic
decarboxylase to form acetaldehyde and CO2.
Pyruvic acid
CO2
Actealdehyde
Decarboxylation
Pyruvic decarboxylase
Pyruic acid
Acetaldehyde + CO2
b) Reduction: Acetaldehyde undergoes reduction in the presence of an enzyme alcohol
dehydrogenase to form ethyl alcohol. The reduced Co enzyme NADH (synthesised in glycolysis)
supplies the hydrogen atom necessary for this reduction
Acetaldehyde
Ethyl alcohol
Reduction
Acetaldehyde +
NADH+H+
alcohol-NAD
oxidoreductase
Ethyl alcohol + NAD+
Mechanism of Anaerobic Respiration
The overall reaction can be represented by following equation.
C6H12O6 2C2 H5OH + 2CO2 + 56 k. cal
Glucose (C6H12O6)
Ethyol alcohol
(2C2H5OH)
Carbon dioxide
(CO2)
Overall reaction
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Alcoholic fermentation is of economic importance in the production of beverages and in raising bread.
Beverages
Bread
Anaerobic respiration occurs inside the living cells.
But fermentation is extracellular carried out by the
zymase enzyme extracted from yeast cells.
Yeast
Lactic Acid Fermentation
In mammalian tissues and lactobacillus pyruvic acid is converted into lactic acid in the presence of
lactic acid dehydrogenase. NADH(glycolysis) supplies the hydrogen atom in this reaction. It causes
spoilage of food.
Pyruvic acid
Lactic acid
Reaction of Lectic acid fermentation
Pyruvic acid + NADH
Lactic acid
dehydrogenase
Lactic acid
Butyric Acid Fermentation
In Bacillus butyricus and clostridium butyricum, pyruvic acid is decarboxylated into aceto acetic acid.
Aceto acetic acid is reduced to butyric acid.
Bacillus butyricus
Pyruvic acid
Acetoacetic acid + CO2
Acetoacetic acid + NADH+H+ Butyric acid + NAD+
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Acetic acid Fermentation Eg: Acetobacter aceti, pyruvic acid is oxidized to form Acetic acid.
Pyruvic acid
Acetic acid
Acetic acid
Pyruvic acid
Reaction of Acetic acid fermentation
Acetobacter aceti
Aerobic Respiration
Aerobic respiration takes place in mitochondria.
Aerobic respiration leads to complete oxidation
of glucose molecule to CO2 and water in the
presence of oxygen. It involves 4 major steps.
Mitochondria
Glycolytic break down of glucose to pyruvic acid.
Oxidative decarboxylation of pyruvic acid to acetyl co A.
Kreb’s cycle.
Terminal oxidation and phosphorylation in respiratory chain
Glycolysis
The conversion of 1 glucose molecule into 2 molecules of pyruvic acid is called glycolysis. The
reactions are described in anaerobic respiration.
Pyruvic acid
Pyruvic acid
Glucose
Reaction of glycolysis
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Oxidative Decarboxylation of Pyruvic Acid to Acetyl CoA
The pyruic acid, generated in the glycolysis cannot enter into kreb’s cycle unless it forms Acetyl coA.
Pyruvic acid enters mitochondrial matrix and undergoes complete biological oxidation and is
converted into Acetyl CoA.
Glucose
Glycolysis
yl
Pyruvic acid
Acet
Co-A
Kreb’s
Cycle
The formation of Acetyl CoA from pyruvic acid involves removal of CO2 and hydrogen and so it is
known as oxidative decarboxylation.
Oxidative decorboxylation
Pyruvic acid reacts with thiamine pyrophosphate to form acetaldehyde. TPP complex and CO2 is
released.
Pyruvic acid
Thiamine pyrophosphate
CO2
a -lipoic acid
Acetaldehyde - TPP complex
NAD+ NADH+H+
Acetyl lipoic acid
co-A
Reduced
a-lipoic acid
Acetyl. Co - A
Kreb’s cycle
Oxidative Decarboxylation of Pyruvic Acid to Acetyl CoA
Pyruvic Acid
TPP & O2
Thiamine pyrophosphate
a-Lipoic acid + Acetaldehyde
Acteladehyde
Acetyl lipoic acid
Acetyl lipoic acid + Co-A
Acetyl Co-A
Formation of Acetyl co A is the connecting link between glycolysis and Kreb’s cycle.
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Kreb’s cycle
Acetyl co A formed from pyruvic acid is the fuel of
Kreb’s cycle which includes a series of oxidation
reactions. The sequence of reactions in this cycle
are described by Sir Hans Krebs in 1937.
Hans krebs
Pyruvate
(3c)
Acetyl coenzyme A
(2c)
Oxaloacetic acid
(4c)
NADH+H+
Citric acid (6c)
H 2O
NAD+
Isocitric acid (6c)
NAD+
Malic acid
(4c)
CITRIC ACID CYCLE
FADH2
FAD
CO2
Succinic acid
(4c)
H 2O
ATP
NADH+H+
Oxalosuccinic acid
Ketoglutaric acid
(3c)
ADP + Pi
Co A
Co A
NAD +
Succinyl Co A
NAD
CO2
H+H +
Kreb’s cycle
In 1953, Krebs was awarded Nobel prize, and the cycle is frequently refered to as krebs cycle in his honour.
The first formed substance in this cycle is citric acid, hence it is also known as citric acid cycle.
Pyruvate
(3c)
Acetyl coenzyme A
(2c)
Oxaloacetic acid
(4c)
NADH+H+
Citric acid (6c)
H 2O
NAD+
Malic acid
(4c)
FADH2
Isocitric acid (6c)
NAD+
T.C.A CYCLE
NADH+H+
Oxalosuccinic acid
FAD
CO2
Succinic acid
(4c)
H 2O
ATP
ADP + Pi
Ketoglutaric acid
(3c)
Co A
Succinyl Co A
CO2
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NAD +
Co A
NAD
H+H +
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Kreb’s cycle includes several biochemical
reactions which takes place in mitochondrical
matrix.
Pyruvate
(3c)
Acetyl coenzyme A
(2c)
Oxaloacetic acid
(4c)
NADH+H+
Citric acid (6c)
H 2O
NAD+
Isocitric acid (6c)
NAD+
Malic acid
(4c)
NADH+H+
CITRIC ACID CYCLE
FADH2
Oxalosuccinic acid
FAD
1. Condensation
2. Isomerisation
3. Oxidation I
4. Decarbohydration
5. Oxidative Decarbohydration II
6. Cleavage
7. Oxidation -III
8. Hydration
9. Oxidation -IV
CO2
Succinic acid
(4c)
H 2O
ATP
ADP + Pi
Ketoglutaric acid
(3c)
Co A
Succinyl Co A
CO2
NAD +
Co A
NAD
H+H +
1. Condensation: Acetyl Co-A (2C) condenses with oxalo acetic acid (4C) to yield citric acid (6C) and
Co. enzyme A. This condensation reaction is catalysed by the enzyme citrate synthetase,a molecule
of water is used in this reaction.
Acetyl Co-A + OAA + H2O
(2c)
(4c)
citrate synthetase
citric acid + Co-A
(6c)
2. Isomerisation: Citric acid loses one molecule of water in the presence of an enzyme aco nitase to
form cis-aconitic acid. Cis- aconitic acid accepts a molecule of water and yields isocitric acid.
Citric acid aconitase cis aconitic acid aconitase isocitric acid
(6c)
(6c)
(6c)
+ H2O
- H2 O
Dehydration
Hydration
Oxidation 1: Isocitric acid undergoes dehydrogenation (oxidation) in the presence of isocitric de
hydrogenase enzyme leading to the formation of oxalosuccinic acid. NAD+ is reduced to NADH + H+
isocitric dehydrogenase
Isocitric acid + NAD+ 19
Oxalosuccinic acid + NADH + H+
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Decarboxylation: Oxalosuccinic (6C) acid releases one molecule of CO2 in the presence of oxalo
succinic decarboxylase enzyme and forms ketoglutaric acid (5C)
Oxalosuccinic acid
(6c)
oxalosuccinic
decarboxylase
a-ketoglutaric acid + CO2
(5c)
(c)
Oxidative decarboxylation (Oxidation): Ketoglutaric acid undergoes oxidation (dehydrogenation)
decarboxylation and condensation with one molecule of Co- A,
Ketoglutaric acid
Leading to the formation of succinyl Co.A in the presence of ketoglutaric dehydrogenase enzyme
NAD+ acts as hydrogen acceptor
aketoglutaric acid
Ketoglutaric acid +
NAD+
+ Co.enzyme A
dehydrogenase
Succinyl co.A + NADH + H+ + CO2
Cleavage:- Succinyl co A splits into succinic acid and co enzyme A by succinic acid thiokinase. The
energy released in this reaction is used to form ATP from ADP and Pi.
Succinic acid
Succinyl Co.A
Because ATP formation is linked directly to conversion of substrate, this reaction is an example of
substrate level phosphorylation.
Succinyl co A + ADP + ip
succinyl
thiokinase
Succinic acid + ATP + Co A
The Krebs cycle in animal mitochondria produces GTP, instead of ATP, which is subsequently
converted to ATP by transphosphorylation
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Pyruvate
(3c)
Acetyl coenzyme A
(2c)
Oxaloacetic acid
(4c)
NADH+H+
Citric acid (6c)
H 2O
NAD+
Isocitric acid
NAD+
Malic acid
(4c)
T.C.A CYCLE
FADH2
NADH+H+
Oxalosuccinic acid
FAD
CO2
Succinic acid
(4c)
H 2O
ATP
Ketoglutaric acid
(3c)
NAD +
Succinyl Co A
ADP + Pi
Co A
Co A
NAD
H+H +
CO2
Oxidation: Succinic acid undergoes oxidation to form Fumaric acid FAD serves as hydrogen acceptor.
FAD is reduced to FADH2. The enzyme which catalyse the reaction is succinic dehydrogenase.
Succinic acid + FAD
succinic
dehydrogenase
Fumaric acid + FADH2
Hydration: Hydration of fumaric acid in the presence of enzyme fumerase leads to the formation of
malic acid.
Fumaric acid + H2O
fumerase
Malic acid.
Oxidation V: Malic acid in the presence of malic dehydrogenase releases 2 hydrogen atoms and
gives rise to oxalo acetic acid. NAD+ acts as hydrogen acceptor and is converted to NADH + H+.
Malic acid + NAD+
Malic
dehydrogenase
Oxaloacetic acid + NADH + H+
Summary equation
Pyruvic acid +
4NAD+
+ 3H2O + FAD + ADP + Pi
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mitochondrial
matrix
3CO2 + 4 NADH +
4H+ + FADH2 + ATP.
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Significance of kreb’s cycle
Kreb’s cycle is a central metabolic pathway playing an important role both in catabolism and
anabolism. In the catabolic role it serves as pathway for oxidation of carbohydrates, fats and proteins.
Fats
Protien
In the catabolic role, the intermediates of this pathway serve as substrate for synthesis of amino acids.
Because the pathway is involved both in catabolism and anabolism, the term amphibolic
(dual purpose) pathway is used to signify Kreb’s cycle.
Electron Transport and oxidative phosphorylation
Oxidation is brought about by removal of a pair of hydrogen (2H) from each intermediate substrate.
Hydrogen pair can dissociate into 2H 2H+ + 2e-
Electron Transport and oxidative phosphorylation
The oxidation process is equated to removal and transport of electrons to molecular O2.
The pairs of hydrogen removed in oxidation steps of glycolysis and kreb’s cycle are not transferred
directly to O2, but are picked up by coenzymes NAD+ and FAD+ to form NADH and FADH2.
NAD+ + H+ NADH
FAD+ + H+ FADH
So NADH and FADH2 are oxidised in a series of reactions called the respiratory chain or electron
transport system, in which electrons from NADH and FADH2 are transfered to molecular O2. The free
energy released during electron transfer in the respiratory chain is made available for ATP synthesis.
This is called Oxidative Phosphorylation.
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RESPIRATION IN PLANTS
NADH FADH ETS
energy
electron carriers
2e- energy
ADP + Pi ATP
ADP + Pi ATP
ETS
energy
electron carriers
2e-
energy
O2
ADP + Pi ATP
ADP + Pi ATP
energy
O2
ADP + Pi ATP
The enzymes and electron carriers (hydrogen carriers) of the respiratory chain are located in the inner
membrane of mitochondria. The include flavor proteins cytochromes, Fe-s proteins and ubiquinone.
Cytocromes Fe-s proteins
Ubiquinone
electron carriers
mitochondria
Flavoproteins: Contain prosthetic groups FMN, or FAD. The prosthetic group can carry 2 hydrogen
groups (2 protons and 2 electrons)
FMN
carry 2 hydrogen groups (2 protons and 2 electrons)
FAD
Cytochromes: Cytochromes are heme containing proteins, they are cyt a1, cyt a3, cyt b, cyt and cyt
c1. They are identified on the basis of light absorption spectra. They are dentified on the basis of
light absorption spectra. They differ from one another according to structure and properties of heme
group.
Cyt a1
Cyt a3
Heme proteins
Cyt b
Cyt c
Cyt c1
Fe- S proteins (Non heme proteins): They play an important role in reductions in biological systems.
They do not exist independently but are associated with other carriers.
Fe- s protiens independently
Not exist
Fe- s protiens + other carriers
involve in biological reactions
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RESPIRATION IN PLANTS
Ubiquinone or CO.Q: Ubiquinone or CO.Q is a mobile carrier that can receive and transfer a pair of hydrogen and electrons.
CO.Q Can receive & transfer of 2H+ + 2e1. The first step involves transfer of hydrogen ions or protons and 2e- from NADH formed in the matrix
in kreb’s cycle. The enzyme NADH dehydrogenase catalyses the removal of 2H from NADH + H+. it is
located in the inner mitochondrial membrane. The 2e- and 2H+ are transferred to FMN. FMN is reduced
to FMNH2 and the co.enzyme NAD+ is oxidized.
NADH dehydrogenage
NADH 2H+ + 2e- + NAD+
FMN + 2e- + 2H+
FMN2
2. From FMN the 2e- are passed to (Fe-s) proteins. It can accept one electron at a time and does not
accept H+. The 2H+ are transferred to inner membrane space. This is 1st step in which a pair of H+ move
out from the matrix across inner mitochondrial membrane to the intermembrane space.
2eFMNH2 Fe- S protien
2H+
3. From reduced Fe-s protein 2e are transferred to Ubiquinone which takes up 2H+ from mitochondrial
matrix. UQ is reduced to UQH2.
UQH2
Fe- s protien + 2e- + 2H+
4. From UQH2 the electrons move to cyt b. the two H+ are transferred outwardly to the inter membrane
space.
Cytb + 2e- + 2H+
UQH2
5. The co.enzyme FAD is reduced to FADH2 is oxidized and UQ is reduced to UQ-H2.
6. From cytochrome b the 2e- move to Fe-S protein.
Cytb
2e-
Fe- s protien
7. From reduced Fe-S protein the 2e- move to cyt c1 when e- are passing from UQ to cyt c1 the 2 H+ are
transported outwardly into inter membrane space.
Fe- s protien
2e24
Cyt c1
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RESPIRATION IN PLANTS
8. The ubiquinone might be involved twice in translocation of protons. 2H+ are transported for each
electron from UQ to cyt b and c, complex.
9. From reduced cyt c, the 2e- move to cyt c
2e-
Cyt - c1
Cyt c
10. From reduced cyt c, the e- are transferred to cytochrome oxidase. It consists of polypeptide, 2
inseparable cyt a and a3 as well as 2 atoms of copper.
Cyt c
2e-
Cyt a - a3
It catalyses the transfer of e- are transferred to a bound O2 molecule that accepts 4H+ and are move
e- forming 2 mol of H2O. water formed at the end of terminal oxidation is called respiratory water.
4H+ + 4e- + O2
Cu
2H2O
It represents only a small fraction of the total water in plant cells.
Matrix
Inner mitochondrial
membrane
Inter membrane space
Electron carriers
Electron transport system
Oxidative phosphorylation
Oxidative phosphorylation is the process in which ATP is formed as a result of transfer of e- from NADH
or FADH2 to O2 via a series of electron carriers.
NADH energy
ADP + Pi ATP
FADH energy
electron carriers
2e- energy
ADP + Pi ATP
electron carriers
2e-
ADP + Pi ATP
energy
O2
ADP + Pi ATP
energy
O2
ADP + Pi ATP
Oxidative phosphorylation is based on chemi-osmotic hypothesis proposed by peter Mithell.
(chemiosmotic hypothesis states that the energy of e- harvested from glucose is used to transport
protons out of the mitochondrial matrix. The return of the protons back into the matrix by diffusion is
coupled to ATP production).
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RESPIRATION IN PLANTS
Production of ATP
Matrix
Glucose 2e
2e-
2e-
2e-
2e-
Oxidative phosphorylation is based on chemiosmotic hypothesis proposed by peter Mitchell.
(chemiosmotic hypothesis states that the energy of e- harvested from glucose is used to transport
protons out of the mitochondrial matrix. The return of the protons back into the matrix by diffusion is
coupled to ATP production).
In the Electron transport system (ETS) the flow of electron leads to the transport of protons (H+) from the
matrix across the inner membrane to its outer side. Proton translocation occurs at 3 sites. This results
in higher proton concentration outside the inner membrane than in the matrix. The inner membrane of
mitochondria is virtually impermeable to protons and thus prevent the return of protons into the matrix.
However, the F0 particle of F0 – F1 particles permits the return of H+ moves down the gradient, energy
is released. The energy rotates F1 particle. Some amount of energy of this rotating F1 particle helps in
combining ADP + Pi. Leading to the formation of ATP. The enerfy from 3H+ moving down the potential
gradient is sufficient to form one ATP molecule.
Matrix
proton
Inner mitochondrial
space
Inter membrane space
Electron transport system
F0 - F1
ADP + ip
ATP
Thus one NADH mol. Which adds 10H+ to the concentration gradient accounts for synthesis of 3 ATP mol
and FADH2 adds 6H+ to the concentration gradient, they account for the synthesis of 2 ATP molecules
each.
NADH + 10H+
3ATP
FADH + 6H+
2ATP
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RESPIRATION IN PLANTS
The Respiratory Balance Sheet
The total yield of ATP from complete oxidation of one molecule of glucose is
Glycolysis
(i) ATP produced by substrate level phosphorylation
1,3-bisphosphoglyceric acid → 3 phosphoglyceric acid
Phosphoenol pyruvic acid → pyruvic acid
(2 x 1 = 2ATP)
(2 x 1 = 2 ATP)
4 ATP
ATP consumed
Glucose to glucose 6-PO4 [6-phosphate]
fructose 6-PO4[6-phosphate] → Fructose 1,6bis PO4 [1,6 bisphosphate]
Net gain of ATP (4-2)
= 2 ATP
(ii) From NADH
G.3 p → 1,3 DPGA (2 x 2 = 4) = 4 ATP
Total ATP produced in Aerobic glycolysis
= 6 ATP
(I) Oxidative phosphoryletion of pyruvic acid
From NADH
Pyruvic acid to acetyl co A (2 x 3 = 6)= 6 ATP
(II) Kreb’s cycle
(i) ATP produced by substrate level phosphoryletion
Succinyl co A → succinic acid (2 x 1 = 2) = 2 ATP
(ii) From NADH
Isocitric acid→oxalo succinic acid
(2 x 3 = 6) = 6 ATP
α keto glutaric acid → succinyl Co A
(2 x 3= 6) = 6 ATP
Malic acid → oxalo acetic acid (2 x 3 = 6) = 6 ATP
(III) From FADH2
Succinic acid → Fumaric acid
(2 x 2 = 4) = 4 ATP
Total ATP produced
36 ATP
One glucose molecule in aerobic results in 36 ATP.
1 ATP = 7.6 K.Cal.
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RESPIRATION IN PLANTS
The chemical energy in k.Cal would be 36 x 7.6 = 273.6 k.Cal.
The total energy in a molecule of glucose is 686 k.Cal.
∴ The process is 686/273 = 40% efficient.
The remaining energy 686 – 273.6 = 412.4 kal would be released as heat energy.
Calculation for the net gain of ATP for every glucose molecule oxidised can be made only on certain
assumptions.
l There is sequential, orderly pathway functioning with glycolysis, TCA cycle and ETS pathway following one after other.
l NADH is synthesised in glycolysis is transferred into mitochondria and undergoes oxidative phosphorylation.
l None is synthesised in glycolysis is transferred into mitochondria and undergoes oxidative phosphorylation.
l None of the intermediates in the pathway are utilized to sythesise other compounds.
l Only glucose is being respired.
But this kind of assumptions are not dvalid in a living system. Yet it is useful to do this exercise extraction and storing energy
Fermentation
Aerobic Respiration
Complete break down of glucose to CO2 and H2O.
1 Partial break down of glucose.
2
Net gain of 2 ATP for each molecule of
Net again is 36 ATP.
glucose.
3 NADH is oxidized to NAD+ slowly.
The reaction is vigorous.
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RESPIRATION IN PLANTS
Amphibolic Pathway
All carbohydrates are first converted into glucose before they are used for respiration. Glucose is the
substrate for respirate other substrates do not enter respiratory pathway in the first step.
Example
Fats
Carbohydrates
Protiens
Fatty acids and glycerol
Simple sugars
e.g glucose
Amino acids
Glucose 6 - phosphate
Fructose 1,6 - bisphosphate
Dihydroxy Acetone Phosphate
Glyceraldehyde 3-phosphate
Pyruvic acid
Acetyl co A
H 2O
Krebs
Cycle
CO2
1. Fats should be broken down into glycerol and fatty acids. If fatty acids were to be respired they should be degraded to acetyl Co.A and enter the pathway. Glycerol would enter pathway after being
converted to phosphoglyceraldehyde.
2. Proteins would be broken down to acetyl Co.A before entering respiratory pathway; when it is
used as substrate. But when the organism needs to synthesis fatty acids, acetyl Co.A would be with
drawn from the respiratory pathway. Hence respiratory pathway is essential both during breakdown and
synthesis of fatty acids.
Similarly during break down and synthesis of protein too, respiratory intermediates form the tissue.
Breaking down processes within the living organisms is catabolism and synthesis is anabolism. Because
the respiratory pathway is involved in both anabolism and catabolism, it would be better to consider the
respiratory pathway as an amphibolic pathway.
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Anabolism
Catabolism
RESPIRATION IN PLANTS
Substrate
Comparison of Respiration and photosynthisis
Respiration
Photosynthisis
1
Oxygen may be absorbed in the process.
Oxygen is liberated.
2
CO2 is evolved as a result of oxidation.
CO2 is absorbed and is fixed inside to form
carbohydrates.
3
It occurs during day and night. Light is not
essential.
It occurs only in the presence of light .
4
During the process, chemical, potential energy is converted to kinetic energy.
IRadiant energy (light) is converted into
potential energy.
5
Chlorophyll is not necessary.
Chlorophyll is necessary.
6
Raw materials are glucose and oxygen.
CO2 and water.
7
Energy is released during the process exothermic.
Energy is conserved – endothermic.
8
It is catabolic process.
Anabolic process.
9
It occurs in cytoplasm and mitochoadria.
It occurs in chloroplast.
10
The organism suffers a loss in weight.
By the process, weight is gained.
11
During aerobic breakdown of one glucose
molecule, 36 ATP are formed .
During the synthesis of glucose 18 ATP are
utilised.
∴ Respiration is a reverse of photosynthesis.
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RESPIRATION IN PLANTS
Respiratory Quotient
The ratio of volume of carbondioxide evolved from the respiratory substrate to the volume of oxygen
absorbed by it is called respiratory quotient.
R.Q. =
Volume of CO2 evolved
Volume of O2 absorbed
The R.Q. depends upon the type of resperatory substrate used during respiration.
I. RQ = 1
When carbohydrates are used as respiratory
substrates, the volume of CO2 evolved is equal to
the volume of O2 absorbed. Hence the volume of
RQ =1
Carbohydrates
C6H12O6 + 6O2 + 6H2O
Oxygen
Glucose
R.Q. =
6CO2 + 12H2O
carbon
dioxide
water
6CO2
6O2
=
water
1 1
=
1
Reaction
The value of RQ in different parts of plant body is usually between 0.97 and 1.17. This indicates that in
plants carbohydrates serve as respiratory substrates.
II. RQ = Less than 1
a) Respiration of fats and proteins: RQ of seeds in which the stored food is in the form of oils(fat) is
less than one.
2C51H98O6+145O2
102CO2+98H2O
Oxygen
Tripalmitin
R.Q. =
102
145
= 0.7
Carbon
dioxide
Water
Reaction
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The propoition of O2 to C is less in fats than in carbohydrates ie., fats are poorer in O2 . Such compounds
require more O2 for complete oxidation.
Oxidation of proteins results in RQ less than one (0.8 - 0.9) since the proportion of O2 to C in such
compounds is less than that of carbohydrates.
b) Respiration of succulents and red leaves:
In plants like opuntia the carbohydrates are not
completely oxidised to CO2 & water. Instead they
are incompletely oxidised to orgnic acid with out
evolution of CO2.
Opuntia
2C6H12O6 + 3O2
3C4H6O5 + 3H2O + 386KCal
Oxygen
Glucose
R.Q. =
Malic acid
CO
O2
=
water
0 0
=
3
Reaction
Plants whose leaves are red by the presence of anthocyanin in their cells. These cells show greater
accumulation of organic acids.
Anthocyanin
III. RQ = More than 1
(a) Respiration of succulents and red leaves
During maturation of fatty seeds, simple
carbohydrates are converted into fats. Oxygen is
eliminated internally in the process.
Fatty seeds
Castor seeds
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This is utilized is respiration with a corresponding liberation of Co2, for which these was no absorption
of O2 from outside. This results in RQ of more than one.
+
Oxygen
Fats
Carbohydrates
Reaction
(b) Respiration of organic acids
In some plants of succulent habits, organic acids from the respiratory substrate. Such compounds are
relatively rich in oxygen as compared with carbohydrates. The RQ of such materials is always more
than one.
2(COOH)2 + O2
4Co2 + 2H2O + 60.2 K.cal
Oxygen
Oxalic acid
R.Q. =
CO2
O2
carbon
dioxide
=
water
4 4
=
1
Reaction
(c) Respiration in the absence of oxygen: (Anaerobic)
Release of Co2 without any corresponding utilisation of atmospheric O2 is characteristic of anaerobic
respiration, which occurs in higher green plants under certain conditions. The R.Q of a plant material is
normally measured with the help of Ganong’s respirometer.
C6 H12 O6
R.Q. =
Zymase
2C2 H5 OH + 2CO2 + 21 K.Cal.
2CO2 2C
=
=∞
0
O2
Plant
Ganong’s respirometer
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RESPIRATION IN PLANTS
Factors Affecting the Rate of Respiration
Rate of respiration indicates the quantity of energy released during biological oxidation of respiratory
substrate in a unit time.
The rate of respiration is high in actively growing tissues which require high quantity of energy .
Example: germinating seeds, meristematic tissues etc.
Germinating seeds
Meristematic tissues
The rate of respiration is less in dormant tissues which do not require high energy.
Example: fruits, seeds, spores etc.
Fruits
Seeds
Spores
When dry seeds and spores are placed at absolute temperature the rate of respiration becomes zero.
Which favourable conditions are available, they germinate, and rate of respiration reaches maximum
level. The rate of respiration is not always uniform but changes based on environmental conditions.The
factors are external and internal factors.
Respiration = 0
spores
Dry seeds
Respiration level
Germinating seeds
Respiration = Maximum level
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RESPIRATION IN PLANTS
External Factors
External Factors are light, temperature, O2, CO2, minerals, water etc.
1. Light: In the presence of light, carbohydrates are synthesisied with the increase in carbohydrate
concentration, the rate of respiration increases.
2. Temperature: Respiration is enzymatically
controlled process. The enzyme activity depends
upon temperature. Plants carry out respiration at a
higher rate at a temperature 25-350c. The minimum
temperature to initiate respiration is 50cat high
temperature the rate of respiration decreases.The
decrease is due to denaturation and inactivation of
enzymes.
Temperature
3. Oxygen: if the concentration of O2 decrease to
a minimum level, the aerobic respiration is affect.
Finally the organism may lose its life.
Oxygen
4. CO2: An increase in conc.of CO2 beyond 1%, the
rate of respiration decreases. This is due stomatal
closure and prevention of gaseous exchange.
CO2
5. Mineral elements: acts as confactors to several enzymes and activate them. Deficiency of minerals,
decrease the rate of respiration.
6. Water: The rate of respiration is low under water
deficit conditions and the rate is high when sufficient
amount of water is available.
Water
INTERNAL
1. Protoplasmic condition:- The rate of respiration
depends upon the quantity of protoplasm. If the
cells have more amount of protoplasm, they show
high rate of respiration.
Protoplasm
Protoplasmic condition
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RESPIRATION IN PLANTS
2. Respiratory substrate:- If respiratory substrate
is carbohydrates, the cells show high rate of
respiration.
Carbohydrates
Respiratory substrate
Dormancy period
3. Dormancy: The rate of respiration is low because
metabolic rate is at minimum level. The growth and
development are suspended temporarily.
Dormancy
SUMMARY
1. Plants require O2 for respiration and give out CO2 . Plants have no specialised organs for gaseous
exchange, but they have stomata & lenticels.
2. The breakdown of complex organic molecules by oxidation, releasing lot of energy is called cellular
respiration.
3. Respiration is of 2 types.
I) Aerobic respiration - takes place in the presence of O2.
II) Anaerobic respiration - takes place in the absence of O2.
4. The initial stage of respiration takes place in the cytoplasm. Each glucose molecule is breakdown to
2moles of pyruvic acid.This process is called glycolysis.
5. The fate of pyruvate depends upon the availability of O2. Under anaerobic condition fermentation
takes place resulting in C2H5OH or lactic acid. In the presence of O2, pyruvic acid is transported to
mitochondria where it is converted to acetyl CO-A,which enters Kreb’s cycle.
6. NADH+H+ & FADH2 are generated in Kreb’s cycle.The energy in these molecules as well as NADH+H+
synthesised during glylcolysis are used to synthesis ATP. This takes place through a system of electron
carriers called electron transport system located on inner membrane of mitochondria.
7. 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 O2 is the acceptor of electrons & it gets
reduced to water.
8. The respiratory process is an amphibolic pathway as it involves both catabolism & anabolism.The
ratio of volume of CO2 evolves from respiratory substrate to the volume of O2 absorbed is called R.Q.
9. The R.Q. depends upon the type of respiratory substrate used during respiration.The rate of respiration
depends upon light, O2, temperature, CO2, mineral elements and water.
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