Respiration In Plants - Class 11 Medical Biology - Extra Questions

A. Louis Pasteur discovered the process of fermentation within the Yeast cells in 1856.

Louis Pasteur discovered fermentation in the year 1857.

Kreb's cycle is a step in the cellular aerobic respiration. It is a series of chemical reactions that takes place in the breakdown of glucose to release energy,  It takes place in the mitochondria. Kreb's cycle is also known as citric acid cycle or tricarboxylic acid cycle. It results in the formation of carbon dioxide and ATPs.

Respiratory Quotient is defined as the ratio of carbon dioxide evolved and oxygen consumed for respiration. A value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. In humans, we mostly, use carbohydrates, proteins and fats as a respiratory substrate.  

Acetyl Coenzyme A is formed by the oxidative phosphorylation of pyruvate molecule in sufficient amount of oxygen is present in the mitochondrial matrix which is catalyzed by an enzyme known as pyruvate dehydrogenase and contain thiamine pyrophosphate as the coenzyme. Carbon dioxide is removed and Acetyl Coenzyme A is formed. This is known as the gateway of Kreb's cycle.

Pyruvic acid ($$CH_3$$CO.COOH) is the end product of glycolysis.

Anaerobic respiration taking place in bacteria and fungi, in which glucose is oxidized into alcohol and $$CO_2$$ or some organic acid, is called fermentation.

Fats are also the source for energy in the cell. Fatty acid metabolism is a crucial metabolism procedure that allows liberation of a great amount of energy.

No oxygen is directly utilized in the Krebs cycle. Oxygen is used in the electron transport chain to generate ATP.

Sir Hans A. Krebs.

The symbols have been replaced as;

Organisms that respire anaerobically are as follows:

• Some bacteria
• Fungi

Krebs cycle reactions begin with the formation of citric acid which contains three (COOH) carboxylic groups. Hence it is called the T.C.A. cycle.

Glycolysis is the first step involved in the process of respiration whereas TCA cycle is the second process in respiration.

Glycolysis is the first phase of respiration in which glucose is broken down through a series of stepwise enzymatic reactions leading to the formation of pyruvate. It is an anaerobic process. At the end of glycolysis, two molecules of pyruvate are formed from 11 molecule of glucose along with the formation of 4 molecules of ATP and 2 molecules of NADH$$_2$$.

In the  Krebs cycle pathway, the metabolite which is used to accept the acetyl group from the acetyl CoA is regenerated at the end of the cycle. The substrate which begins the cycle is oxaloacetate.

Karen's cycle events:

(a)

Aerobic Respiration	Anaerobic Respiration
1. Aerobic respiration takes place in the presence of free oxygen.	1. Anaerobic respiration takes place in the absence of free oxygen.
2. The first step of this process (glycolysis) takes place in the cytoplasm while the second step (Krebs cycle) is carried out in mitochondria.	2.The complete process is carried out outside the mitochondria i.e., in the cytoplasm.
3. Glucose is completely oxidized into carbon dioxide and water.	3. Glucose is incompletely oxidized into carbon dioxide and ethyl alcohol.
4. 38 molecules of ATP are produced by the complete oxidation of one gram-mole of glucose.	4. Only 2 molecules of ATP are formed in this process.

Answer:

• Respiration is the breakdown of complex substances into simpler ones to produce an energy-rich molecule called ATP (Adenosine Triphosphate). So the process is called the catabolic process and the pathway is known as a catabolic pathway. Proteins or fatty acids are broken down to form acetyl-CoA and further processes of respiration occur. This is catabolism.
• When the body requires fatty acids or proteins, the same acetyl-CoA is utilized and fatty acids are manufactured. This process of synthesis is termed anabolism. 
• Thus we can say respiration includes both catabolism and anabolism due to which it is rightly called an amphibolic pathway. 

The puffy appearance of Idli and Dosa dough is due to carbon dixoide released by microbes during fermentation. Likewise, baking industries use Baker's yeast that causes rise of batter of cakes and pastries during the process. It is also due to released carbon dioxide during alcohol fermentation.

Respiration is the process where the food is oxidized in the presence of oxygen by the certain respiratory enzyme and it leads to the production of 38 ATP. 

The ratio of the volume of CO$$_2$$ given out to the volume of O$$_2$$ consumed in respiration is called respiratory quotient. In anaerobic respiration, R.Q is infinite because CO$$_2$$ is evolved without taking O$$_2$$.

The process by which the glucose (6C compound) is split into two molecules of pyruvic acid (3C compound) is called glycolysis or EMP pathway. It occurs in the cytoplasm. 

Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms, typically involving effervescence and the giving off of heat.

Used: 1 glucose, 4 ADP, 4 phosphate groups, 2 ATP, 2 $$NAD^+$$ Produced : 2 pyruvic acid, 4 ATP, 2 phosphate groups, 2 ADP, 2 $$NADH^+$$ $$H^+$$, 2H2O.

Anaerobic respiration takes place in the absence of oxygen. It is also known as fermentation which converts the sugar to acid, gases and alcohol. In ethanolic fermentation, the pyruvate formed at the end of glycolysis under anaerobic condition forms ethanol and carbon dioxide.

Respiratory quotient depends on the substrate used for the process. It is because the ratio is defined as the CO$$_2$$ molecules eliminated to that of O$$_2$$ consumed. The value of RQ for carbohydrate is one as the amount of carbon dioxide evolved is equal to the amount of oxygen consumed. When the respiratory substrate is incompletely oxidised the value of R.Q. = 0. Generally, organic acids have their R.Q. value greater than 1 and fats have the R.Q. which is less than 1. R.Q for proteins is less than 1.

The process by which the glucose (6C compound) is split into two molecules of pyruvic acid (3C compound) is called glycolysis or EMP pathway. The glucose is broken down anaerobically to form pyruvic acid. The pyruvic acid enters the Krebs cycle. Glycolysis occurs in the cytoplasm of the cell. 

The major steps of the Krebs cycle are given in the diagram. 

The given diagram illustrates the key steps for Krebs cycle.

Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time. Its value can be one, zero, more than 1 or less than one.

• Acetyl CoA is a 2 carbon compound formed from pyruvic acid after oxidative decarboxylation in link reaction.
  
• Malic acid is a 4 carbon compound. It has a R.Q of >1.
• Pyruvic acid, the end product of glycolysis is a 3 carbon compound.
• Glucose is a 6 carbon carbohydrate that acts as the organic substrate for glycolysis.

Respiratory Quotient ( R. Q) is the ratio of the volume of carbon dioxide release to the volume of oxygen taken in respiration in the given period of time at standard temperature and pressure.

Anaerobic respiration	Fermentation
It is an intracellular process.	It may be an extracellular process.
It occurs in the absence of oxygen.	It requires a small amount of oxygen.
CO$$_2$$ is not released.	CO$$_2$$ is released as the by-product.
It uses either endogenous or exogenous non-oxygen compounds as terminal electron acceptors.	In this, energy is produced from organic compounds using endogenous electron acceptors.

Glycolysis	Citric acid cycle
It occurs in the cytoplasm.	It occurs in the mitochondria.
It is the first step of respiration.	It is the second step of respiration.
Glucose is broken down to form 2 molecules of puruvate.	Water and carbon dioxide are formed from pyruvate.
It results in the liberation of 2 ATP and 2 NADH.	Two molecules of acetyl Co-A oxidize to give 6 molecules of NADH and 2 molecules of FADH$$_2$4.
It is not connected with oxidative phosphorylation.	It is connected with oxidative phosphorylation.

• When a process involves both anabolism and catabolism then the process is known as an amphibolic pathway. 
• In respiration, various complex molecules like carbohydrates, fatty acids, proteins are broken down into simpler molecules, this is catabolism. 
• On the other hand, during the formation of fatty acids, acetyl CoA is withdrawn from the respiratory pathway. 
• Likewise, amino acids also are withdrawn to produce proteins, this being anabolism. 

Respiratory Quotient is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration under standard temperature and pressure.

Acetyl-CoA is formed from Acetate and coenzyme A.

Respiratory substrate is the substrate required for cellular respiration. The energy is derived from the respiratory substrate through oxidation. Carbohydrates, fats, and proteins are examples of respiratory substrates. 

Glycolysis is the metabolic process by which glucose is converted into pyruvate. This is the foundation for both aerobic and anaerobic cellular respiration. It results in the formation of energy-rich molecule ATP. It occurs in 10 steps.

During glycolysis, 2 molecules of NADH are formed.

The respiratory quotient (RQ) is used in the calculations of basal metabolic rate (BMR). It is the ratio of the volume of carbon dioxide produced by the body to the volume of oxygen consumed by the body in respiration over a period of time. It depends on the type of respiratory substrate used during respiration. RQ of fat is about 0.7. RQ of carbohydrate is 1. RQ of organic acids is above 1. RQ of protein is 0.9. RQ value gives information about the source of energy an animal is using.

Carbohydrates- Polyhydroxy aldehydes or ketones and their derivatives or as substances that yield one of these compounds on hydrolysis.

Anaerobic respiration in plants is shown by reaction number 5 indicated by the formation of ethanol.

• At the end of the glycolysis the break down of one molecule of glucose gives 2 pyruvic acid which are formed in the cytoplasm.
• This pyruvic acid is required to enter the mitochondria to take part in kerb cycle but it cannot enter as pyruvic acid so it reacts with the Coenzyme A before entering the kerb cycle.
• The pyruvic acid undergoes oxidative decarboxylation where it reacts with coenzyme A and forms Acetyl CoA , one molecule of Carbon dioxide and one molecule of NADH.
• This Acetyl CoA reacts with the oxaloacetic acid and forms citric acid which under go reactions of the kerb cycle and ends with the regeneration of the oxaloacetic acid.
• Therefore the correct statement is "Acetyl CoA is formed from Pyruvic acid and coenzyme A"
  

glycolysis, Krebs cycle, electron transport chain 

Glycolysis, Krebs cycle, electron transport system.

A value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8.

The diagram represents the significant steps of aerobic respiration in the presence of oxygen.

Formation of acetyl coenzyme A.

The citric acid cycle (CAC) – also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle[1][2] – is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into adenosine triphosphate (ATP) and carbon dioxide. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically. In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion. In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol with the proton gradient for ATP production being across the cell's surface (plasma membrane) rather than the inner membrane of the mitochondrion. The overall yield of energy-containing compounds from the TCA cycle is three NADH, one FAD(2H), and one GTP.

Total 38 ATPs are produced by complete oxidation of one molecule of glucose. Out of 38 ATPs, glycolysis produces 2 ATP, Kerb's cycle produces 2 ATP and electron transport chain produces 34 ATP. In glycolysis, one molecule of glucose produces 2 molecules of PEP (phosphoenolpyruvic acid). Also, 2 molecules of ATP are produced from conversion of 2 molecules of PEP into 2 molecules of pyruvic acid.

• Respiration is that the biochemical process in living organisms involving the assembly of energy. This is generally finished with the intake of oxygen and this leads to the discharge of greenhouse gas, water, and ATP. 
• Aerobic respiration is that the method of cellular respiration that takes place within the presence of oxygen gas to provide energy from food. 
• This sort of respiration is common in most plants and animals, birds, humans, and alternative mammals.

The respiratory quotient can be defined as the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed by an organism, tissue or a cell and is given by; RQ = CO$$_2$$ evolved/ O$$_2$$ consumed. It denotes the amount of substrate used by a tissue, cell or an organism.

Fermentation is a metabolic process in which an organism converts a carbohydrate such as starch or sugar,  into an alcohol or an acid. For example, yeast performs fermentation to obtain energy by converting sugar into alcohol. Bacteria perform fermentation, converting carbohydrates into lactic acid.

RQ slightly more than unity is found when organic acids are broken down as respiratory substrates under aerobic conditions,e.g.,

The important events of aerobic respiration are as follows: 
(a) Complete oxidation of pyruvate happens in the matrix of mitochondria. This happens by step-wise removal of all the hydrogen atoms. Finally, 3 molecules of carbon dioxide are left. 
(b) This event happens in the inner membrane of mitochondria. Electrons removed as part of the hydrogen atoms are passed on to molecular oxygen. Synthesis of ATP takes place simultaneously. 

(a) A stands for volume of $$CO_2$$ evolved and B stands for volume of $$O_2$$ consumed. 
(b) Carbohydrates have R.Q. = 1; while protein and fat have R.Q. less than 1. No substrate has R.Q. > 1. 

Kreb's cycle is also known as the citric acid cycle because the first product formed during the cycle is citric acid. Citric acid is formed by the condensation of an acetyl group with oxaloacetic acid and water. 

In the Electron Transport System, $$NADH+H^+$$ and $$FADH_2$$ are energy carriers and are utilized to produce ATP. Electrons are transferred through a sequence of events and finally to the oxygen molecule. The oxygen molecule works like the final acceptor of hydrogen to form water molecules. Although oxygen comes at the end but it is critical for aerobic respiration. Oxidative phosphorylation involves a chain of events in which electrons are transferred through electron gradient. This is like flow of water through a tap. Until the tap is not open to allow the first drop of water, subsequent drops of water cannot flow down. Oxygen acting as hydrogen acceptor is like a receptacle collecting the first electron so that subsequent flow of electrons through ETS could be maintained. Hence, presence of oxygen is necessary to create a gradient through which electrons could be transferred passively. 

During TCA cycle, glucose is broken down which shows catabolic pathway. Additionally, $$FADH_2$$ and ATP are synthesized during this cycle. Thus, both catabolic and anabolic processes are taking place during TCA cycle. Hence, TCA cycle can said to be amphibolic. 

1. Fermentation:
Anaerobic respiration takes place in bacteria and fungi, in which glucose is incompletely oxidized into alcohol and $$CO_2$$ or organic acid is called fermentation.

2. Oxidative break-down of pyruvic acid:
Pyruvic acid formed as an end product of glycolysis enters mitochondria and in the matrix region of mitochondria, it is completely oxidized into $$CO_2$$ and $$H_2O$$ in the presence of Oxygen.
This is called an oxidative breakdown of pyruvic acid. Before entry into mitochondria, pyruvic acid is converted into Acetyl Coenzyme-A by decarboxylation and dehydrogenase enzyme. This complex reaction is catalyzed by five co-factors.
These are Coenzyme A; NAD+; Mg++; Thymine pyrophosphate (TPP) and Lipoic acid.
Acetyl Coenzyme – Acetyl CoA formed in this reaction acts as a connecting link between glycolysis and Krebs cycle.

3. Respiratory Quotient:
The ratio of the volume of $$CO_2$$ released in respiration to the volume of $$O_2$$ used is called respiratory quotient. The value of R.Q. of different respiratory substrates is as follows:
R.Q. of carbohydrate = 1.0
R.Q. of fats = 0.7
R.Q. of proteins = 0.8
R.Q. of organic acids = 1.33
R.Q. of succulents = zero
Anaerobic respiration R.Q. = infinite (∝).

4. Chemiosmotic theory of Mitchell:
This theory was proposed by Mitchell to explain ATP formation during respiration and photosynthetic process. According to this theory, transport of positively charged protons (Hydrogen ions) across the membrane of mitochondria or chloroplasts takes place through the enzymes involved in these processes via an electron transport system. Due to this, the electrochemical gradient is created on two sides of the membrane. The energy released during the passive transport of protons is utilized in formation of ATP.

The process of breakdown of one molecule of glucose into two molecules of pyruvic acid through a series of biochemical reactions is called glycolysis. The whole process can be expressed by the following equation.
1.The term Glycolysis has originated from the Greek work “Glycose”, (meaning sugar) and Lysis (meaning break down, dissolve, analysis), and means the breakdown of sugar. This is a complex biochemical process and is completed in ten steps.
2.Different steps of Glycolysis were worked out by G Embden, Otto Meyerhof, and J.Parnas in 1930, hence this process is also called EMP path. This is completed in the cytoplasm of the cell and $$O_2$$ is not used in the process.

3.Glycolysis takes place by the same reactions in all types of organisms and is found in both aerobic and anaerobic respiration.

4.It can be defined as, “Breakdown of a glucose molecule into pyruvic acid through a series of biochemical reactions along with the release of energy is called glycolysis”.

5.All the 10 biochemical reactions occurring during glycolysis can be studied under three heads.

1. Phosphorylation of Glucose
2. Splitting of phosphorylated hexose molecule in two molecules of phosphoglyceraldehyde
3. Formation of two molecules of pyruvic acid.

1. Phosphorylation of Glucose:

• In the first step of glycolysis one molecule of glucose changes in glucose-6-phosphate by use of one ATP molecule in the presence of hexokinase enzyme.
• In the presence of isomerase enzyme, Glucose-6-phosphate changes into fructose-6-phosphate.
• Fructose-6-phosphate using one molecule of ATP further changes into fructose 1-6-bisphosphate in the presence of enzyme phosphofructokinase.
• 
  2. Splitting of Phosphorylated Hexose molecule in two molecules of Phosphoglyceraldehyde:
  

• In this reaction, fructose 1-6-diphosphate breaks down in one molecule of phosphoglyceraldehyde (3-PGAL) and one molecule of dihydroxyacetone phosphate (DHAP) in the presence of aldolase enzyme.
  
• These two compounds are interconvertible in the presence of triphosphate isomerase enzyme.
• Of these compounds, oxidation takes place of only 3-PGAL. Hence as 3-PGAL is oxidized, DHAP keeps on changing to 3-PGAL.

3. Formation of two molecules of Pyruvic Acid:

3-phosphoglyceraldehyde (3PGAL) molecules are oxidized and converted into two molecules of pyruvic acid by the following reactions.

• Formation of 1-3 di-phosphoglyceric acid:
  3-PGAL molecule reacts with $$H_3PO_4$$ in the presence of enzyme dehydrogenase and forms 1-3 di-phosphoglyceraldehyde. This is oxidized to form 1-3 di-phosphoglyceric acid. This is called the oxidative step. Hydrogen ions released in this reaction reduce NAD+ to NADH + H+ which generates ATP through electron transport chain in case of aerobic respiration.
  
  
• Formation of 3-phosphoglyceric acid from 1-3 di-phosphoglyceric acid:
  In this reaction, one phosphate group is removed from 1-3 di-phosphoglyceric acid in the presence of enzyme di-phosphoglycerokinase. ADP is converted into ATP and 3-phosphoglyceric acid is formed.

• The isomerization of 3-phosphoglyceric acid into 2-phosphoglyceric acid: 3-phosophoglyceric acid is changed to its isomeric form 2-phosphoglyceric acid in the presence of enzyme phosphoglyceromutase.
  
• Formation of 2-phosphoenolpyruvate from 2phosphoglyceric acid: 2-phosphoglyceric acid changes to 2-phosphoenolpyruvate by losing one molecule of water in the presence of enzyme enolase.
  
• Formation of Pyruvic acid from 2-phosphoenolpyruvate:
  In the presence of enzyme pyruvic kinase, 2-phosphoenolpyruvate loses one phosphate and forms pyruvic acid and ATP molecule.

The name of end products formed by complete oxidation of glucose are $$CO_2$$, $$H_2O$$ and energy (ATP).

The respiratory substrate in the germinating seeds of castor is fat and its R.Q. is 0.7.

The value of R.Q. depends upon the nature of the respiratory substrates. The value of R.Q. is variable according to the substrate. The substrates having more oxygen in comparison to carbon, need less oxygen for oxidation and hence their R.Q. is more than 1, whereas those substrates where $$O_2$$ is less than carbon, require more $$O_2$$ for their oxidation and their R.Q. is less than one.
R.Q. of carbohydrates = 1.0
R.Q. of fats = 0.7 (<1)
R.Q. of proteins = 0.8 (<1)
R.Q. organic acids = 1.33 (>1)

	Aerobic respiration	Fermentation
1	Oxygen is used for deriving energy.	It occurs in the absence of oxygen.
2	It occurs in the cytoplasm and mitochondria.	It occurs in the cytoplasm.
3	End products are carbon dioxide and water.	End products are ethyl alcohol and carbon dioxide.
4	Complete oxidation of the respiratory substrate takes place.	Incomplete oxidation of the respiratory substrate takes place
5	38ATP molecules are produced during aerobic respiration.	Only 2 ATP molecules are produced.

The major steps in aerobic respiration and the sites where they occur are listed in the given table :

Aerobic respiration is divided into four phases, namely glycolysis, TCA cycle, ETS and oxidative phosphorylation. During aerobic respiration, ATP is produced in a stepwise manner. In this, the product of one phase forms the substrate of the other phase. Also, several molecules produced are involved in other biochemical processes. The respiratory substrates enter and withdraw from pathway as per requirement. ATP is utilized wherever required and enzymatic rates are generally controlled. This stepwise release of energy makes the system more efficient in obtaining and storing energy.

(a) List of enzymes involved in Krebs cycle includes:

Erythrocytes perform anaerobic respiration only because it lacks mitochondria in which Krebs' cycle takes place. So aerobic respiration is not possible.

Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms, typically involving effervescence and the giving off of heat. Both fermentation and glycolysis are processes of converting complex molecules such as sugars and carbohydrates into simple forms. Fermentation uses yeast or bacteria in the process of conversion whereas glycolysis does not. When sufficient oxygen is not present in the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by reduction of pyruvate to lactate.

C$$_6$$H$$_1$$$$_2$$O$$_6$$ $$\rightarrow$$ 2C$$_2$$H$$_5$$OH + 2CO$$_2$$ + 2ATP

The ratio of the volume of $$CO_2$$ released to the volume of $$O_2$$ used in respiration is called respiratory quotient. It is measured with the help of Ganong’s respirometer.

The value of R.Q. is helpful in determining the type of substrate used and the type of respiration. The value of R.Q. varies with the type of substrate used in respiration. This can be explained by the following:
1. R.Q. of carbohydrates:
When carbohydrate is a respiratory substrate and it is completely oxidized, its R.Q. is always unit or 1 (one). In this process, the volume of $$CO_2$$ released is always equal to the volume of $$O_2$$  used.

2. R.Q. of fats:
In oilseeds (Mustard, Groundnut, Cotton seeds, etc.) fats are used as a respiratory substrate during germination. In fat molecules, the amount of  $$O_2$$ is less in comparison to carbohydrates. Hence fats require more oxygen for their oxidation and so the value of R.Q. of fats is always less than one.

3. R.Q. of proteins:
Protein molecules also have an amount of $$O_2$$  less in comparison to the amount of carbon like fats. Hence oxidation of proteins also requires more of $$O_2$$ and hence the value of R.Q. of proteins is also less than one (<1), (0.7 -0.9). Proteins act as a respiratory substrate only in the absence of carbohydrates and fats.
Example: In Human proteins are used as respiratory substrate after prolonged fasting and its continuance is indicative of the death of Human.

4. R.Q. of organic acids:
In some plants, organic acids act as respiratory substrates. The molecules of organic acids have an amount of oxygen more than carbon. Hence oxidation of organic acids requires much less oxygen and release $$CO_2$$ in a large amount. Hence the value of R.Q. of organic acids is always more than one (>1) (4.0)

Respiratory Quotient of citric acid and malic acid is 1.14 and 1.33 respectively.

5. R.Q. of succulent or fleshy plants:
In fleshy or succulent plants such as Opuntia etc. although carbohydrate is used as a respiratory substrate it is not completely oxidized. As a result, intermediate compounds are formed but $$CO_2$$ is not released. Hence in these plants, the value of R.Q. is zero.

6. R.Q. in Anaerobic Respiration:
In anaerobic respiration, $$CO_2$$ is released but $$O_2$$ is not used hence in this type of respiration the value of R.Q. = ∝.

It is observed that lesser is the value of R.Q. more is the energy released in the process. Hence more energy is released from one molecule of fat whereas energy released in the oxidation of organic acid and during anaerobic respiration is much less.

Kreb's cycle is a cyclic process that occurs in the mitochondrial matrix, in the presence of oxygen. 
Step 1: The cycle starts with the condensation of an acetyl group with oxaloacetic acid and water to form citric acid, with a release of $$CoA$$ molecule. The enzyme involved in this conversion is citrate synthase. 
Step 2: Citrate then isomerises to form isocitrate. 
Step 3: The synthesis of citrate is followed by two successive steps of decarboxylation — first, the formation of a-ketoglutaric acid, and then, that of succinyl-$$CoA$$. 
Step 4: This succinyl-$$CoA$$ gets oxidised to form malic acid, which in turn gets converted into oxaloacetic acid, thereby allowing the cycle to continue. 

Answer:  2 molecules of ATP are produced by substrate-level phosphorylation

• Substrate level phosphorylation (SLP): The metabolic reaction wherein the transfer of high energy phosphate group from a substrate directly to ADP or simply the transfer of energy from high energy intermediate to a lower energy intermediate is called substrate-level phosphorylation.
• In the citric acid cycle (Krebs cycle, Tricarboxylic acid cycle) SLP is coupled with the removal of CoA from succinyl-CoA to produce succinate, in the presence of Succinyl-CoA synthetase.
• This leads to the transfer of high energy to form one GTP which leads to the formation of ATP.

Hence, 2 molecules of ATP are produced by substrate-level phosphorylation, since, one glucose is broken down into 2 pyruvate molecules and each pyruvate molecule enters the TCA cycle.