CBSE Class 11 Biology VBQs Respiration in Plants

Question. Respiratory quotient (R.Q.) is
(a) volume of O2 evolved/volume of CO2 consumed
(b) volume of CO2 evolved/volume of O2 consumed
(c) volume of O2 consumed/volume of CO2 evolved
(d) volume of CO2 consumed/volume of O2 evolved.

Answer : B

Question. In glycolysis, enzyme enolase produces
(a) phosphoglyceric acid
(b) phosphoenol pyruvate
(c) phosphoglyceraldehyde
(d) pyruvate.

Answer : B

Question. In which one of the following do the two names refer to tricarboxylic acid cycle?
(a) a-ketoglutaric acid and Krebs’ cycle
(b) Malic acid cycle and Kornberg cycle
(c) Citric acid cycle and Krebs’ cycle
(d) Oxaloacetic acid and Kornberg cycle

Answer : C

Question. The last product of glycolysis is degraded to CO2 and H2O in
(a) matrix of chloroplasts
(b) cytoplasm
(c) matrix of mitochondria
(d) inner membrane of mitochondria.

Answer : C

Question. The reaction involved in reduction of NAD+is
(a) Glucose Glucose 6–Phosphate
(b) Fructose 1, 6–diphosphate PGAL+DHAP
(c) Glucose 6–Phosphate Fructose 6– Phosphate
(d) Glyceraldehyde 3-phosphate 1, 3-biphosphoglycerate.

Answer : D

Question. In electron transport system (ETS) which of the following cytochromes reacts with oxygen?
(a) Cyt a3
(b) Cyt b
(c) Cyt b3
(d) Cyt b6

Answer : A

Question. The mobile electron carrier that transfers electrons between
(a) complex I and II
(b) complex II and III
(c) complex III and IV
(d) complex I and IV.

Answer : C

Question. Electron transport system (ETS) is present in
(a) inner mitochondrial membrane
(b) mitochondrial matrix
(c) chlorophyll
(d) cytosol.

Answer : A

Question. Which product of glycolysis is consumed in alcoholic fermentation?
(a) NADH
(b) ATP
(c) ATP and NADH
(d) CO2

Answer : A

Question. Common phase between aerobic and anaerobic modes of respiration is
(a) Krebs’ cycle
(b) EMP/glycolysis
(c) oxidative phosphorylation
(d) PPP.

Answer : A

Question. Which one is correct sequence in glycolysis?
(a) G-6-P → PEP → 3-PGAL → 3-PGA
(b) G-6-P → 3-PGAL → 3-PGA → PEP
(c) G-6-P → PEP → 3-PGA → 3-PGAL
(d) G-6-P → 3-PGA → 3-PGAL → PEP

Answer : B

Question. Refer to the given equation.
2(C51H98O6) + 145O2 → 102CO2 + 98H2O + Energy The R.Q. in this case is
(a) 1
(b) 0.7
(c) 1.45
(d) 1.62.

Answer : B

Question. The above figure indicates the inter-relationship among metabolic pathways. Now identify A to D.   (Img 58 )
        A     B      C      D
(a) Protein Acetyl CoA Fat DHAP
(b) Fat DHAP Protein Acetyl CoA 
(c) Acetyl CoA Fat DHAP Protein
(d) Fat DHAP Acetyl CoA Protein

Answer : B

Question. Amount of energy released during hydrolysis of a high energy bond of ATP is
(a) 73 kcal mol–1
(b) 0.73 kcal mol–1
(c) 3.4 kcal mol–1
(d) 7.3 kcal mol–1.

Answer : D

Question. During the process of respiration which of the following are released as a product?
(a) CO2, H2O and O2
(b) CO2, O2 and energy
(c) CO, H2O and energy
(d) CO2, H2O and energy

Answer : D

Question. Respiration is
(a) anabolic + exergonic
(b) catabolic + exergonic
(c) catabolic + endergonic
(d) anabolic + endergonic.

Answer : B 

Question. Study the given flow chart and identify A-D.
Glucose
Acetyl CoA
1, 3 bisphosphoglyceric acid
A B C D
(a) A–3-phosphoglycerate, B–2-phosphoglycerate,
C–pyruvic acid, D–phosphoenolpyruvate
(b) A–2-phosphoglycerate, B–3-phosphoglycerate,
C–pyruvic acid, D–phosphoenolpyruvate
(c) A–phosphoenolpyruvate, B–2-phosphoglycerate
C–3-phosphoglycerate, D–pyruvic acid
(d) A–3-phosphoglycerate, B–2-phosphoglycerate,
C–phosphoenolpyruvate, D–pyruvic acid

Answer : D

Question. In both lactic acid and alcoholic fermentation ATP released is less in amount as
(a) not all of energy is trapped in high energy bonds of ATP
(b) complete oxidation of organic substances  occur in the presence of oxygen.
(c) no enzyme is used in this process
(d) fermentation is energy consuming process.

Answer : A

Question. Select the incorrect options regarding anaerobic respiration or fermentation?
(a) Occurs inside the mitochondria
(b) Partial breakdown of glucose occurs
(c) Net gain of only 2 ATP molecules
(d) None of these

Answer : A

Question. The fruits stored in refrigerator or cold storage maintain their flavour and taste for longer period due to
(a) presence of excess of carbon dioxide
(b) non-availability of oxygen
(c) slower rate of respiration
(d) presence of excess humidity.

Answer : C

Question. Various electron carriers are arranged in ETS in the order of their
(a) decreasing energy level
(b) increasing energy level
(c) decreasing stability level
(d) increasing stability level.

Answer : A

Assertion & Reasoning Based MCQs

For question numbers 51-60, two statements are given-one labelled Assertion and the other labelled Reason.
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.

Question. Assertion : Respiratory pathway is an amphibolic pathway.
Reason : In respiration, there is breakdown of many substances (catabolism) and synthesis of many substances (anabolism) by respiratory
intermediates.

Answer : A

Question. Assertion : Substrate level phosphorylation is present in glycolysis.
Reason : Substrate level phosphorylation causes synthesis of ATP.

Answer : B

Question. Assertion : The pentose phosphate pathway is not a mainline pathway for the oxidation of glucose.
Reason : PPP generates energy in the form of ATP.

Answer : C

Question. Assertion : Oxidative phosphorylation is the synthesis of energy rich ATP.
Reason : The enzyme required for ATP synthesis is called ATP synthase.

Answer : B

Question. Assertion : Electron transport chain is also called cytochrome system.
Reason : Electron transport chain involves redox reactions.

Answer : B

Case Study Questions Respiration in Plants Class 11 Biology

Read the following passage and answer the questions from 46 to 50 given below.
Anaerobic respiration is the exclusive mode of respiration in parasitic worms, many prokaryotes, several unicellular eukaryotes and moulds. Based on major organic product formed, anaerobic respiration is divided into alcoholic fermentation and lactic acid fermentation. Anaerobic respiration cannot continue indefinitely (except in some micro-organisms) because of accumulation of poisonous compounds and less availability of energy per gram mole of food broken.

Question. Anaerobic respiration takes place in
(a) mitochondrion
(b) nucleus
(c) cytoplasm
(d) vacuole.

Answer : C

Question. Identify A and B in the given reaction.
Pyruvic acid + NADH Lactate dehydrogenase FMN, 'A' lactic acid + B.
(a) Fe3+, NADH2
(b) Mg2+, FADH2
(c) Zn2+, NAD+
(d) Fe+, ATP

Answer : C

Question. Select the correct option for the production of very little energy during anaerobic respiration?
(I) Incomplete breakdown of respiratory substrate.
(II) Electron transport chain is present.
(III) Oxygen is used for receiving electrons and protons.
(IV) NADH produced during glycolysis is used up
(a) I and IV
(b) II and III
(c) I only
(d) I and II

Answer : A

Question. In alcoholic fermentation the set of enzyme required are
(a) pyruvic are acid oxygenase and alcohol dehydrogenase
(b) lactate dehydrogenase and Phosphokinase
(c) phospho and pyruvate carboxylase and lactate dehydrogenase
(d) pyruvic acid decarboxylase and alcohol dehydrogenase.

Answer : D

Question. Which one of the following is complex V of the ETS?
(a) NADH dehydrogenase
(b) ATP synthase
(c) Succinate dehydrogenase
(d) Ubiquinone

Answer : B

Very Short Answer Type Questions  

Question. Enumerate the first step in cellular respiration.
Answer. Gylcolysis, which involves breaking down of sugar to pyruvic acid is the first step in cellular respiration.

Question. Why does anaerobic respiration/fermentation yield less energy than aerobic respiration?
Answer. It happens due to incomplete oxidation of the substrate.

Question. List two instances where lactic acid is formed by fermentation.
Answer. Instance where lactic acid is formed by fermentation are (i) During fermentation by lactic acid bacteria. (ii) During strenuous exercise, in the striated muscles in humans.

Question. At which step of respiration, hydrogen of NADH2 is used?
Answer. The hydrogen atoms accepted by NADH2 during glycolysis are introduced to route I of ETS. In this route 3 ATP molecules are produced.

Question. Some amino acids also enter the Krebs’ cycle after their deamination. Name two amino acids and the compounds formed by them.
Answer. Amino acids enter Krebs’ cycle directly as glutamate that forms a-ketoglutarate and aspartate that forms oxaloacetate after their deamintion.

Short Answer Type Questions

Question. (i) What is the end product of glycolysis in aerobes and where does this process occur?
(ii) List the conditions under which fermentation occurs in plant cells.
Answer. (i) End product of glycolysis is pyruvic acid Glucose + 2ADP + 2Pi + 2NAD → 2 Pyruvic acid + 2ATP + 2NaDH2 It occurs in cytoplasm, i.e., outside the mitochondria. (ii) Fermentation in plant cells occurs in absence of oxygen.

Question. What are the main steps in aerobic respiration? Where does it take place?
Answer. The main steps in aerobic respiration are as follows (i) Glycolytic breakdown of glucose into pyruvic acid. (ii) Oxidative decarboxylation of pyruvic acid to acetyl CO-A (acetyl coenzyme-A). (Link reaction) (iii) Oxidative and cyclic degradation of activated acetate derived from pyruvate to produce energy (Kreb’s cycle). Glycolysis takes place in cytoplasm whereas Link reaction and Kreb’s cycle takes place in matrix of mitochondria.

Question. What would be the RQ value of yeast if it were to respire glucose anaerobically?
Answer. The RQ will be infinity (∞) during anaerobic respiration. In anaerobic respiration, CO₂ is evolved but oxygen is not used. Therefore, RQ, in such a case, will be infinite.

Question. Write a short note on pyruvate dehydrogenasecomplex. 
Answer. The pyruvate dehydrogenase complex is a multimolecular aggregate of three enzymes: pyruvate decarboxylase whose prosthetic group is coenzyme thiamine pyrophosphate (TPP); dihydrolipoyl transacetylase (prosthetic group is lipoic acid) and dihydrolipoyl dehydrogenase (prosthetic group is flavin adenine dinucleotide FAD) and it also requires Mg2+. It carries out the given decarboxylation reaction that acts as a gateway step as it links glycolysis with Krebs’ cycle : Pyruvate + NAD+ + CoA Pyruvate dehydrogenase Acetyl CoA + NADH + H+ + CO2.

Question. (a) Which complex of ETS possess two copper centres?
(b) Name any three inhibitors that block electron transport by acting on NADH dehydrogenase.
Answer. (a) ETS complex IV comprises of cytochrome c oxidase that contains cytochrome a and cytochrome a3. The latter possesses two copper centres that help in transfer of electrons to oxygen. (b) The three inhibitors that block electron transport by acting on NADH dehydrogenase are : (i) Rotenone - a toxic plant substance (ii) Amytal - a barbiturate drug (iii) Piercidin - an antibiotic resembling ubiquinone

Question. What do you understand by substrate level phosphorylation? At which step in glycolysis,substrate level phosphorylation occurs?
Answer. The direct synthesis of ATP from metabolites is called substrate level phosphorylation. (i) In glycolysis, ATP is synthesised from 1, 3-biphosphoglycerate. The reaction is as follows : 1, 3 biphosphoglycerate + ADP Phosphoglycerate kinase Mg2+ 3-phosphoglycerate + ATP (ii) Another step at which substrate level phosphorylation occurs in glycolysis is during formation of pyruvate from phosphoenol pyruvate (PEP). PEP + ADP Pyruvate kinase Mg2+, K+ Pyruvate + ATP

Question. Anaerobic respiration produces very little energy as compared to aerobic respiration. Give reasons.
Answer. Anaerobic respiration produces very little energy because : (i) There is incomplete breakdown of respiratory substrate. (ii) At least one of the products of anaerobic respiration is organic. It can be further oxidised to release energy. (iii) NADH produced during glycolysis is often used up. (iv) ATP formation does not occur during regeneration of NAD+. (v) Electron transport chain is absent. (vi) Oxygen is not used for receiving electrons and protons.

Long Answer Type Questions 

Question. In the following flow chart, replace the symbols a,b,c and d with appropriate terms.
Briefly explain the process and give any two applications of it.   (Img 63)
Answer. a = Glyceraldehyde 3-phosphate
b = Phosphoenol pyruvic acid
c = Ethanol
d = Lactic acid
The given metabolic pathway in figure is anaerobic respiration or fermentation. 
Anaerobic respiration is an enzyme mediated energy liberating catabolic process of step-wise but incomplete breakdown of organic substrate without using oxygen as an oxidant. Energy is liberated during breaking of bonds between various types of atoms. The common products of anaerobic respiration are CO₂, ethyl alcohol and lactic acid.
C₆H₁₂O₆ → Enzymes 2CO₂ + 2C₂H₅OH + 59 kcal Ethyl alcohol
C₆H₁₂O₆ → Enzymes 2C₃H6O₃ + 36 kcal Lactic acid It is of two types:
(i) Alcoholic fermentation : It takes place in two steps:

It produces lactic acid from pyruvate and does not release carbon dioxide.
Applications of anaerobic respiration are as follows : 
(i) It has important role in brewing (Saccharomyces cerevisiae), baking, vinegar and milk industries (Streptococcus lactis). Vinegar is produced by fermentation of sugars in the
presence of acetic acid bacteria (Acetobacter aceti).
(ii) Production of industrial alcohols and organic acids like citric acid and malic acid.

Question. Respiratory pathway is believed to be a catabolic pathway. However, nature of TCA cycle is amphibolic. Explain.
Answer. The amphibolic nature of TCA cycle can be explained by the following reasons :
Amphibolic pathway is the one which is used for both breakdown (catabolism) and build-up (anabolism) reactions.
Respiratory pathway is mainly a catabolic process which serves to run the living system by providing energy. The pathway produces a number of intermediates. Many of them are raw materials for building up both primary and secondary metabolites.
(i) Acetyl CoA is helpful not only is using fatty acids in  Krebs’ cycle but is also raw material for synthesis of fatty acids, steroids, terpenes, aromatic compounds and carotenoids.
(ii) a-ketoglutarate is organic acid which forms glutamate (an  important amino acid) on amination.
(iii) Oxaloacetate on amination produces aspartate (anotherimportant amino acid).
(iv) Both aspartate and glutamate are components of proteins. Pyrimidines and alkaloids are other products.
(v) Succinyl CoA forms cytochromes and chlorophyll.

Question. Draw a schematic representation of citric acid cycle.
Answer. A schematic representation of citric acid cycle (Krebs’ cycle) is given below 

1. All living organisms need energy for carrying out daily life activities → the process of breathing is very much connected to the process of release of energy from food. → All the energy required for ‘life’ processes is obtained by OXIDATION OF ‘FOOD’.

2. The PROCESS OF PHOTOSYNTHESIS → Light energy convert into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch.

3. Animals are HETEROTROPHIC, i.e., they obtain food from plants

4. SAPROPHYTES like fungi are dependent on dead and decaying matter.

5. Ultimately all the food that is respired for life processes comes from photosynthesis.

6. Photosynthesis 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).

7. 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.

8. 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.

9. 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 stepwise reactions controlled by enzymes, and it is trapped as chemical energy in the form of ATP.

10. 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
• Carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell.

11. Plants require O₂ for respiration to occur and they also give out CO₂ . Hence, plants have systems in place that ensure the availability of O₂ . Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose.

12. Why plant don’t have evolved respiratory system like animals?
• Each plant part takes care of its own gas-exchange needs.
• Very little transport of gases from one plant part to another.
• Plants do not present great demands for gas exchange.
• Roots, stems and leaves respire at rates far lower than animals do.
• During photosynthesis availability of O₂ is not a problem in these cells since O₂ is released within the cell.
• The 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. ‘

13. 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.

14. The complete combustion of glucose, which produces CO₂ and H₂ O as end products, yields energy most of which is given out as heat.

15. The plant cell 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.

16. During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products.

17. Anaerobic organisms →
• First cells on this planet lived in an atmosphere that lacked oxygen.
• Even among present-day living organisms, several are adapted to anaerobic conditions.
• Some of these organisms are FACULTATIVE ANAEROBES, while in others the requirement for anaerobic condition is OBLIGATE ANAEROBES.
• 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.

18. GLYCOLYSIS
• The term glycolysis → Greek words, glycos for sugar, and lysis for splitting.
• The scheme of glycolysis → given by Embden, Meyerhof, and Parnas
• Also known as → EMP pathway.
• In anaerobic organisms, it is the only process in respiration.
• Glycolysis occurs in the cytoplasm of the cell
• Present in all living organisms.
• In this process, glucose undergoes partial oxidation
✓ Form two molecules of pyruvic acid
✓ Oxygen not consumed
✓ CO₂ not released
• In plants, this glucose is derived from sucrose
✓ which is the end product of photosynthesis, or from storage carbohydrates.
✓ Sucrose is converted into glucose and fructose by the enzyme, invertase,
✓ 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.
• In glycolysis, A CHAIN OF TEN REACTIONS, under the control of different enzymes, takes place to produce pyruvate from glucose.

• REMEMBER THE STEPS
✓ At which utilisation or synthesis of ATP and NADH + H+ take place.
✓ ATP IS UTILISED AT TWO STEPS:
1. first in the conversion of glucose into glucose 6-phosphate and
2. 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).
• There is one step where NADH + H+ is formed from NAD+ ;
✓ 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.
• How many ATP molecules are directly synthesised in this pathway from one glucose molecule → Total 4 ATP
• PYRUVIC ACID → the KEY PRODUCT OF GLYCOLYSIS.
• What is the metabolic fate of pyruvate?
✓ This depends on the cellular need.
✓ THERE ARE THREE MAJOR WAYS –
1. LACTIC ACID FERMENTATION,
2. ALCOHOLIC FERMENTATION
3. AEROBIC RESPIRATION.
• Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
• For the complete oxidation of glucose to CO₂ and H₂O → organisms adopt Krebs’ cycle (aerobic respiration); This requires O₂ supply.

19. FERMENTATION
• Best example is yeast
• Incomplete oxidation of glucose
• Under anaerobic conditions
• By sets of reactions → Pyruvic acid is converted to CO₂ and ethanol.
• The enzymes required are:
✓ pyruvic acid decarboxylase
✓ alcohol dehydrogenase
• Some bacteria produce lactic acid from pyruvic acid.
• 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 when one molecule of glucose is fermented to alcohol or lactic acid → 2 ATP
• 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? → Around 4 to 9% (Not more than 12%)
✓ Beer → 4% to 8%
✓ Wine → 8 to 11%
• How do you think alcoholic beverages of alcohol content greater than this concentration are obtained? → By distillation
✓ Whisky/ Rum/ Vodka / Scotch/Gin → 36% to 60%
• What then is the process by which complete oxidation of glucose and extract the energy stored to synthesise a larger number of ATP molecules needed for cellular metabolism? → Aerobic respiration (Krebs cycle & ETS)
• This type of respiration is most common in higher organisms.

20. AEROBIC RESPIRATION
• Take place within the mitochondria
• For this → 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 CO₂ .
✓ The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂ with simultaneous synthesis of ATP.
• THE FIRST PROCESS → In the matrix of the mitochondria (Kreb’s cycle)
• THE SECOND PROCESS → On the inner membrane of mitochondria (ETS)
• Pyruvate 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.
• During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (from one glucose).
• 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.

21. TRICARBOXYLIC ACID CYCLE
• The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid
✓ Acceptor → OAA
✓ First formed chemical → CITRIC ACID (TCA)
✓ Catalysed by the enzyme CITRATE SYNTHASE
✓ 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 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.
• There are three points in the kreb’s cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH₂ .
• 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 FADH₂ respectively.

22. Electron Transport System (ETS) and Oxidative Phosphorylation
• These steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH₂ .
• Through the electron transport system → the electrons are passed on to O₂ resulting in the formation of H₂ O.
• The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS)
• It is present in the inner mitochondrial membrane.
• Electrons from NADH 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 FADH₂ (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 1 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 a3 , 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 phospha 
• 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 FADH₂  produces 2 molecules of ATP.
• 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.
• 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.
• For the PRODUCTION OF PROTON GRADIENT required for phosphorylation in respiration → The energy of oxidation-reduction utilised so the process is called OXIDATIVE PHOSPHORYLATION.
• The energy released during the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V).
✓ This complex consists of two major components, F1 and F0.
✓ The F1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate.
✓ F0 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 F1 component for the production of ATP.
• For each ATP produced, 2H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.

23. THE RESPIRATORY BALANCE SHEET
• There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
• Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO₂ and H₂O.
• 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.

24. 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 do not enter the respiratory pathway at the first step.
• 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 ome stage within the Krebs’ cycle or even as pyruvate or acetyl CoA.
• Since respiration involves breakdown of substrates → the respiratory process are catabolic process and the respiratory pathway as a catabolic pathway.
• Many compounds that would be withdrawn from the respiratory pathway for the synthesis of the various substrates. →
✓ To synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it.
✓ 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.

25. RESPIRATORY QUOTIENT 
• The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
• The respiratory quotient depends upon the type of respiratory substrate used during respiration.
• Carbohydrates → RQ 1
• Fats → RQ is less than 1.
• Proteins → RQ is 0.9.