Respiration in Plants Class 11 Notes

Cellular respiration is the process by which cells break down organic molecules (mainly glucose) to release energy in the form of ATP. It can be aerobic (with O₂) or anaerobic (without O₂, i.e. fermentation). In eukaryotes, glycolysis occurs in the cytoplasm, while the Krebs cycle and ETS occur in the mitochondria.

• Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH.
• Link reaction (mitochondrial matrix): pyruvate → acetyl CoA + CO₂ + NADH.
• Krebs cycle / TCA cycle (mitochondrial matrix): acetyl CoA is fully oxidised to CO₂; produces NADH, FADH₂ and ATP.
• Electron Transport System and Oxidative Phosphorylation (inner mitochondrial membrane): NADH and FADH₂ donate electrons to generate a proton gradient that drives ATP synthase — the bulk of ATP production.

Glycolysis (Embden–Meyerhof–Parnas pathway) is the universal first step of glucose breakdown, occurring in the cytoplasm of all living cells. It is anaerobic — it does not require oxygen. A 6-carbon glucose is split into two 3-carbon pyruvate molecules through a 10-step pathway.

Energy investment and payoff

• Energy investment phase: 2 ATP are consumed to phosphorylate glucose (glucose → fructose-1,6-bisphosphate), making it reactive.
• Energy payoff phase: the 6-carbon sugar is split into two 3-carbon molecules, each of which is oxidised and converted to pyruvate. In the process 4 ATP (by substrate-level phosphorylation) and 2 NADH are produced.
• Net gain per glucose: 2 ATP + 2 NADH + 2 pyruvate.

When oxygen is unavailable (anaerobic conditions), pyruvate from glycolysis is further metabolised by fermentation to regenerate NAD⁺ from NADH, allowing glycolysis to continue producing a small amount of ATP.

• Alcoholic fermentation: pyruvate → acetaldehyde + CO₂ → ethanol (C₂H₅OH), catalysed by pyruvate decarboxylase and alcohol dehydrogenase. Occurs in yeast and some bacteria.
• Lactic acid fermentation: pyruvate is directly reduced to lactate (CH₃CH(OH)COOH) by lactate dehydrogenase, regenerating NAD⁺. Occurs in animal muscle cells during intense exercise and in certain bacteria (Lactobacillus).
• Fermentation yields only 2 ATP per glucose (from glycolysis) — far less efficient than aerobic respiration.

Krebs cycle / TCA cycle

The Krebs cycle (tricarboxylic acid cycle) takes place in the mitochondrial matrix. Acetyl CoA (2C) combines with oxaloacetate (OAA, 4C) to form citrate (6C). Through eight enzymatic steps citrate is oxidised and decarboxylated, regenerating OAA. Each turn produces 3 NADH, 1 FADH₂ and 1 GTP (equivalent to 1 ATP). Since one glucose yields 2 acetyl CoA, two turns of the cycle occur per glucose.

• Key intermediates: citrate (6C) → isocitrate → α-ketoglutarate (5C) → succinyl CoA → succinate → fumarate → malate → OAA (4C).
• Two decarboxylation steps release 2 CO₂ per turn (one at isocitrate dehydrogenase, one at α-ketoglutarate dehydrogenase).
• Net per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 GTP + 4 CO₂.

The ETS is a series of protein complexes (Complex I–IV) embedded in the inner mitochondrial membrane. NADH donates electrons at Complex I; FADH₂ donates at Complex II. Electrons pass through ubiquinone (CoQ), the cytochrome bc₁ complex, cytochrome c, and finally to Complex IV (cytochrome c oxidase), where O₂ is the terminal electron acceptor, forming H₂O.

• As electrons flow through Complexes I, III and IV, protons (H⁺) are pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient (proton motive force).
• Protons flow back into the matrix through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP — this is chemiosmosis (Peter Mitchell's hypothesis).
• Oxygen is essential as the final electron acceptor: O₂ + 4H⁺ + 4e⁻ → 2H₂O. Without O₂ the ETS stalls, the proton gradient collapses, and oxidative phosphorylation ceases.

The respiratory balance sheet tracks ATP production through each phase of aerobic respiration per molecule of glucose. It is theoretical and assumes ideal conditions; actual yields may be slightly lower due to membrane leakage and energy costs of metabolite transport.

• Glycolysis: 2 ATP + 2 NADH → ~5 ATP (via shuttle to mitochondria).
• Link reaction: 2 NADH → ~5 ATP.
• Krebs cycle (2 turns): 2 GTP (2 ATP) + 6 NADH (~15 ATP) + 2 FADH₂ (~3 ATP).
• Total: ~36–38 ATP per glucose. The range depends on whether the glycerol-phosphate shuttle (~36 ATP) or the malate-aspartate shuttle (~38 ATP) is used to transfer cytoplasmic NADH into mitochondria.

• Carbohydrates (glucose): RQ = 1 (6CO₂/6O₂ = 1).
• Fats: RQ ≈ 0.7 (more O₂ is required per carbon for oxidation of fats).
• Proteins: RQ ≈ 0.9.
• Anaerobic respiration in yeast: RQ is infinite (CO₂ released but no O₂ consumed).
• Organic acids (e.g. oxalic acid): RQ > 1 (more CO₂ released than O₂ consumed).

Example: Calculate the total number of ATP molecules produced from complete aerobic oxidation of one molecule of glucose, assuming the malate-aspartate shuttle.

Solution: Glycolysis yields 2 ATP (net) + 2 NADH. Using the malate-aspartate shuttle, each NADH yields ~2.5 ATP → 2 × 2.5 = 5 ATP. Link reaction yields 2 NADH → 5 ATP. Krebs cycle (2 turns): 2 GTP = 2 ATP, 6 NADH → 15 ATP, 2 FADH₂ → 3 ATP. Total = 5 + 5 + 2 + 15 + 3 = 30 ATP from oxidative phosphorylation + 2 ATP from glycolysis + 2 GTP = 34? Recalculating: 2 (glycolysis ATP) + 5 (glycolysis NADH) + 5 (link NADH) + 2 (Krebs GTP) + 15 (Krebs NADH) + 3 (Krebs FADH₂) = 32 ATP. Different textbooks give 36–38 depending on the shuttle and rounding; the key is that glycolysis NADH and Krebs FADH₂ contribute differently.

Example: A fat molecule with the formula C₅₇H₁₁₀O₆ is completely oxidised. Why is its RQ lower than that of glucose?

Solution: Fats have a higher proportion of carbon and hydrogen relative to oxygen compared to carbohydrates. More O₂ molecules are needed to fully oxidise the extra C–H bonds to CO₂ and H₂O. For a typical fat, RQ ≈ 0.7 because the volume of CO₂ released is only about 70% of the O₂ consumed. Glucose (C₆H₁₂O₆) already contains sufficient oxygen for its own oxidation, giving RQ = 1.

Exercises (NCERT Q1–Q12)

• 1) Differentiate between (a) Respiration and Combustion (b) Glycolysis and Krebs’ cycle (c) Aerobic respiration and Fermentation
• 2) What are respiratory substrates? Name the most common respiratory substrate.
• 3) Give the schematic representation of glycolysis?
• 4) What are the main steps in aerobic respiration? Where does it take place?
• 5) Give the schematic representation of an overall view of Krebs’ cycle.
• 6) Explain ETS.
• 7) Distinguish between the following: (a) Aerobic respiration and Anaerobic respiration (b) Glycolysis and Fermentation (c) Glycolysis and Citric acid Cycle
• 8) What are the assumptions made during the calculation of net gain of ATP?
• 9) Discuss “The respiratory pathway is an amphibolic pathway.”
• 10) Define RQ. What is its value for fats?
• 11) What is oxidative phosphorylation?
• 12) What is the significance of step-wise release of energy in respiration?

Answers at a glance

1. 1
   Q1 → (a) Respiration and combustion. Respiration is a biochemical process occurring inside the living cell, as a series of enzyme-controlled steps catalysed by the enzymes of the cell, whereas combustion is a chemical process, a physical reaction of a substance with oxygen, not catalysed by the cell enzymes in that sense. Respiration is slow, controlled and stepwise, the energy being released in small instalments so that it can be trapped in the form of ATP, whereas combustion is rapid, violent and highly exothermic, all the energy being released at once as heat. Respiration is therefore energy-efficient, conserving part of the energy in ATP, whereas combustion converts the energy entirely into heat and conserves none, which is why glucose oxidised in a cell yields usable energy and the same glucose burnt in air yields only heat. Respiration occurs at a comparatively low temperature and its rate is regulated by the cell according to the availability of the substrate, of the ADP and of the oxygen, whereas combustion requires a high temperature to start and once started proceeds to completion. And respiration occurs in living cells only, being enzymatic, whereas combustion may be brought about in dead matter and needs no living system. It should be noted that the overall chemical equation of the oxidation of glucose is broadly the same in both, glucose plus six oxygen giving six carbon dioxide and six water with energy, so the difference lies not in the chemistry but in the route, the control and the fate of the energy. (b) Glycolysis and Krebs cycle. The site is the cytoplasm for the glycolysis and the mitochondrial matrix for the Krebs cycle, the whole of the rest of the aerobic respiration, that is the pyruvate oxidation and the Krebs cycle, occurring in the mitochondrion. The nature is that the glycolysis is anaerobic and needs no oxygen, taking place whether or not the oxygen is present, whereas the Krebs cycle is aerobic and cannot proceed without the oxygen, since the NADH and the FADH2 it forms must be reoxidised by the electron transport system. The substrate is the glucose or the starch hydrolysed to glucose for the glycolysis, and the acetyl CoA, the two-carbon compound formed from the pyruvate by the pyruvate oxidation, for the Krebs cycle. The carbon accounting is that one molecule of the six-carbon glucose gives two molecules of the three-carbon pyruvate in the glycolysis, whereas one turn of the Krebs cycle, which occurs twice for one glucose, accepts one molecule of the two-carbon acetyl CoA and releases two molecules of the one-carbon carbon dioxide. In the sequence the glycolysis is the first and preparatory step and the Krebs cycle is the final common oxidative pathway, the Krebs cycle being also the point at which the carbohydrate, the fat and the protein all converge, since the acetyl CoA derived from the fat and the protein enters it. The yield differs, the glycolysis giving a net 2 ATP and 2 NADH and one turn of the Krebs cycle giving 3 NADH, 1 FADH2 and 1 ATP. (c) Aerobic respiration and fermentation. The oxygen is required in the aerobic respiration and not in the fermentation. The oxidation is complete in the aerobic respiration, the substrate being fully oxidised to carbon dioxide and water, and partial in the fermentation, the substrate being only partly broken down to a product such as the lactic acid or the ethanol that still contains unreleased energy. The site is the mitochondrion, its matrix and inner membrane, for the aerobic respiration, and the cytoplasm for the fermentation, since both the fermentation and the glycolysis that precedes it are cytoplasmic with soluble enzymes. The energy yield is large in the aerobic respiration, some 38 ATP per glucose in a eukaryotic cell, and very small in the fermentation, a net 2 ATP, so most of the energy of the substrate remains locked in the products. The end products are carbon dioxide and water in the aerobic respiration, lactic acid in the lactic acid fermentation of the animal muscle and of the bacterium, and ethanol with carbon dioxide in the alcoholic fermentation of the yeast and of the plant. The relation between them is that the fermentation is the anaerobic respiration and uses the same glycolysis as the aerobic one, the difference lying in the fate of the pyruvate, which is oxidised completely in the aerobic respiration and reduced to an organic end product in the fermentation.
2. 2
   Q2 → The respiratory substrates are the organic substances that are oxidised in the cells to release the energy required for the cellular activities, so they are the fuels of the cell, and they are broken down to simpler substances, ultimately the carbon dioxide and the water, with the energy captured as ATP. They fall into three classes, in the order of the preference of the cell. The carbohydrates are the most preferred and the most commonly used, and the chief carbohydrate substrate is glucose, which is produced by the hydrolysis of the starch, the glycogen and the other polysaccharides, and which is also the sugar of the blood, so that the cells have a constant supply of it. The fats come second and are used when the carbohydrate is not available, being stored in the adipose tissue as the triglycerides, and they yield rather more energy per unit mass, about 9 kcal a gram against the 4 kcal of a carbohydrate, so they serve as the reserve fuel. The proteins come last and are used only when both the carbohydrate and the fat are exhausted, being the structural proteins and the enzymes of the cell, so their use for energy is necessarily a last resort. The most common respiratory substrate is glucose. It is the most readily available, since it is transported as the glucose of the blood and is constantly supplied to the cells from the digestion of the dietary carbohydrate; it is the substrate for which the whole enzyme machinery of the glycolysis is present and has evolved, so it is the direct fuel of the glycolysis; and it is the substrate whose oxidation gives the classical respiratory quotient of 1, since equal volumes of carbon dioxide and oxygen are then involved. It is worth adding the comparison of the energy yields, that a gram of carbohydrate gives about 4 kcal, a gram of fat about 9 kcal and a gram of protein about 5.6 kcal, so the fat is the more economical store of energy although the glucose is by far the more convenient fuel and the one actually used in the great majority of the respirations of the body.
3. 3
   Q3 → The glycolysis is represented in the following schematic form, in which the whole pathway may be drawn in two phases, the preparatory and the payoff. In the preparatory or energy-investment phase, one molecule of the six-carbon glucose is phosphorylated at the sixth carbon by the hexokinase, at a cost of one ATP, to the glucose 6-phosphate, which is isomerised to the fructose 6-phosphate and then phosphorylated at the first carbon by the phosphofructokinase, at a cost of a second ATP, to the fructose 1,6-bisphosphate, so that 2 ATP are used per glucose. The fructose 1,6-bisphosphate is then cleaved by the aldolase into two molecules of the three-carbon triose phosphate, the dihydroxyacetone phosphate and the glyceraldehyde 3-phosphate, and the dihydroxyacetone phosphate is isomerised to a second glyceraldehyde 3-phosphate, so that one glucose yields two molecules of the three-carbon glyceraldehyde 3-phosphate. In the payoff or energy-return phase, each of these two is oxidised and phosphorylated by the glyceraldehyde 3-phosphate dehydrogenase, using inorganic phosphate and the NAD plus, to form the 1,3-bisphosphoglycerate and reducing one NAD plus to NADH plus H plus, so that 2 NADH are formed per glucose, and each 1,3-bisphosphoglycerate is then phosphorylated by the phosphoglycerate kinase to the 3-phosphoglycerate, yielding 2 ATP per glucose, that is 4 ATP for the two trioses. The 3-phosphoglycerate is then converted to the 2-phosphoglycerate by the phosphoglycerate mutase, a molecule of water is removed by the enolase to form the phosphoenolpyruvate, which is the highest-energy compound of the glycolysis, and the phosphoenolpyruvate is phosphorylated by the pyruvate kinase to pyruvate, giving a further 2 ATP per glucose. The net yield is therefore boxed as 2 minus 2 minus 2 plus 2 plus 2 plus 2, that is a net 2 ATP of direct energy and 2 NADH per molecule of glucose. The site is the cytoplasm, the cytosol, and the whole glycolysis is anaerobic, requiring no oxygen, and it is the same in both the aerobic and the anaerobic respiration, since only the subsequent fate of the pyruvate differs. If the 2 NADH are oxidised by the aerobic route through the NADH dehydrogenase of the matrix they yield 2 times 3, that is 6, further ATP, so the aerobic yield of the glycolysis is 8 ATP, whereas in the fermentation the yield is the direct 2 ATP alone. The glycolysis is thus the classic example of substrate-level phosphorylation, the ATP being made by the direct transfer of a phosphate from the substrate, here the 1,3-bisphosphoglycerate and the phosphoenolpyruvate, to the ADP and not by the electron transport chain.
4. 4
   Q4 → The main steps of the aerobic respiration are four, and each has its own site. The first is the glycolysis, the Embden-Meyerhof-Parnas pathway, which is the anaerobic phase common to both the aerobic and the anaerobic respiration, and it takes place in the cytoplasm. The second is the oxidation of the pyruvate, the link or transition reaction, in which the two molecules of pyruvate produced by the glycolysis are oxidatively decarboxylated to give two molecules of acetyl CoA with the release of carbon dioxide, and it takes place in the mitochondrial matrix. The third is the Krebs cycle, also called the citric acid cycle or the tricarboxylic acid cycle, in which the acetyl CoA is completely oxidised to carbon dioxide, and it takes place in the mitochondrial matrix. The fourth is the oxidative phosphorylation, the electron transport system, in which the reduced coenzymes, the NADH and the FADH2, are reoxidised and the energy of the electrons that pass down the chain is used to synthesise ATP, and it takes place on the inner membrane of the mitochondrion, at the cristae. The whole pathway therefore condenses to one line, glucose in the cytoplasm giving two pyruvate with 2 ATP and 2 NADH; the two pyruvate in the matrix giving two acetyl CoA with 2 NADH and no ATP of their own; the two acetyl CoA in the matrix giving four carbon dioxide with 6 NADH, 2 FADH2 and 2 ATP; and the 10 NADH, two of them of glycolytic origin, together with the 2 FADH2, being oxidised on the inner membrane to give six water and 34 ATP, giving a total of 38 ATP for each glucose in a eukaryotic cell. Two observations complete the picture. Only the glycolysis occurs in the cytoplasm and needs no oxygen; the link reaction, the Krebs cycle and the oxidative phosphorylation all require the oxygen. And the compartmentation is functionally significant rather than merely structural, since the Krebs cycle and the electron transport system are held apart in the matrix and in the membrane respectively, it being the impermeability of the inner membrane to the NADH that forces the respiratory hydrogen formed in the cytosol to be carried in by the shuttle systems, and it being the fact that the NADH delivers its electrons at the first complex while the FADH2 delivers them at the second and therefore further down the chain, that fewer protons are pumped and fewer ATP made, so that the NADH yields three ATP and the FADH2 only two. Finally, the respiration of a eukaryotic cell is in the mitochondrion for the functional reason that the pathway generates the reduced coenzymes in the matrix and the protons used by the ATP synthase are pumped across the inner membrane, whereas the bacterial cell has no such organelle, so all of its aerobic respiration occurs in the cytoplasm and the plasma membrane.
5. 5
   Q5 → The overall view of the Krebs cycle may be represented as a ring of the following form. The entry is the condensation of the four-carbon oxaloacetic acid, the acceptor of the cycle, with the two-carbon acetyl CoA, derived from the pyruvate, catalysed by the citrate synthase, to give the six-carbon citric acid, the citrate, and this is the only step in the cycle at which the carbon number increases and the only step at which the carbon of the acetyl group enters. The citric acid is then converted through the isocitric acid, the six-carbon, and the isocitric acid is oxidatively decarboxylated by the isocitrate dehydrogenase, releasing the first molecule of carbon dioxide and reducing one NAD plus to NADH plus H plus. The resulting five-carbon alpha-ketoglutaric acid is oxidatively decarboxylated by the alpha-ketoglutarate dehydrogenase complex, releasing the second carbon dioxide and reducing a second NAD plus to NADH plus H plus, and giving the four-carbon succinyl CoA, and the conversion of the succinyl CoA to the succinate is the one substrate-level phosphorylation of the cycle, the phosphate of the coenzyme being transferred to the GDP to make a GTP, or an ATP. The succinate is then oxidised to the fumarate by the succinate dehydrogenase, which is a flavoprotein and passes the hydrogen to the FAD, making the FADH2, the fumarate is hydrated to the malate, and the malate is oxidised by the malate dehydrogenase to regenerate the oxaloacetic acid, so closing the ring, and at the same time reducing a third NAD plus to NADH plus H plus. Written in full, the ring is therefore the oxaloacetic acid, 4C, plus the acetyl CoA, 2C, giving the citric acid, 6C, giving the isocitric acid, 6C, losing a CO2 to the alpha-ketoglutaric acid, 5C, losing a CO2 to the succinyl CoA, 4C, making a GTP, then the succinate, 4C, the fumarate, 4C, the malate, 4C, and back to the oxaloacetic acid, 4C, so that the carbon count returns to four. The accounting per turn is that one molecule of the two-carbon acetyl CoA enters, two molecules of the carbon dioxide leave, and three molecules of the NADH, one of the FADH2 and one of the ATP or the GTP are formed; since one glucose yields two acetyl CoA, the cycle turns twice for each glucose, giving for one glucose 4 CO2, 6 NADH, 2 FADH2 and 2 ATP, in addition to the 2 NADH and 2 ATP of the glycolysis and the 2 NADH of the link reaction, which makes no ATP of its own. It is to be noted that the two carbon dioxide released in a turn are not the two carbons of the acetyl CoA that has just entered, but come from the oxaloacetic acid that accepted it, so that the acetyl group is completely oxidised only over the course of several successive turns, and that the cycle is amphibolic, since it also serves the biosynthesis of the succinyl CoA in the porphyrin synthesis and of the oxaloacetate and the alpha-ketoglutarate in the amino acid synthesis.
6. 6
   Q6 → The ETS, the electron transport system, also called the electron transport chain, is the fourth and the last step of the aerobic respiration, and it is the machinery by which the energy of the electrons in the reduced coenzymes, the NADH and the FADH2, is converted into ATP, so it is the site of the oxidative phosphorylation. Its location is the inner membrane of the mitochondrion, at the cristae, and in a bacterial cell the plasma membrane, and the reason the membrane is the site is that the ATP synthase is embedded in it and the energy of the electrons is used to pump the protons from the matrix into the intermembrane space, creating an electrochemical gradient across the membrane which then drives the synthesis of the ATP. The mechanism is as follows. The electron of the NADH is handed to the first complex, the NADH dehydrogenase or the complex I, while the electron of the FADH2 is passed to the FAD of the second complex, the succinate dehydrogenase or the complex II, which is the only component of the chain that does not pump protons, being the same enzyme as that of the Krebs cycle and receiving the electrons from the FAD of the FADH2 formed within that cycle, and this is why the two carriers finally yield different amounts of ATP. From the first and the second complexes the electrons pass to the ubiquinone or the coenzyme Q, a mobile lipid-soluble carrier, then to the third complex, the cytochrome bc1 complex, then to the cytochrome c, a second mobile carrier, then to the fourth complex, the cytochrome c oxidase or the complex IV, and finally to the terminal acceptor, the molecular oxygen, which combines with the protons of the matrix to form water, so the oxygen is the last acceptor in the chain and this is why the process is aerobic and why the absence of the oxygen stops the chain. The transfer of the electrons from one component to the next releases energy at each step, and this energy is used to pump the protons from the matrix into the intermembrane space, the first, the third and the fourth complexes pumping, about ten protons per pair of electrons in the NADH case and about six in the FADH2 case, the electrons in the latter entering at the second complex and so bypassing the first. Since the membrane is otherwise impermeable to the protons, they accumulate in the intermembrane space and create a proton motive gradient, the matrix becoming negative and alkaline, and the protons then flow back into the matrix through the F0F1 ATP synthase, which spans the membrane, the flow turning the rotor of the synthase and so driving the catalytic formation of ATP from the ADP and the inorganic phosphate at the F1 head, the ATP then leaving the mitochondrion in exchange for the ADP and the phosphate. The yields are about 34 ATP per glucose in a eukaryotic cell, since the ten NADH, two of glycolytic and eight of matrix origin, give 30 ATP at three each and the two FADH2 give four at two each. The chain itself consists of the coenzyme Q, the cytochrome bc1 complex, the cytochrome c and the cytochrome c oxidase in a definite sequence within the membrane, the cytochrome c being the component on the outer surface and the cytochrome c oxidase on the inner, so the electrons cross the membrane from one to the other and the cytochrome c is the mobile shuttle between them; and the difference of three ATP for the NADH against two for the FADH2 is not a matter of the number of the electrons but of the point at which they enter the chain, since the FADH2 enters at the second complex and forgoes the pumping at the first.
7. 7
   Q7 → (a) Aerobic respiration and anaerobic respiration. The aerobic respiration requires the molecular oxygen as the terminal acceptor of the electron transport chain, whereas the anaerobic respiration does not, and it is the absence of the oxygen that makes the fermentation the available route. The oxidation is complete in the aerobic respiration, the substrate being fully oxidised to carbon dioxide and water, whereas in the anaerobic respiration it is only partly broken down, the carbon dioxide being formed without the oxygen being used and the products being organic compounds such as the lactic acid or the ethanol that still hold most of the energy. The site is the mitochondrion, its matrix and inner membrane, in the aerobic respiration, and the cytoplasm alone in the anaerobic respiration. The energy yield is about 38 ATP per glucose in a eukaryotic cell against a net 2 in the anaerobic respiration, so the aerobic releases some nineteen times as much energy. The enzymes differ too, the aerobic using the oxidoreductases of the membrane and the anaerobic the soluble enzymes of the cytoplasm. And the two are related rather than alternative, since the glycolysis is the common first phase of both, so the anaerobic respiration is not a different route but the same route with its last two steps replaced by the fermentation. (b) Glycolysis and fermentation. The glycolysis is the normal, universal and obligate pathway of the glucose breakdown, occurring in every living cell of every organism and requiring no oxygen, whereas the fermentation is not universal but is confined to particular organisms, the yeasts, the bacteria and the animal muscle, and is an adaptation to the absence of oxygen. The product of the glycolysis is the pyruvate, a three-carbon acid that still holds much unreleased energy and is not the end of the matter, whereas the product of the fermentation is the lactic acid or the ethanol with carbon dioxide, and this is the end of the matter, so the fermentation is the final step of the anaerobic respiration and the glycolysis the first. Both occur in the cytoplasm, but with different and specific enzymes, the kinases and the dehydrogenases of the pathway in the glycolysis and the lactic acid dehydrogenase, the pyruvate decarboxylase and the alcohol dehydrogenase in the fermentation. The energy yield differs completely, the glycolysis giving a net 2 ATP and the fermentation giving none at all, so the whole ATP of the anaerobic respiration is that of the glycolysis; the fermentation step thus makes no ATP of its own and is not energy-conserving in that sense, and what it does instead is to reoxidise the NADH to NAD plus so that the glycolysis can continue. The fate of the NADH is the counterpart, since the NADH is made in the glycolysis and consumed and reoxidised in the fermentation, so the fermentation is the sink for the reduced hydrogen of the glycolysis. And the total yield of the anaerobic respiration is therefore only the 2 ATP of the glycolysis, which is why it is some nineteen times less efficient than the aerobic respiration. (c) Glycolysis and citric acid cycle. The site is the cytoplasm for the glycolysis and the mitochondrial matrix for the cycle; the nature is that the glycolysis is anaerobic and needs no oxygen whereas the cycle is aerobic and cannot proceed without it, the NADH and the FADH2 it forms having to be reoxidised by the electron transport system; the substrate is the glucose for the glycolysis and the acetyl CoA for the cycle; the carbon accounting is two molecules of the three-carbon pyruvate from one six-carbon glucose against one molecule of the two-carbon acetyl CoA and two molecules of carbon dioxide per turn; the position in the sequence is first and preparatory for the glycolysis and final common oxidative pathway for the cycle, which is also the point at which the carbohydrate, the fat and the protein metabolisms converge; and the yield is a net 2 ATP and 2 NADH against 3 NADH, 1 FADH2 and 1 ATP per turn. It should be added that the glycolysis occurs in the cytoplasm of every living cell, prokaryote and eukaryote alike, whereas the citric acid cycle is a feature of the aerobic eukaryote, the bacteria performing the analogous reactions in the plasma membrane and the cytoplasm, so the two are not equally universal.
8. 8
   Q8 → The calculation of the net gain of ATP is made on the following assumptions. First, that the entire NADH produced in the glycolysis is transported into the mitochondrion and is oxidised there, so that each yields three ATP; but the inner membrane of the mitochondrion is impermeable to the NADH, so its reducing equivalents must be carried across by the shuttles, and the malate-aspartate shuttle transfers them so as to give three ATP while the glycerol 3-phosphate shuttle transfers them to the FAD of the second complex and gives only two, so this first assumption is a simplification and the glycolytic NADH does not always yield three. Second, that the two NADH and the two FADH2 produced in the Krebs cycle yield three and two ATP respectively. Third, that the two NADH produced in the oxidation of the pyruvate to the acetyl CoA also yield three ATP each, the pyruvate dehydrogenase reaction yielding two NADH. And fourth, that the ATP made by substrate-level phosphorylation, the two in the glycolysis and the two in the Krebs cycle, is taken at its face value and is added to the oxidative phosphorylation, no correction being made for the ATP used in the transport of the metabolites across the mitochondrial membranes. On these assumptions the ideal total is 2 ATP of substrate-level phosphorylation in the glycolysis, plus 2 times 3 for the 2 NADH of the glycolysis, that is 6, plus 2 ATP and 2 NADH at the link reaction, that is 2 plus 6, plus 6 times 3 for the 6 NADH of the Krebs cycle, that is 18, plus 6 times 2 for the 2 FADH2 of the Krebs cycle, that is 4, giving 2 plus 6 plus 2 plus 6 plus 18 plus 4, that is 38 ATP for each glucose in a eukaryotic cell. The practical total is less, and is given as 36, because the glycolytic NADH in the muscle of the animal is carried in by the glycerol 3-phosphate shuttle and yields only two ATP each instead of three, so 4 in place of 6, and the arithmetic becomes 2 plus 4 plus 2 plus 6 plus 18 plus 4, that is 36. The prokaryotic figure differs again, for the bacterial cell has no mitochondrion and no impermeable inner membrane, so the glycolytic NADH is oxidised directly on the plasma membrane at its full value of three and there is no loss on the transport, and the total is correspondingly higher. It is worth adding the caveat that these totals are accounting figures based on the assumed ratios of three ATP for each NADH and two for each FADH2, ratios that are calculated from the assumed energies of about four kilojoules for the ATP and about two and a half kilojoules for the proton and are not exact, the stoichiometry of the chain being now thought to be nearer two and a half ATP for the NADH and one and a half for the FADH2, which would give a figure nearer thirty than thirty-eight. So the 38 is the classical textbook figure obtained on the assumptions stated, the 36 is the corresponding figure for the shuttle of the animal muscle, and neither is a directly measured yield.
9. 9
   Q9 → An amphibolic pathway is one that serves both the catabolic, or degradative and energy-releasing, function and the anabolic, or biosynthetic, function, that is a pathway into which material enters in order to be broken down and a pathway out of which the intermediates are withdrawn in order to build up larger molecules, and the respiratory pathway is amphibolic in this sense. The amphibolic pathway is the Krebs cycle, the glycolysis and the electron transport system being purely catabolic. In the catabolic role the Krebs cycle is the final common oxidative pathway of the three classes of the respiratory substrate, for the acetyl CoA of the fat and of the protein and the pyruvate of the carbohydrate all converge upon it, and the cycle completely oxidises the carbon of the acetyl group to carbon dioxide over the course of several successive turns, releasing the six NADH, the two FADH2 and the two ATP of the two turns for each glucose to be passed to the electron transport system, the six NADH giving 6 times 3 ATP and the two FADH2 giving 2 times 2 ATP, that is 18 plus 4, or 22 more ATP, so that in this direction the cycle is purely catabolic. In the anabolic role, which is the point of the question, the intermediates of the cycle are themselves the raw materials of several biosyntheses, and it is their withdrawal for that purpose that makes the pathway amphibolic. The two-carbon acetyl CoA, the very entry of the cycle, is the precursor of the fatty acids and of the cholesterol and so of the steroids, and this synthesis occurs in the cytosol, the acetyl CoA being made in the matrix of the mitochondrion but exported, chiefly as the citrate, to be used by the cytosolic fatty acid synthase and by the cholesterol synthesis. The succinyl CoA is the precursor of the porphyrins and so of the haem of the haemoglobin, of the cytochromes and of the chlorophyll, its formation being the first committed step of the porphyrin synthesis. The oxaloacetic acid is transaminated to the aspartate, one of the twenty amino acids and so a building material of the proteins, and it also yields the oxaloacetate from which the pyruvate and the phosphoenolpyruvate, and so the sugars, are formed in the gluconeogenesis. And the alpha-ketoglutaric acid is transaminated to the glutamate, another of the twenty amino acids, and so serves in the synthesis of the amino acids and hence of the proteins. Several of the intermediates are therefore, directly or after a single transamination, the starting points of the fatty acids, the cholesterol, the porphyrins and the amino acids. The consequence, and the completion of the answer, is that if these intermediates are continuously withdrawn for the biosynthesis the cycle would run out of them and would stop, so the cell must replenish them, and this is done by the anaplerotic, or filling-up, reactions, the most important being the carboxylation of the pyruvate to the oxaloacetic acid by the pyruvate carboxylase with the biotin and the ATP and the carbon dioxide, which is a carboxylation and so the opposite of the link reaction, that one removing a carbon as carbon dioxide to give the acetyl CoA and this one adding a carbon as carbon dioxide to give the oxaloacetic acid. A second group of reactions, the cataplerotic, or emptying-up, reactions, removes the intermediates, and the two groups are kept balanced within the cell so that the cycle runs at a constant rate. The respiratory pathway is therefore amphibolic because it is not merely a degradative sequence but a metabolic crossroads, the point at which the breakdown of the food and the building of the new cell material meet, and the balance between the two uses of its intermediates is what keeps the metabolism of the cell in a steady state.
10. 10
    Q10 → The RQ, the respiratory quotient, is the ratio of the volume of the carbon dioxide released to the volume of the oxygen consumed during respiration, so that it equals the volume of the carbon dioxide divided by the volume of the oxygen, both measured under the same conditions. It is a dimensionless ratio and, since it is a function of the substrate that is being oxidised, it is used to identify the respiratory substrate. For a carbohydrate the RQ is 1, since the complete oxidation of the glucose consumes six volumes of the oxygen and releases six volumes of the carbon dioxide, glucose plus 6O2 giving 6CO2 plus 6H2O. For a protein it is about 0.9, being less than unity because the protein contains a nitrogen that is excreted rather than respired and because it is relatively poorer in hydrogen than a carbohydrate, so less oxygen is required. For a fat it is about 0.7, being much below unity for two reasons, that the fats are much poorer in oxygen than the carbohydrates and much richer in hydrogen, so that a very large volume of oxygen is required to oxidise them completely while comparatively little carbon dioxide is released. The composition of a typical fat illustrates the point: the tripalmitin, C51H98O6, is oxidised according to 2C51H98O6 plus 145O2 giving 102CO2 plus 98H2O, so 145 volumes of the oxygen are consumed against 102 volumes of the carbon dioxide released, and the quotient is 102/145, that is 0.7. It follows that the fats are the least oxygen-demanding of the respiratory substrates, giving the most energy for the least oxygen, which is why they are the preferred fuel of the hibernating animal and of the migratory bird, of the whale and of the seal in their deep and prolonged dives, and of the bird in flight at high altitude, where the oxygen supply is the limiting factor. The value is the theoretical one for the complete oxidation of a pure fat, an animal respiring a mixed diet giving a figure between the extremes of 1 and 0.7. The practical use of the RQ is diagnostic, since the quotient is used to identify the substance being oxidised in a patient, and a quotient of about 0.7 in the diabetic coma of the uncontrolled case indicates that the fat is being oxidised.
11. 11
    Q11 → Oxidative phosphorylation is the synthesis of the ATP by the phosphorylation of the ADP with the inorganic phosphate, the energy for that phosphorylation being supplied by the oxidation of the reduced coenzymes, the NADH and the FADH2, through the electron transport system. The word oxidative refers to the oxidation of the coenzymes and the word phosphorylation to the attachment of the phosphate to the ADP, and the two processes are coupled, the oxidation of the coenzyme driving the phosphorylation of the ADP. It is the fourth step of the aerobic respiration and it is the major site of the ATP production, yielding about 34 of the 38 ATP formed for each glucose in a eukaryotic cell. Its site is the inner mitochondrial membrane, at the cristae, where both the electron transport system and the ATP synthase are located, and in a bacterial cell the plasma membrane, since the bacteria have no mitochondrion; and the site is not incidental, because a membrane is required to hold the proton gradient and the ATP synthase must be embedded in it, which is why an artificial membrane system reconstituting both the respiratory chain and the synthase will carry out the oxidative phosphorylation in the test tube, this being the classic demonstration that no soluble factor is required. The mechanism is as follows. The electrons of the NADH and the FADH2 are passed down the chain, through the complex I, the coenzyme Q, the complex III, the cytochrome c and the complex IV, to the oxygen, which is the terminal acceptor and forms the water, and at three points of the chain the energy released by the transfer is used to pump the protons from the matrix into the intermembrane space, so that a proton motive gradient is set up across the membrane with the matrix becoming negative and alkaline. The protons then return to the matrix through the F0F1 ATP synthase, and the flow turns its rotor and drives the formation of the ATP from the ADP and the inorganic phosphate. The sequence is therefore the oxidation of the coenzyme by the transfer of the electrons, the pumping of the protons to create the gradient, and the return of the protons through the synthase to make the ATP; and it is to be noted that the ATP is not made by a direct transfer of the phosphate from the substrate, as in the substrate-level phosphorylation of the glycolysis and of the Krebs cycle, but indirectly through the gradient, and that is the essential distinction between the two. Its significance is that it is the mechanism by which the bulk of the energy of the food is converted into the ATP that the cell uses, the 34 ATP being made here against the 4 made by substrate-level phosphorylation, so it is some eight times more efficient than that mechanism, and it is the only stage at which the oxygen of the air is consumed, so it is the point at which the cell is sensitive to a lack of the oxygen; and since its substrates are the NADH and the FADH2 only, it can run only while these are available, which is precisely why the fermentation, which regenerates the NAD plus by reoxidising the NADH, is what permits the cell to make its 2 ATP in the absence of the oxygen.
12. 12
    Q12 → The significance of the step-wise release of the energy in respiration is that it enables the cell to trap the energy of the substrate in the form of the ATP rather than to lose it as heat, and this is the whole difference between respiration and combustion. The energy is not released in one step but in a series of enzyme-controlled steps, each releasing a small and manageable portion of it, and at a few of these steps the released energy is used to make ATP, so a very large part of the energy of the substrate is conserved in a form the cell can spend. The trapping occurs in two ways. In the substrate-level phosphorylation, which occurs twice in the glycolysis and once per turn of the Krebs cycle, the phosphate is transferred directly from a high-energy substrate of the pathway, the 1,3-bisphosphoglycerate and the phosphoenolpyruvate in the glycolysis and the succinyl CoA in the cycle, to the ADP, so that the ATP is made without any membrane and without any gradient. In the oxidative phosphorylation, which occurs in the fourth step, the energy is not released at the substrate at all but in the transfer of the electrons down the respiratory chain, and the components of the chain, the complex I to the complex IV of the transport chain together with the coupled complex V, the ATP synthase, and with the ubiquinone and the cytochrome c as the mobile carriers, are arranged so that the energy released at each transfer can be used to pump the protons, so that one large release is broken into several smaller ones, and those smaller releases are precisely what makes it possible to build the proton gradient. The step-wise release is therefore what makes the oxidative phosphorylation possible at all, for a single large release could not have been harnessed in this way. The benefits that follow are several. The cell gains usable energy, 38 molecules of ATP for each molecule of the glucose, and the whole of the metabolism of the cell runs on the ATP, so nothing could proceed without it. The cell saves the energy that would otherwise be lost as heat, so that respiration is efficient where combustion is not, combustion of the same glucose liberating the whole energy at once and conserving none. The release is controlled, each step being catalysed by a specific enzyme, so the rate can be regulated by the availability of the ADP, of the substrate and of the oxygen, and the respiration is switched down in a resting cell and up in an active one. The intermediates, being produced in small steps rather than consumed in one, are also available as the raw materials of the biosynthesis, which is the amphibolic role, so the step-wise nature of the pathway is what makes it usable for both purposes. And because the energy is not liberated as a single burst of heat, the temperature of the cell is not raised, and the cell is able to run this chemistry at a constant and survivable temperature.

MCQ Questions

1 Mark Questions

1. 1
   Define cellular respiration. (Ans: oxidation of organic compounds to release energy stored as ATP)
2. 2
   Where does the electron transport system (ETS) operate? (Ans: inner mitochondrial membrane)
3. 3
   Write the respiratory quotient (RQ) of carbohydrates. (Ans: 1.0)
4. 4
   Which molecule is the energy currency of the cell? (Ans: ATP)

2 Mark Questions

1. 1
   Differentiate between aerobic and anaerobic respiration on any two points.
2. 2
   What is an amphibolic pathway? (Ans: a pathway that is both catabolic and anabolic — the respiratory pathway (Krebs) provides intermediates for biosynthesis)
3. 3
   Write the summary equation of aerobic respiration of glucose. (Ans: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (36–38 ATP))

3 Mark Questions

1. 1
   Describe glycolysis, distinguishing the investment and payoff phases. (Ans: 2 ATP invested; 4 ATP + 2 NADH + 2 pyruvate produced in cytosol)
2. 2
   Describe the Krebs (TCA) cycle mentioning the number of CO₂, NADH, FADH₂ and ATP released per turn.
3. 3
   Describe the electron transport system and chemiosmotic ATP synthesis. (Ans: NADH/FADH₂ donate electrons, O₂ is terminal acceptor, proton gradient drives ATP synthase)

5 Mark Questions

1. 1
   Give a detailed account of aerobic respiration: glycolysis, pyruvate oxidation (link reaction), Krebs cycle and the electron transport chain, summing the ATP yield per glucose molecule. (Ans: 2 in glycolysis + 2 in Krebs + 32–34 in ETS = 36–38)
2. 2
   Explain fermentation in plants, including the reactions catalysed by enzymes, and state why the respiratory quotient changes in anaerobic conditions. (Ans: pyruvate → acetaldehyde → ethanol with NAD⁺ regeneration, 2 ATP per glucose)