NCERT Solutions for Class 11 Biology Chapter 12: Respiration in Plants

Concept — why the three pairs look alike but are not. Respiration is the controlled, enzyme-driven oxidation of food inside living cells, in which the energy of C–C bonds is released in small instalments and trapped as ATP. All three pairs below compare steps of this one story (or its non-living look-alike, combustion), so the differences lie in where the reaction occurs, whether oxygen is needed, how much energy is trapped and what the end products are.

(a) Respiration and combustion

The complete combustion of glucose, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy, has the same overall equation as aerobic respiration; the real difference is that a cell oxidises glucose not in one step but in several small steps, some of which are just large enough for the energy to be coupled to ATP synthesis.

(b) Glycolysis and Krebs’ cycle (TCA cycle)

(c) Aerobic respiration and fermentation

Extra depth. Glycolysis is the common first stage of both aerobic respiration and fermentation: only after pyruvic acid is formed does the pathway split depending on O₂ supply and on the organism. This is why glycolysis is considered the most ancient pathway — the first cells lived in an oxygen-free atmosphere and all organisms still keep the enzymes to oxidise glucose partially without O₂.

Conclusion: Respiration differs from combustion in being enzyme-controlled, step-wise and ATP-generating; glycolysis (cytoplasm, anaerobic, glucose → 2 pyruvate) differs from the Krebs’ cycle (matrix, oxygen-dependent, acetyl CoA → CO₂); and aerobic respiration oxidises glucose completely with a high ATP yield whereas fermentation is partial with a net gain of only 2 ATP.

Concept. The breaking of C–C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy, is called respiration. The compounds that are oxidised during this process are known as respiratory substrates.

• Carbohydrates are the usual substrates. Usually carbohydrates are oxidised to release energy. All carbohydrates are first converted into glucose before they are respired — in plants this glucose is derived from sucrose, the end product of photosynthesis, or from storage carbohydrates such as starch. Sucrose is split by the enzyme invertase into glucose and fructose.
• Other substrates under certain conditions. Proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions. They do not enter the pathway at the first step: fats are broken to glycerol and fatty acids (fatty acids → acetyl CoA; glycerol → PGAL), and proteins are degraded by proteases to amino acids which, after deamination, enter at pyruvate, acetyl CoA or a step of the Krebs’ cycle.
• Most common substrate: glucose. Glucose is the favoured (most common) respiratory substrate. In living organisms the substrates are often more than one — pure proteins or fats are never used alone.

Extra depth. Energy in a respiratory substrate is not released free in the cell or in one step; it is released in slow, enzyme-controlled steps and trapped in ATP. Because ATP, not the substrate itself, is what the cell uses, ATP is called the energy currency of the cell, while the carbon skeletons produced are used as precursors for biosynthesis of other molecules.

Conclusion: Respiratory substrates are the organic compounds oxidised in respiration to release energy; carbohydrates, and above all glucose, are the most common substrate, while fats, proteins and organic acids serve under certain conditions.

Concept. The term glycolysis originated from the Greek words glycos (sugar) and lysis (splitting). The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, so it is often called the EMP pathway. It occurs in the cytoplasm of all living organisms; glucose undergoes partial oxidation to form two molecules of pyruvic acid through a chain of ten reactions, each controlled by a different enzyme.

1. Entry of sugar. Sucrose is converted into glucose and fructose by invertase. Glucose (and fructose) is phosphorylated to glucose-6-phosphate by hexokinase, using 1 ATP (first ATP utilised).
2. Isomerisation. Glucose-6-phosphate isomerises to fructose-6-phosphate.
3. Second phosphorylation. Fructose-6-phosphate → fructose-1,6-bisphosphate using a second ATP (second ATP utilised).
4. Splitting. Fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL); from here on every step occurs twice per glucose molecule.
5. Oxidation (NADH formed). PGAL is oxidised, with inorganic phosphate, to 1,3-bisphosphoglycerate (BPGA); two hydrogen atoms (two redox equivalents) are transferred to NAD⁺ forming NADH + H⁺ — the only NADH-forming step.
6. First ATP gain. BPGA → 3-phosphoglyceric acid (PGA); the energy is trapped by forming ATP (substrate-level phosphorylation).
7. Rearrangement and dehydration. PGA → 2-phosphoglycerate → phosphoenolpyruvate (PEP) with loss of H₂O.
8. Second ATP gain. PEP → pyruvic acid, with synthesis of another ATP.

Extra depth. NCERT asks: how many ATP molecules are directly synthesised from one glucose? Four are made and two are spent, so the net direct gain is two ATP. Pyruvic acid is the key product, and its fate depends on the cell: lactic acid fermentation, alcoholic fermentation or aerobic respiration through the Krebs’ cycle.

Conclusion: Glycolysis is a ten-step cytoplasmic pathway converting one glucose into two pyruvic acid with a net gain of 2 ATP and 2 NADH + H⁺.

Concept. Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water and a large amount of energy present in the substrate. It is the most common type of respiration in higher organisms and, in eukaryotes, takes place within the mitochondria (after glycolysis in the cytoplasm).

The two crucial events (NCERT): (i) the complete oxidation of pyruvate by the stepwise removal of all hydrogen atoms, leaving three molecules of CO₂; and (ii) the passing on of the electrons removed as part of the hydrogen atoms to molecular O₂ with simultaneous synthesis of ATP.

Extra depth. Note that the first process (removal of hydrogen/CO₂) occurs in the matrix, whereas the second (ETS) is located on the inner membrane. During the link reaction two NADH are produced from two molecules of pyruvic acid (one glucose). O₂ acts only at the terminal stage but is vital because it removes hydrogen from the system and so drives the whole process.

Conclusion: Aerobic respiration comprises glycolysis, oxidative decarboxylation of pyruvate, the Krebs’ cycle and ETS with oxidative phosphorylation; the last three occur in the mitochondria (matrix and inner membrane).

Concept. The acetyl CoA formed from pyruvate enters a cyclic pathway, the tricarboxylic acid (TCA) cycle, more commonly called Krebs’ cycle after Hans Krebs, who first elucidated it. It runs in the matrix of the mitochondria and oxidises the acetyl group completely to CO₂, while capturing the hydrogen as NADH + H⁺ and FADH₂.

1. Link reaction (before the cycle). Pyruvic acid (3C) + CoA + NAD⁺ → acetyl CoA (2C) + CO₂ + NADH + H⁺, catalysed by pyruvic dehydrogenase (with Mg²⁺).
2. Condensation. Acetyl CoA (2C) combines with oxaloacetic acid (OAA, 4C) and water to form citric acid (6C); the enzyme is citrate synthase and CoA is released.
3. Isomerisation. Citrate is isomerised to isocitrate.
4. First decarboxylation. Isocitrate → α-ketoglutaric acid (5C) + CO₂; NAD⁺ → NADH + H⁺.
5. Second decarboxylation. α-Ketoglutaric acid → succinyl-CoA (4C) + CO₂; another NADH + H⁺.
6. Substrate-level phosphorylation. Succinyl-CoA → succinic acid; a molecule of GTP is synthesised, which is converted to GDP with simultaneous synthesis of ATP from ADP.
7. Regeneration of OAA. Succinic acid → (FAD⁺ → FADH₂) → malic acid → (NAD⁺ → NADH + H⁺) → oxaloacetic acid, ready to accept another acetyl group.

Extra depth. NCERT notes that there are three points where NAD⁺ is reduced to NADH + H⁺ and one point where FAD⁺ is reduced to FADH₂. Continued oxidation of acetyl CoA requires continued replenishment of oxaloacetic acid and regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂ — which is done by the electron transport system. Counting the link reaction, one glucose gives CO₂ together with eight NADH + H⁺ and two FADH₂ besides two ATP.

Conclusion: The Krebs’ cycle completely oxidises acetyl CoA to CO₂ in the mitochondrial matrix, yielding NADH + H⁺, FADH₂ and GTP/ATP, and regenerates oxaloacetic acid so the cycle continues.

Concept. The electron transport system (ETS) is the metabolic pathway through which electrons pass from one carrier to another. It is present in the inner mitochondrial membrane and releases and utilises the energy stored in NADH + H⁺ and FADH₂ by passing their electrons finally to oxygen, which forms water.

1. Complex I — NADH dehydrogenase. Electrons from NADH produced in the matrix during the citric acid cycle are oxidised by NADH dehydrogenase and transferred to ubiquinone located within the inner membrane.
2. Complex II — succinate dehydrogenase (FADH₂). Ubiquinone also receives reducing equivalents via FADH₂ generated during oxidation of succinate in the cycle.
3. Complex III — cytochrome bc₁. The reduced ubiquinone (ubiquinol) is oxidised, passing electrons to cytochrome c.
4. Cytochrome c. A small protein attached to the outer surface of the inner membrane that acts as a mobile carrier between complexes III and IV.
5. Complex IV — cytochrome c oxidase. It contains cytochromes a and a₃ and two copper centres; it passes electrons to O₂, which is reduced (with H⁺) to H₂O.
6. Complex V — ATP synthase. As electrons pass through complexes I to IV the energy released is coupled to ATP synthase for synthesis of ATP from ADP and inorganic phosphate.

ATP yield. Oxidation of one molecule of NADH gives rise to 3 ATP, while that of one FADH₂ produces 2 ATP (NCERT values). Oxygen acts as the final hydrogen (electron) acceptor; though its role is limited to the terminal stage, it is vital because it drives the whole process by removing hydrogen from the system.

Extra depth — why “oxidative phosphorylation”. In photophosphorylation the light energy builds the proton gradient, but in respiration the energy of oxidation–reduction is used for the same purpose; hence the process is called oxidative phosphorylation. (See Q11.)

Conclusion: The ETS is a chain of membrane-bound carriers (complexes I–IV, ubiquinone, cytochrome c) that moves electrons from NADH/FADH₂ to O₂ and powers ATP synthase (complex V) in the inner mitochondrial membrane.

Concept. Anaerobic respiration and fermentation both start with glycolysis but differ from aerobic respiration in what happens to pyruvic acid. Pyruvic acid is handled in three ways by different cells: lactic acid fermentation, alcoholic fermentation or aerobic respiration.

(a) Aerobic respiration and anaerobic respiration

(b) Glycolysis and fermentation

(c) Glycolysis and citric acid cycle

Extra depth. In both lactic and alcoholic fermentation, NADH + H⁺ is reoxidised to NAD⁺; this keeps glycolysis running even without oxygen. Yeast poisons itself when alcohol reaches about 13 per cent, which limits natural fermentation; stronger drinks are obtained by distillation.

Conclusion: Aerobic respiration completely oxidises glucose with O₂ and high ATP yield, anaerobic respiration/fermentation does not; glycolysis is cytoplasmic and universal, fermentation and the citric acid cycle are alternative fates of pyruvate under anaerobic and aerobic conditions respectively.

Concept. It is possible to calculate the net gain of ATP for every glucose molecule oxidised, but NCERT stresses that this “can remain only a theoretical exercise”, because the calculation holds only on certain assumptions.

1. Sequential, orderly pathway. There is a sequential, orderly pathway functioning, with one substrate forming the next, and glycolysis, TCA cycle and ETS following one after another.
2. Glycolytic NADH is transferred and oxidised. The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
3. No diversion of intermediates. None of the intermediates in the pathway is utilised to synthesise any other compound.
4. Only glucose is respired. No other alternative substrates enter the pathway at any of the intermediary stages.

Why the assumptions fail in a living cell. All pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means. Yet the exercise is useful to appreciate the beauty and efficiency of the living system in extracting and storing energy — hence a net gain of 38 ATP per glucose in aerobic respiration.

Conclusion: The net 38 ATP per glucose rests on four assumptions — sequential pathways, transfer of glycolytic NADH to mitochondria, no diversion of intermediates and glucose as sole substrate — none of which strictly holds in a living cell.

Concept. Breaking down processes within a living organism are catabolism; synthesis is anabolism. Since respiration involves the breakdown of substrates, it has traditionally been considered a catabolic process and the respiratory pathway a catabolic pathway. NCERT asks whether this understanding is correct — and shows that it is not the whole truth.

• Catabolic entry of substrates. Glucose is the favoured substrate and all carbohydrates are first converted into glucose. Fats are broken into glycerol and fatty acids; fatty acids are degraded to acetyl CoA and glycerol enters after conversion to PGAL. Proteins are degraded by proteases to amino acids, which after deamination enter at pyruvate, acetyl CoA or a Krebs’ cycle step.
• Anabolic withdrawal of the same intermediates. It is these very compounds that would be withdrawn from the respiratory pathway for the synthesis of the said substrates. When the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the pathway; similarly, during breakdown and synthesis of proteins, respiratory intermediates form the link.
• Conclusion of the argument. The respiratory pathway therefore comes into the picture both during breakdown and during synthesis, i.e., it serves both catabolism and anabolism, and is better described as an amphibolic pathway (amphi = both) rather than purely catabolic.

Extra depth. The carbon skeletons produced during respiration are used as precursors for biosynthesis of other molecules in the cell (stated in the chapter introduction). This is why one pathway can connect carbohydrate, fat and protein metabolism, as shown in the interrelationship diagram.

Conclusion: Because respiratory intermediates are both used up for energy and withdrawn for biosynthesis of fats, proteins and carbohydrates, the respiratory pathway is amphibolic, not merely catabolic.

Definition. 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.

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed

Value for fats. When fats are used in respiration, the RQ is less than 1. For tripalmitin, taken as the example substrate:

2(C₅₁H₉₈O₆) + 145 O₂ → 102 CO₂ + 98 H₂O + energy

RQ = 102 CO₂ ÷ 145 O₂ = 0.7

Fats contain much less oxygen in their molecules than carbohydrates, so a larger volume of external O₂ is needed for their complete oxidation than the CO₂ evolved — hence RQ is below 1.

Extra depth. RQ depends on the type of respiratory substrate. In living organisms the substrates are often more than one — pure proteins or fats are never used as respiratory substrates — so the observed RQ of a tissue is usually between these values.

Conclusion: RQ is the ratio of CO₂ evolved to O₂ consumed; for fats it is about 0.7 (less than 1).

Definition. Oxidative phosphorylation is the synthesis of ATP from ADP and inorganic phosphate by ATP synthase, using the energy released as electrons are passed from NADH + H⁺ and FADH₂ through the electron transport system to molecular oxygen. It takes place on the inner mitochondrial membrane.

• Why “oxidative”. Unlike photophosphorylation where it is light energy that is utilised for the production of the proton gradient, in respiration it is the energy of oxidation–reduction that is utilised for the same process; hence the name oxidative phosphorylation.
• Mechanism — chemiosmosis. The energy released during electron transport is utilised in synthesising ATP with the help of ATP synthase (complex V), as explained by the chemiosmotic hypothesis.
• Structure of ATP synthase. It consists of two major components, F₁ and F₀. The F₁ headpiece is a peripheral membrane protein complex containing the site for synthesis of ATP from ADP and inorganic phosphate. F₀ is an integral membrane protein complex forming the channel through which protons cross the inner membrane.
• Proton flow. The passage of protons through the channel is coupled to the catalytic site of F₁. For each ATP produced, 4H⁺ pass through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient.
• Role of oxygen. O₂ is the ultimate electron acceptor and is reduced to water; it drives the whole process by removing hydrogen from the system.

Yield. One NADH gives 3 ATP and one FADH₂ gives 2 ATP.

Extra depth. The proton gradient is established across the inner mitochondrial membrane (high H⁺ in the intermembrane space, low in the matrix); ATP synthase turns this electrochemical gradient into chemical energy. Together, glycolysis, the Krebs’ cycle and oxidative phosphorylation give the theoretical net gain of 38 ATP per glucose (see Q8).

Conclusion: Oxidative phosphorylation is the ATP synthesis driven by the redox energy of the ETS, with O₂ as final acceptor, occurring through ATP synthase in the inner mitochondrial membrane.

Concept. The complete combustion of glucose produces CO₂ and H₂O as end products, but most of the energy is given out as heat. If the energy is to be useful, the cell must be able to use it to synthesise other molecules it requires.

• Avoids loss as heat. The strategy of the plant cell is to catabolise glucose in such a way that not all the liberated energy goes out as heat; oxidising glucose not in one step but in several small steps lets some steps be just large enough for the energy released to be coupled to ATP synthesis.
• Energy is trapped as ATP. Because the energy is released in a series of slow step-wise reactions controlled by enzymes, it is trapped as chemical energy in ATP, which is broken down whenever and wherever energy is needed. Energy released in respiration cannot be used directly; it is used to synthesise ATP, the energy currency of the cell.
• Control by enzymes. Each step is controlled by a specific enzyme, so the rate of energy release matches cellular needs and the cell is protected from sudden temperature rise that would damage it.
• Carbon skeletons for biosynthesis. The intermediate compounds produced act as carbon skeletons and precursors for biosynthesis of other molecules (amphibolic role).

Extra depth. Both the intermediates and the ATP obtained step by step serve the cell: ATP supplies energy wherever it is needed, and respiratory intermediates (e.g., acetyl CoA) are drawn off for fatty acid and protein synthesis. In contrast, a fire converts the same glucose into heat and light only. This is also why fermentation, with its minimal 2 ATP, wastes most of the chemical energy still locked in ethanol or lactic acid.

Conclusion: Step-wise release lets the cell capture energy as ATP and intermediates for biosynthesis instead of wasting it as heat.