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

Answer

Respiration is a biological, enzymatically controlled, stepwise oxidation of organic compounds in living cells releasing energy as ATP. Combustion is a rapid, uncontrolled, non-enzymatic oxidation of organic matter releasing all energy at once as heat and light; no ATP is produced.

(b) Glycolysis and Krebs' cycle

Ans. Glycolysis occurs in the cytoplasm without O₂; glucose
⇒ 2 pyruvate + 2 ATP + 2 NADH. Krebs' cycle occurs in the mitochondrial matrix; acetyl-CoA is completely oxidised to CO₂ producing 3 NADH, 1 FADH₂, and 1 ATP per turn. Krebs' cycle requires O₂ indirectly and releases CO₂, while glycolysis does not.

(c) Aerobic respiration and Fermentation

Ans. Aerobic respiration requires O₂, completely oxidises glucose to CO₂ and H₂O yielding ~36–38 ATP per glucose via glycolysis + Krebs' cycle + ETS. Fermentation occurs without O₂; glucose is incompletely oxidised to ethanol + CO₂ (yeast) or lactic acid (muscles), yielding only 2 ATP; it occurs entirely in the cytoplasm.

Answer

Respiratory substrates are organic compounds oxidised during cellular respiration to release energy. They include carbohydrates, fats, proteins, and organic acids. The most common respiratory substrate is glucose.

Answer

Glucose (6C)
⇒ Glucose-6-phosphate
⇒ Fructose-6-phosphate
⇒ Fructose-1,6-bisphosphate
⇒ 2 × Glyceraldehyde-3-phosphate (3C)
⇒ 2 × 1,3-bisphosphoglycerate
⇒ 2 × 3-phosphoglycerate
⇒ 2 × 2-phosphoglycerate
⇒ 2 × Phosphoenolpyruvate
⇒ 2 × Pyruvate (3C). Net gain: 2 ATP and 2 NADH. No CO₂ is released. Takes place in cytoplasm.

Answer

(1) Glycolysis — cytoplasm; glucose
⇒ 2 pyruvate + 2 ATP + 2 NADH. (2) Pyruvate oxidation (link reaction) — mitochondrial matrix; pyruvate
⇒ acetyl-CoA + CO₂ + NADH. (3) Krebs' cycle — mitochondrial matrix; acetyl-CoA completely oxidised to CO₂, producing NADH, FADH₂, and ATP. (4) Electron Transport System (ETS) / Oxidative phosphorylation — inner mitochondrial membrane; NADH and FADH₂ donate electrons; O₂ is reduced to H₂O; proton gradient drives ATP synthesis via chemiosmosis — the main source of ATP.

Answer

Acetyl-CoA (2C) + Oxaloacetate (4C)
⇒ Citrate (6C)
⇒ Isocitrate
⇒ α-Ketoglutarate (5C) + CO₂ + NADH
⇒ Succinyl-CoA (4C) + CO₂ + NADH
⇒ Succinate + ATP
⇒ Fumarate + FADH₂
⇒ Malate
⇒ Oxaloacetate + NADH. Net per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP. The cycle runs twice per glucose.

Answer

The Electron Transport System (ETS) is located on the inner mitochondrial membrane. NADH and FADH₂ donate electrons to ETS complexes (I
⇒ ubiquinone
⇒ III
⇒ cytochrome c
⇒ IV). Electrons finally reduce O₂ to H₂O. As electrons flow through Complexes I, III, and IV, protons are pumped into the intermembrane space, creating a proton gradient. These protons re-enter the matrix through ATP synthase (Complex V), driving ATP synthesis by chemiosmosis (oxidative phosphorylation). NADH yields ~2.5 ATP; FADH₂ yields ~1.5 ATP.

Answer

Aerobic respiration requires O₂, completely oxidises glucose to CO₂ + H₂O, yields ~36–38 ATP, and occurs in cytoplasm and mitochondria. Anaerobic respiration (fermentation) requires no O₂, incompletely oxidises glucose to ethanol + CO₂ or lactic acid, yields only 2 ATP, and occurs entirely in the cytoplasm.

(b) Glycolysis and Fermentation

Ans. Glycolysis is the first stage of both aerobic and anaerobic respiration; glucose
⇒ 2 pyruvate + 2 ATP + 2 NADH. Fermentation is the anaerobic process that follows glycolysis; it regenerates NAD⁺ by reducing pyruvate to ethanol + CO₂ (yeast) or lactic acid (animals) using NADH, but produces no additional ATP.

(c) Glycolysis and Citric acid Cycle

Ans. Glycolysis occurs in the cytoplasm, does not require O₂, produces 2 ATP and 2 NADH, and converts 6C glucose into two 3C pyruvate molecules. The Krebs'/citric acid cycle occurs in the mitochondrial matrix, requires O₂ indirectly, and completely oxidises acetyl-CoA to CO₂ producing 3 NADH, 1 FADH₂, and 1 ATP per turn.

Answer

Assumptions include: (i) complete oxidation of glucose under ideal conditions; (ii) cytoplasmic NADH from glycolysis is available to the mitochondrial ETS (via shuttle); (iii) P/O ratio — each NADH yields ~2.5 ATP, each FADH₂ ~1.5 ATP; (iv) no energy lost to transport processes; (v) constant temperature, pH, and no competing reactions. In reality, actual ATP yield is lower and variable (~30 ATP).

Answer

A pathway is amphibolic when it serves both catabolism and anabolism. Respiratory intermediates are used in biosynthesis: pyruvate
⇒ amino acids/fatty acids; acetyl-CoA
⇒ fatty acids/sterols; α-ketoglutarate and oxaloacetate
⇒ amino acids; succinyl-CoA
⇒ porphyrins/chlorophyll; glucose-6-phosphate
⇒ carbohydrates. Thus, the respiratory pathway is not purely catabolic but also provides carbon skeletons for building molecules, making it amphibolic.

Answer

Respiratory Quotient (RQ) = Volume of CO₂ evolved / Volume of O₂ consumed. For carbohydrates, RQ = 1. For fats, which are more reduced (contain more H relative to O), more O₂ is required for complete oxidation and proportionally less CO₂ is produced. Hence, RQ for fats is less than 1 (approximately 0.7).

Answer

Oxidative phosphorylation is the synthesis of ATP by ATP synthase using the energy of a proton electrochemical gradient generated by the ETS. As electrons flow through the ETS complexes on the inner mitochondrial membrane, protons are pumped to the intermembrane space, creating a proton motive force. Protons flow back into the matrix through ATP synthase, driving ATP synthesis from ADP + Pi. O₂ serves as the final electron acceptor, forming H₂O.

Answer

Step-wise energy release: (1) allows efficient capture of energy as ATP rather than heat; (2) prevents sudden temperature rises that would damage cellular components; (3) allows regulation of energy release according to metabolic needs; (4) provides intermediates for biosynthetic (amphibolic) pathways; (5) overall efficiency is much higher than direct combustion, maximising utilisation of energy stored in glucose.

Answer

NAD⁺ is an electron carrier that accepts electrons (and H⁺) from substrates during glycolysis, the link reaction, and the Krebs' cycle, becoming NADH. NADH then donates electrons to the ETS to drive oxidative phosphorylation and is regenerated to NAD⁺, allowing continued respiration. Without NAD⁺ regeneration, glycolysis would halt.

Answer

Substrate-level phosphorylation transfers a phosphate group directly from a metabolic intermediate to ADP (in glycolysis and Krebs' cycle), without the ETS; it does not require O₂ and yields small amounts of ATP. Oxidative phosphorylation uses the proton gradient generated by the ETS to synthesise ATP via ATP synthase; it requires O₂ and produces the bulk of ATP in aerobic respiration.

Answer

Both regenerate NAD⁺. Yeast has pyruvate decarboxylase (converts pyruvate to acetaldehyde + CO₂) and alcohol dehydrogenase (reduces acetaldehyde to ethanol using NADH), producing ethanol. Muscle cells lack pyruvate decarboxylase; instead, lactate dehydrogenase reduces pyruvate directly to lactic acid using NADH. The enzyme present determines the product.

Answer

The Krebs' cycle provides carbon skeletons for biosynthesis: α-ketoglutarate and oxaloacetate for amino acid synthesis, succinyl-CoA for porphyrins and haem, citrate for fatty acid synthesis. This amphibolic role makes it a central hub of metabolism connecting catabolism with anabolism.

Answer

The P/O ratio is the number of ATP molecules synthesised per pair of electrons transferred to O₂. NADH has a P/O of ~2.5 (≈2.5 ATP); FADH₂ has ~1.5 (≈1.5 ATP). FADH₂ yields less because it donates electrons at Complex II, bypassing Complex I, pumping fewer protons and generating a smaller gradient. This ratio indicates the efficiency of oxidative phosphorylation.