Previous year questions: Respiration in Plants

These ten questions come from the NCERT Exemplar chapter on respiration in plants, the only official previous year source available for Class 11 Biology. The set covers glycolysis, the location of the citric acid cycle and the electron transport system, the ATP accounting, the respiratory quotient and the amphibolic pathway. Marks follow the convention used across this book: 1 mark for a multiple choice, 2 marks for a very short answer, 3 marks for a short answer and 5 marks for a long answer. The chapter reference in each heading points to the item in the Exemplar.

Q1 (NCERT Exemplar, 1 mark; Chapter 14, Multiple Choice 1)

The ultimate electron acceptor of respiration in an aerobic organism is which of these?

Answer: Oxygen is the final electron acceptor of the electron transport chain. At complex IV, cytochrome oxidase passes electrons to oxygen, and each oxygen atom takes two protons to form water. Cytochrome is a carrier rather than the final acceptor, hydrogen is carried by NADH and FADH2, and glucose is the fuel, not the acceptor.

Q2 (NCERT Exemplar, 1 mark; Chapter 14, Multiple Choice 2)

Phosphorylation of glucose during glycolysis is catalysed by which enzyme?

Answer: Hexokinase transfers a phosphate from ATP to glucose at the start of glycolysis, and this first investment of ATP is what traps the sugar inside the cell. Phosphoglucomutase shifts a phosphate between carbon positions, phosphoglucoisomerase converts glucose 6-phosphate to fructose 6-phosphate, and phosphorylase breaks down glycogen stores rather than phosphorylating free glucose. The hexokinase step also shows why glycolysis needs ATP before it can make any: the investment phase runs before the payoff phase.

Q3 (NCERT Exemplar, 1 mark; Chapter 14, Multiple Choice 4)

The electron transport system is located in which part of the mitochondrion?

Answer: The system sits on the inner mitochondrial membrane, where the protein complexes and the ATP synthase are embedded. The outer membrane is freely permeable and carries no respiratory chain, the intermembrane space holds the proton pool, and the matrix holds the enzymes of the citric acid cycle.

Q4 (NCERT Exemplar, 1 mark; Chapter 14, Multiple Choice 6)

Mitochondria are called the powerhouses of the cell. Which observation supports this?

Answer: The statement that mitochondria synthesise ATP directly supports the powerhouse description, because ATP is the energy currency used by the cell. A double membrane, the presence of Krebs cycle enzymes and their occurrence in most eukaryotic cells are all true of mitochondria but do not by themselves show why they are called powerhouses. The double membrane is structural, the cycle enzymes explain only one stage of oxidation, and the widespread occurrence says nothing about energy output. The ATP claim is the one that connects the organelle to the usable energy supply of the cell.

Q5 (NCERT Exemplar, 2 marks; Chapter 14, Very Short Answer 1)

Energy is released during the oxidation of compounds in respiration. How is this energy stored and released as and when needed?

Answer: ATP holds the usable share of that energy in its terminal phosphate linkage. Breaking the last phosphate off ATP gives ADP and inorganic phosphate and frees energy for cellular work such as making new molecules, moving ions and contracting muscle. The cell then rebuilds ATP from ADP and Pi with energy from respiration, so a single ATP molecule is recycled many times. This is why ATP is called the energy currency: it is not a long term store like starch but a small spendable unit that is carried to wherever work is being done.

Q6 (NCERT Exemplar, 2 marks; Chapter 14, Very Short Answer 7)

The product of glycolysis under hypoxia in skeletal muscle and the product of anaerobic fermentation in yeast are respectively what?

Answer: In skeletal muscle under hypoxia, pyruvate is reduced to lactic acid, while in yeast it is converted to ethanol and carbon dioxide. Both reactions use the NADH made in glycolysis to regenerate NAD+, which allows glycolysis to continue without oxygen. The two routes are called lactic acid fermentation and alcoholic fermentation.

Q7 (NCERT Exemplar, 3 marks; Chapter 14, Short Answer 11)

ATP produced during glycolysis is a result of substrate level phosphorylation. Explain.

Answer: In substrate level phosphorylation, a phosphate group is transferred directly from a phosphorylated substrate molecule to ADP, producing ATP without any electron transport chain. In glycolysis, the phosphate is passed to ADP at the steps where 1,3-bisphosphoglyceric acid is converted to 3-phosphoglyceric acid and where phosphoenol pyruvic acid is converted to pyruvic acid. This direct transfer is why the ATP of glycolysis is described as substrate level, and it contrasts with oxidative phosphorylation, where the proton gradient drives ATP synthesis.

Q8 (NCERT Exemplar, 3 marks; Chapter 14, Short Answer 12)

Name the step in the TCA cycle where substrate level phosphorylation occurs.

Answer: This is the step in which succinyl CoA becomes succinic acid. When the CoA group leaves, the bond that held it breaks and the energy released attaches a phosphate to GDP, giving GTP. GTP can hand that phosphate to ADP and become ATP, and this is the only direct formation of a nucleoside triphosphate in one turn of the cycle. The rest of the ATP of aerobic respiration comes later, from the carriers passed to the electron transport chain.

Q9 (NCERT Exemplar, 3 marks; Chapter 14, Short Answer 17)

The energy yield in ATP is higher in aerobic respiration than in anaerobic respiration. Explain.

Answer: In aerobic respiration glucose is oxidised completely to carbon dioxide and water, and the electrons from twelve carrier molecules are passed down the electron transport chain, which makes a large number of ATP through oxidative phosphorylation. In anaerobic respiration the glucose is only partly broken down: pyruvate is converted to lactic acid or ethanol and carbon dioxide, and the NADH made in glycolysis is used up in that conversion instead of reaching the chain. The energy left in the products of fermentation is not captured, so the yield stays at the two ATP of glycolysis. Another way to put the comparison is in terms of the electron carriers: aerobic respiration sends ten NADH and two FADH2 to the chain and harvests their energy in many small phosphorylation steps, while fermentation sends none. The difference is not in the fuel but in how completely it is oxidised and how many chances the cell gets to capture energy along the way. A useful exam habit is to state the actual ATP numbers for both routes and then explain the gap in terms of oxygen, complete oxidation and the presence or absence of an electron transport chain.

Q10 (NCERT Exemplar, 5 marks; Chapter 14, Long Answer 4)

Enumerate the assumptions made in making the respiratory balance sheet. Are these assumptions valid in a living system? Compare fermentation and aerobic respiration in this context.

Answer: The balance sheet assumes that the pathway runs in a strict order from glycolysis through the link reaction and the citric acid cycle to the electron transport system. It assumes that the NADH made in glycolysis reaches the mitochondrion and is used fully in the chain, with no energy spent on the transport. It assumes that none of the pathway intermediates is withdrawn for biosynthesis, and it fixes the ATP yield of each NADH and FADH2. It also assumes that respiration alone uses the glucose and that only one substrate is oxidised at a time. These assumptions are not fully valid in a living cell: intermediates are withdrawn for synthesis, the carriers are not used with perfect efficiency, protons leak across the membrane and several substrates burn together, so the real yield is lower than the theoretical value. Fermentation is a much simpler case, because it has no electron transport chain and no proton gradient; its yield of two ATP per glucose is close to the actual value and does not depend on the same assumptions. Aerobic respiration gives far more ATP per glucose, but its realised output is lower than the balance sheet figure and is not the same in every tissue. The comparison shows that ATP accounting is a model, useful for organising the pathway but not a measurement of the living cell. For fermentation the model and the measurement nearly agree, which makes fermentation a good reference case when explaining why the aerobic figure is only a ceiling and not a promise.