MPSC 2021 Prelims Paper-I · Question 60 of 100

Correct — option (1), the Calvin cycle. Read the stem before the options: it asks which of the four is NOT involved in the respiratory process, and 'not' is printed in bold in the English column and 'नाही' in bold in the Marathi. The question is inverted, so the three stages that ARE part of respiration have to be discarded and the outsider is the answer. The three that belong are easy to place once the pathway is held in order. Cellular respiration begins with glycolysis in the cytoplasm, where a six-carbon glucose molecule is split into two three-carbon pyruvate molecules with a small net yield of ATP and some NADH; this stage needs no oxygen and occurs in every living cell. Pyruvate then enters the mitochondrion, is decarboxylated to acetyl coenzyme A, and that acetyl group is fed into the citric acid cycle in the mitochondrial matrix, where it is oxidised completely to carbon dioxide while the energy released is captured as NADH and FADH₂. Those reduced carriers deliver their electrons to the electron transport chain on the inner mitochondrial membrane, where the electrons pass down a series of carriers to oxygen, the final acceptor, producing water; the energy released along the way pumps protons across the membrane and the resulting gradient drives ATP synthase. Glycolysis, the citric acid cycle and the electron transport chain are therefore the three stages of aerobic respiration in sequence, and all three are eliminated by the stem's negation. The Calvin cycle belongs to the opposite process. It is the light-independent stage of photosynthesis, running in the stroma of the chloroplast, in which carbon dioxide is fixed onto a five-carbon acceptor by the enzyme RuBisCO and reduced to sugar using the ATP and NADPH generated by the light reactions in the thylakoid membranes. It was worked out by Melvin Calvin with Andrew Benson and James Bassham, and Calvin received the Nobel Prize in Chemistry in 1961 for it. So the Calvin cycle builds carbohydrate from carbon dioxide using light energy, while respiration dismantles carbohydrate to carbon dioxide and releases energy — the two are near mirror images, and only one of them is what this question asks about. The row naming the Calvin cycle is option (1).

• (2)Citric acid cycle — The citric acid cycle is the second stage of aerobic respiration and therefore cannot answer a question asking what is NOT involved in it. Also called the Krebs cycle, after Hans Krebs, who worked it out and shared the Nobel Prize in Physiology or Medicine in 1953, and sometimes the tricarboxylic acid cycle, it runs in the mitochondrial matrix. An acetyl group delivered by coenzyme A joins a four-carbon acceptor to form the six-carbon citrate that gives the cycle its name, and through a series of oxidations and two decarboxylations the four-carbon acceptor is regenerated, releasing two molecules of carbon dioxide and loading three NADH, one FADH₂ and one high-energy phosphate for each acetyl group entering. Those reduced carriers are the cycle's real product — the ATP made directly in it is negligible beside what the electron transport chain later extracts from them. A candidate selects this row essentially only by failing to invert the stem, since the cycle's place in respiration is in every textbook account.
• (3)Glycolysis — Glycolysis is the first stage of respiration in every organism, so it too is eliminated by the negation in the stem. It takes place in the cytoplasm rather than in the mitochondrion, requires no oxygen, and converts one molecule of glucose into two of pyruvate with a net gain of two ATP and two NADH. It is also called the Embden-Meyerhof-Parnas pathway, and its universality is one of the standard arguments for its antiquity: bacteria, plants and animals all use it. Its independence from oxygen is what makes anaerobic respiration and fermentation possible — when oxygen is unavailable, the pyruvate produced by glycolysis is converted to lactic acid in animal muscle or to ethanol and carbon dioxide in yeast, so that the pathway can keep running at a much lower energy yield. A candidate might reach for this row on the impression that glycolysis is somehow separate from 'true' respiration because it happens outside the mitochondrion; the location is different, but the pathway is respiration's opening stage.
• (4)Electron transport chain — The electron transport chain is the final stage of aerobic respiration, sited on the inner mitochondrial membrane, and so cannot be the item excluded by the stem. It is where the great majority of the ATP is made: NADH and FADH₂ from the earlier stages hand their electrons to a series of carrier complexes, the electrons pass step by step to oxygen, which combines with hydrogen to form water, and the energy released at each step pumps protons into the intermembrane space, creating a gradient whose collapse through ATP synthase drives phosphorylation. There is a subtlety in this option worth knowing, because it is the kind of thing that turns into a later question: an electron transport chain also exists in photosynthesis, on the thylakoid membrane, where light-excited electrons move through carriers and generate ATP by a very similar mechanism. The term is therefore shared between the two processes — but the chain is unquestionably part of respiration as well, which is what this stem asks about, so the row is discarded either way.

Respiration and photosynthesis are the two great energy pathways of the living world and it helps to learn them as opposites. Photosynthesis, in green plants, algae and cyanobacteria, uses light energy to build carbohydrate from carbon dioxide and water, releasing oxygen; respiration, in effectively all living cells, oxidises carbohydrate back to carbon dioxide and water, releasing the energy as ATP. Each has two halves and the halves are often confused across the two processes. Photosynthesis has the light reactions in the thylakoid membranes of the chloroplast, where water is split, oxygen is released and ATP and NADPH are made, and the light-independent Calvin cycle in the stroma, where those products are used to fix carbon dioxide onto ribulose bisphosphate through the enzyme RuBisCO and reduce it to sugar. Variants of the fixation step give the C3, C4 and CAM plants, adaptations to heat and water stress in which the Calvin cycle itself remains the final common pathway. Respiration has glycolysis in the cytoplasm, then in the presence of oxygen the link reaction, the citric acid cycle in the mitochondrial matrix and oxidative phosphorylation on the inner mitochondrial membrane; in the absence of oxygen it stops at fermentation, producing lactic acid in muscle or ethanol and carbon dioxide in yeast, with a small fraction of the aerobic yield. Both processes converge on the same currency: ATP, the molecule in which cells hold energy for immediate use. The location of each stage is worth memorising along with its name, since a large share of examination questions in this area are location questions in disguise.

Biology in MPSC papers leans heavily on plant and human physiology, and respiration is among the most frequently examined topics in it because the pathway has named stages, named locations, named products and a named discoverer for almost every part. This item is the simplest shape the topic supports: three stages of one pathway and one stage of another, with a negative stem to make the sorting non-automatic. That construction is common enough to be worth naming — a list in which three members belong to one family and the fourth to a neighbouring family, with the ask inverted so that the outsider is wanted. The discipline is always the same. Circle the negating word and write 'NOT' beside the question number, then classify each option before looking at any of them as a candidate answer. Five of this paper's seven negative stems print the negation in bold in both columns, this one among them, while one bolds it in the English column only and one bolds it in neither — so the emphasis is a courtesy rather than a safeguard and should never be relied on. It is also worth noticing a small coincidence in this paper: an earlier question in this same block offers Charles Krebs, a modern Canadian ecologist, among its options, while the cycle in this question is named after Hans Krebs, the German-born biochemist who described it. The two are unrelated, and papers that ask about both in one sitting are not unusual.

• Aerobic respiration has three stages in sequence: glycolysis in the cytoplasm, the citric acid cycle in the mitochondrial matrix, and the electron transport chain with oxidative phosphorylation on the inner mitochondrial membrane; a link reaction converting pyruvate to acetyl coenzyme A joins the first to the second.
• The Calvin cycle is not part of respiration at all: it is the light-independent stage of PHOTOSYNTHESIS, running in the stroma of the chloroplast, where carbon dioxide is fixed by the enzyme RuBisCO and reduced to sugar using the ATP and NADPH produced by the light reactions.
• Melvin Calvin, working with Andrew Benson and James Bassham, established the cycle and received the Nobel Prize in Chemistry in 1961; Hans Krebs described the citric acid cycle and shared the Nobel Prize in Physiology or Medicine in 1953.
• Glycolysis, also called the Embden-Meyerhof-Parnas pathway, needs no oxygen and converts one glucose into two pyruvate with a net yield of two ATP and two NADH, which is why fermentation to lactic acid or to ethanol and carbon dioxide can continue when oxygen is absent.
• In the electron transport chain oxygen is the final electron acceptor and combines with hydrogen to form water, while the proton gradient built across the inner mitochondrial membrane drives ATP synthase — the stage that produces most of the ATP.
• An electron transport chain also operates in photosynthesis, on the thylakoid membrane, so the term is shared between the two processes even though the chain in this question's option set is unquestionably part of respiration.

• Missing the negation in the stem and choosing a stage that genuinely belongs to respiration, which is what three of these four rows are for
• Confusing the Calvin cycle with the citric acid cycle because both are cycles named after their discoverers and both handle carbon dioxide, in opposite directions
• Treating glycolysis as somehow outside respiration because it happens in the cytoplasm rather than in the mitochondrion
• Assuming a term belongs to only one process, when an electron transport chain operates in photosynthesis as well as in respiration
• Learning the stages without their locations, when a large share of questions on this topic are location questions wearing a different label

Respiration and photosynthesis reach MPSC papers in four shapes. The first is a location item — where glycolysis occurs, where the citric acid cycle runs, where the light reactions take place — and it is answered from a single memorised column. The second is the classification item used here, sorting named stages into one process or the other, often with a negative stem. The third is a product or yield item, asking which gas is released, what the final electron acceptor is, or how many ATP a stage produces. The fourth attaches a name to a pathway, since Krebs, Calvin and Mitchell are all asked by name. One page carrying the stages of both processes with their locations, inputs, outputs and discoverers answers all four, and it is among the highest-yield pages a candidate can prepare for the science section.

No directly related past PYQ was found.

• practice — not a real PYQ

  The Calvin cycle takes place in which part of the cell ?

  • (a)The mitochondrial matrix
  • (b)The stroma of the chloroplast
  • (c)The thylakoid membrane
  • (d)The cytoplasm

  Answer(b) The stroma of the chloroplast — the Calvin cycle is the light-independent stage of photosynthesis, in which carbon dioxide is fixed by RuBisCO and reduced to sugar using the ATP and NADPH made by the light reactions. Those light reactions occur on the thylakoid membranes, which is the third option and the commonest confusion; the mitochondrial matrix is where the citric acid cycle of respiration runs; and the cytoplasm is where glycolysis takes place.

• practice — not a real PYQ

  In aerobic respiration, what is the final acceptor of electrons in the electron transport chain ?

  • (a)Carbon dioxide
  • (b)NAD⁺
  • (c)Oxygen
  • (d)Pyruvate

  Answer(c) Oxygen — electrons delivered by NADH and FADH₂ pass down the carrier complexes of the inner mitochondrial membrane and are finally accepted by oxygen, which combines with hydrogen ions to form water. That is why the pathway is called aerobic and why it halts without oxygen. NAD⁺ is an electron carrier within the chain's supply line rather than its terminus, carbon dioxide is a waste product of the citric acid cycle rather than an acceptor, and pyruvate is the product of glycolysis and the input to the link reaction.