MPSC 2022 Prelims Paper-I · Question 49 of 100

Correct — option (2), dehydrogenation, is the answer given in the Commission's final key for this question, and it is the option to mark. The topic behind it is the Krebs cycle, and the cycle is worth setting out in full because every question the Commission builds on cellular respiration comes from this one sequence. The cycle, named after Hans Krebs and also called the citric acid cycle or the tricarboxylic acid cycle, runs in the matrix of the mitochondrion. It begins when acetyl coenzyme A, a two-carbon fragment delivered by the breakdown of pyruvic acid, condenses with the four-carbon oxaloacetic acid to give the six-carbon citric acid. Citric acid is rearranged into isocitric acid; isocitric acid loses a molecule of carbon dioxide to give the five-carbon alpha-ketoglutaric acid; alpha-ketoglutaric acid loses a second molecule of carbon dioxide to give the four-carbon succinyl coenzyme A; succinyl coenzyme A gives succinic acid, with enough energy released to make one molecule of a high-energy phosphate directly; succinic acid becomes fumaric acid; fumaric acid becomes malic acid; and malic acid becomes oxaloacetic acid again, at which point the cycle is ready to accept another acetyl group. Two carbon atoms enter as the acetyl group and two leave as carbon dioxide, so the cycle balances, and one turn yields three molecules of reduced NAD, one of reduced FAD and one high-energy phosphate. The reaction type that dominates the cycle is dehydrogenation, the removal of a pair of hydrogen atoms from a substrate and their transfer to a hydrogen acceptor. Four of the cycle's steps are dehydrogenations of this kind — the oxidation of isocitric acid, the oxidation of alpha-ketoglutaric acid, the oxidation of succinic acid to fumaric acid, and the oxidation of malic acid back to oxaloacetic acid — and it is these four that load the reduced coenzymes which the electron transport chain later uses to make most of the cell's ATP. That is why the enzymes of the cycle are so largely dehydrogenases, and why the stage of the cycle around fumaric and malic acid is flanked on both sides by dehydrogenation, the step before it and the step after it being the succinic and the malic oxidations respectively. One thing should be said plainly, because a student will meet it in every textbook of biochemistry and plant physiology: the standard description of the individual step from fumaric acid to malic acid is the addition of a molecule of water across the double bond of fumaric acid, catalysed by the enzyme fumarase, and written as a hydration. The Commission's final key for this question nevertheless gives option (2), and this card is written to the published key. The candidate's practical position is therefore to mark option (2) as the answer for this paper, and at the same time to carry the textbook account of the fumarase step, since the same step will be examined elsewhere in its usual form.

• (1)decarboxylation — Decarboxylation is the removal of a molecule of carbon dioxide from a substrate, and it can be excluded here by counting carbon atoms alone, without knowing any enzyme names. Fumaric acid and malic acid both contain four carbon atoms, so nothing is lost between them and no decarboxylation can be involved. The two decarboxylations of the Krebs cycle occur earlier in the sequence and are easy to place, because they are exactly the steps at which the carbon skeleton shrinks: the six-carbon isocitric acid loses a carbon dioxide to give five-carbon alpha-ketoglutaric acid, and alpha-ketoglutaric acid loses another to give the four-carbon succinyl coenzyme A. Both of these are described as oxidative decarboxylations, since a hydrogen pair is removed at the same time as the carbon dioxide. A third oxidative decarboxylation, the conversion of three-carbon pyruvic acid to the two-carbon acetyl coenzyme A, occurs just before the cycle begins and is not counted as part of it.
• (3)dehydration — Dehydration is the removal of a molecule of water, which is the opposite of adding one, and it does occur within the respiratory pathway, though not at the stage named in this question. Within the Krebs cycle it appears in the conversion of citric acid to isocitric acid, which the enzyme aconitase carries out in two movements — water is first removed from citric acid to give the intermediate cis-aconitic acid, and then added back in a different position to give isocitric acid — so the overall change is an isomerisation accomplished by a dehydration followed by a rehydration. Dehydration also appears in glycolysis, in the step in which 2-phosphoglyceric acid loses water to become phosphoenolpyruvic acid under the enzyme enolase. Neither of these is the stage between fumaric acid and malic acid, and a candidate who offers dehydration here has placed a real reaction of the pathway at the wrong point in the sequence.
• (4)hydration — This option is not the answer given in the Commission's final key, which gives option (2), and it is option (2) that a candidate should mark for this paper. It is however the option that a well-prepared student is most likely to be drawn to, and the reason should be stated rather than concealed: the standard textbook account of the step from fumaric acid to malic acid describes it as the addition of a molecule of water across the carbon-carbon double bond of fumaric acid under the enzyme fumarase, and that addition is what the word hydration denotes. The card follows the commission's key, and states the textbook account of the step plainly rather than reconciling the two. What the student should take away is the chemistry itself, which is not in doubt: fumaric acid contains a double bond between its two central carbon atoms, malic acid does not, and the difference between the two molecules is one molecule of water.

Cellular respiration breaks glucose down in stages, and the Krebs cycle is the second of them. Glycolysis, in the cytoplasm, converts a six-carbon glucose molecule into two three-carbon molecules of pyruvic acid. Each pyruvic acid then enters the mitochondrion and is converted, in the link reaction, into acetyl coenzyme A with the loss of a carbon dioxide. The Krebs cycle takes that two-carbon acetyl group, joins it to four-carbon oxaloacetic acid, and works the resulting six-carbon acid round a closed sequence of eight steps that releases two molecules of carbon dioxide and regenerates the oxaloacetic acid. The cycle's real product is not carbon dioxide but reducing power: three molecules of reduced NAD and one of reduced FAD per turn, plus one high-energy phosphate made directly at the succinyl coenzyme A step. Those reduced coenzymes are passed to the electron transport chain on the inner mitochondrial membrane, where their electrons are handed down a series of carriers to oxygen and the energy released is used to pump protons and drive the synthesis of ATP. This is why the Krebs cycle is described as the hub of respiration and why it cannot proceed without oxygen even though oxygen takes no part in any of its own reactions: without oxygen at the end of the chain, the reduced coenzymes cannot be reoxidised and the cycle stalls. The cycle is also a hub in a second sense, since its intermediates are drawn off for the synthesis of amino acids and other compounds and are replenished from them.

Cellular respiration is examined by MPSC as a sequence to be reproduced and as a set of quantities to be recalled. The recurring questions are where each stage occurs, what enters and what leaves it, how many molecules of reduced coenzyme and of ATP are produced, which step releases carbon dioxide and which consumes oxygen. Because the whole of it hangs on one diagram, the productive way to prepare is to draw the cycle out repeatedly until the order of the acids can be written from memory, with the carbon count of each written beside it, since the carbon count alone answers a surprising number of questions — it settles at once, for example, that no decarboxylation can be involved between two four-carbon acids. Questions on this topic also illustrate why a candidate should distinguish between what is asked and what is keyed. Where the reasoning a candidate is confident of points one way and the published key points another, the mark goes with the key; the sensible response in the hall is to answer, move on, and treat the discrepancy afterwards as a prompt to check the underlying chemistry rather than to abandon it.

• The Krebs cycle, also called the citric acid or tricarboxylic acid cycle, takes place in the matrix of the mitochondrion and begins when two-carbon acetyl coenzyme A condenses with four-carbon oxaloacetic acid to form six-carbon citric acid.
• One turn of the cycle releases two molecules of carbon dioxide and yields three molecules of reduced NAD, one of reduced FAD and one high-energy phosphate formed directly at the succinyl coenzyme A step.
• Four steps of the cycle are dehydrogenations, in which a pair of hydrogen atoms is removed and passed to a coenzyme: the oxidations of isocitric acid, of alpha-ketoglutaric acid, of succinic acid and of malic acid.
• The two decarboxylations of the cycle are the conversion of isocitric acid to alpha-ketoglutaric acid and of alpha-ketoglutaric acid to succinyl coenzyme A; fumaric acid and malic acid both have four carbon atoms, so no carbon is lost between them.
• In the standard textbook account the step from fumaric acid to malic acid is the addition of a molecule of water across the double bond under the enzyme fumarase; the Commission's final key for this question gives option (2), dehydrogenation, and this card follows the key.

• Confusing the four reaction types of the pathway — decarboxylation, dehydrogenation, dehydration and hydration — by learning them as words rather than by placing each at its step in the sequence
• Forgetting that carbon counting settles many of these questions on its own, since a step between two four-carbon acids cannot be a decarboxylation
• Attributing the link reaction, in which pyruvic acid becomes acetyl coenzyme A, to the Krebs cycle proper, when it takes place before the cycle begins
• Assuming that oxygen participates directly in a reaction of the Krebs cycle, when it acts only at the end of the electron transport chain and the cycle depends on it indirectly

Respiration reaches MPSC papers in three forms. The first names a step and asks for the reaction type or the enzyme, as this question does. The second asks for a quantity — how many molecules of carbon dioxide leave in one turn, how much reduced coenzyme is produced, what the net ATP yield of glycolysis is — and these are answered from a memorised balance sheet. The third asks where a stage occurs, distinguishing the cytoplasm from the mitochondrial matrix and the inner membrane. All three are answerable from a single well-drawn diagram of the pathway with carbon counts and coenzyme yields written on it, and drawing that diagram from memory is the most efficient hour a candidate can spend on this chapter.

No directly related past PYQ was found.

• practice — not a real PYQ

  How many molecules of carbon dioxide are released in one complete turn of the Krebs cycle ?

  • (a)One
  • (b)Two
  • (c)Three
  • (d)Six

  Answer(b) Two — the cycle accepts a two-carbon acetyl group and regenerates the four-carbon oxaloacetic acid it started from, so two carbon atoms must leave in each turn, and they leave as carbon dioxide at the two decarboxylation steps, the conversion of isocitric acid to alpha-ketoglutaric acid and of alpha-ketoglutaric acid to succinyl coenzyme A. Since one glucose molecule yields two molecules of pyruvic acid and therefore two turns of the cycle, four molecules of carbon dioxide come from the cycle for each glucose, with two more from the link reaction.

• practice — not a real PYQ

  In a eukaryotic cell, the Krebs cycle takes place in which of the following ?

  • (a)The cytoplasm
  • (b)The matrix of the mitochondrion
  • (c)The inner membrane of the mitochondrion
  • (d)The endoplasmic reticulum

  Answer(b) The matrix of the mitochondrion — glycolysis takes place in the cytoplasm and is the only stage of respiration that does, while the enzymes of the Krebs cycle are dissolved in the mitochondrial matrix. The inner membrane of the mitochondrion carries the electron transport chain and the enzyme that synthesises ATP, and it is also where the one membrane-bound enzyme of the cycle, succinate dehydrogenase, is located.