MPSC 2021 Prelims Paper-I · Question 64 of 100

Correct — option (2). Read the stem carefully first: it asks which of the four is NOT an intermediate step in respiration, and the word 'not' is printed in bold in the English column and 'नाही' in bold in the Marathi one, so the paper itself is warning that the answer is the odd one out. Three of the four arrows describe real reactions of glycolysis, the first stage of respiration; the keyed one describes a reaction that runs the other way. Glycolysis, also called the Embden-Meyerhof-Parnas pathway, takes place in the cytoplasm and converts one molecule of glucose into two of pyruvate through a fixed sequence of ten enzyme-catalysed steps. It opens with an investment phase in which ATP is spent: glucose is phosphorylated to glucose-6-phosphate by hexokinase; glucose-6-phosphate is rearranged to fructose-6-phosphate by phosphoglucose isomerase; and fructose-6-phosphate is phosphorylated a second time, by phosphofructokinase, to fructose-1,6-bisphosphate, the compound older books call fructose-1,6-diphosphate. That third reaction is the committed and effectively irreversible step of the whole pathway, and it consumes the second molecule of ATP. The six-carbon sugar is then split by aldolase into two three-carbon pieces, and the pay-off phase begins, in which the cell recovers four ATP and two NADH. The keyed option asks the candidate to accept fructose-1,6-diphosphate turning back into fructose-6-phosphate. In a cell that reaction does happen, but it is not part of respiration. It is a hydrolysis catalysed by a different enzyme, fructose-1,6-bisphosphatase, and it belongs to gluconeogenesis — the pathway by which the liver and kidney build glucose out of lactate, glycerol and amino acids when the body needs to raise its blood sugar. Gluconeogenesis is not simply glycolysis in reverse; it retraces most of the same steps but has to bypass the three irreversible ones with different enzymes, and this is one of those bypasses. So the two directions are catalysed by different enzymes, are separately regulated, and serve opposite purposes: glycolysis breaks glucose down to release energy, gluconeogenesis builds it up at the cost of energy. Putting a biosynthetic bypass reaction into a list of respiratory intermediates is exactly the sort of inversion this stem is built on, and option (2) is therefore the one that does not belong. Two printing points deserve mention because they can unsettle a reader: the paper writes 'Fructose 1-6 diphosphate' in this option, and in option (3) it writes the same compound as 'Fructose 1,6-phosphate', dropping the prefix altogether — which, read literally, names a different substance, since a 1,6-phosphate would carry one phosphate group and the intermediate carries two. Both are the paper's own spellings of fructose-1,6-bisphosphate and are reproduced here as printed.

• (1)Glucose → glucose-6-phosphate — This is a genuine step, and in fact the very first one of glycolysis. Hexokinase transfers a phosphate group from ATP to glucose to give glucose-6-phosphate, and although it costs the cell an ATP, the investment does two useful things: it makes the sugar reactive enough for the reactions that follow, and it traps it inside the cell, because the plasma membrane carries transporters for glucose but not for the charged phosphorylated form. In the liver the same reaction is carried out by glucokinase, an isoenzyme with a lower affinity for glucose that only becomes active when blood sugar is high. Since the arrow describes a real intermediate step, it cannot be the answer to a stem asking for the one that is not.
• (3)Fructose-6-phosphate → Fructose 1,6-phosphate — This is the third step of glycolysis and, so far as regulation goes, the most important one in the pathway. Phosphofructokinase transfers a phosphate from ATP to fructose-6-phosphate to produce fructose-1,6-bisphosphate — the compound this option prints as 'Fructose 1,6-phosphate' — and once that has happened the sugar is committed to being broken down, because the reaction is effectively irreversible. Phosphofructokinase is accordingly the pace-setting enzyme of glycolysis: it is inhibited by ATP and citrate when the cell has plenty of energy, and stimulated by AMP and ADP when it does not. It is a real intermediate step, and it is the forward version of the reaction the keyed option prints backwards, which is precisely the pair the examiner has set against each other.
• (4)1-3 di PGA → 3PGA — This too is a real step, from the pay-off phase of glycolysis. 1,3-diphosphoglycerate, written in this option as 1-3 di PGA, hands its high-energy phosphate group to ADP to give ATP and 3-phosphoglycerate, and the enzyme is phosphoglycerate kinase. The step matters for a reason worth remembering separately: it is an example of substrate-level phosphorylation, in which ATP is made by direct transfer of a phosphate group from a substrate, without any involvement of the electron transport chain or of oxygen. Because two three-carbon molecules are travelling down the pathway from each glucose, this step yields two ATP, and a second substrate-level phosphorylation later in the pathway yields two more, which is how glycolysis comes to have a net gain of two ATP after the two spent in the investment phase.

Cellular respiration is the controlled oxidation of food to release energy as ATP, and it proceeds in stages. Glycolysis, the Embden-Meyerhof-Parnas pathway, takes place in the cytoplasm, needs no oxygen, and converts one glucose into two molecules of pyruvate with a net yield of two ATP and two NADH. Its ten steps fall into an investment phase, in which two ATP are spent to make fructose-1,6-bisphosphate, and a pay-off phase, in which the six-carbon sugar is split into two three-carbon fragments that are oxidised to pyruvate with the recovery of four ATP and two NADH. Three of the ten steps are irreversible — those catalysed by hexokinase, by phosphofructokinase and by pyruvate kinase — and these are the control points. If oxygen is available, pyruvate enters the mitochondrion, where the link reaction converts it to acetyl coenzyme A with the release of carbon dioxide and the reduction of NAD, and the acetyl group is then oxidised completely in the Krebs or citric acid cycle. The NADH and FADH2 generated in these stages are oxidised by the electron transport chain in the inner mitochondrial membrane, and the energy released is used to phosphorylate ADP in the process of oxidative phosphorylation, oxygen serving as the final electron acceptor and being reduced to water. If oxygen is not available, pyruvate is instead reduced by fermentation — to lactic acid in vertebrate muscle, or to ethanol and carbon dioxide in yeast — which regenerates the NAD needed to keep glycolysis running. Running in the opposite direction is gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors, which reuses most of the glycolytic enzymes but replaces the three irreversible steps with separate bypass reactions, one of them the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate.

Respiration is a fixed part of MPSC's biology section, and the Commission asks it at two levels. The easier level is the arithmetic and the geography of the process — where glycolysis happens, where the Krebs cycle happens, how many ATP each stage yields, which stage releases carbon dioxide, which requires oxygen. The harder level, of which this question is an example, tests whether the candidate has the sequence of named intermediates in order rather than a general description of the pathway. Questions of this second kind are usually built by taking a real sequence and altering one item, and the alteration is often a reversal rather than an invention, because a reversed arrow still names two genuine compounds and so survives a quick glance. The defence is to learn glycolysis as a directed chain — glucose, glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-bisphosphate, the three-carbon fragments, 1,3-diphosphoglycerate, 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, pyruvate — with the arrows fixed, and to know that reversing a phosphorylation step takes you out of respiration and into gluconeogenesis. The second habit this question rewards is reading the stem to the end. It is one of the negative stems in this paper, and although the word 'not' is set in bold here, a candidate should never depend on the typography to notice a negation.

• Glycolysis takes place in the cytoplasm, 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.
• The opening steps are glucose to glucose-6-phosphate by hexokinase, glucose-6-phosphate to fructose-6-phosphate by phosphoglucose isomerase, and fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase, the committed and rate-limiting step of the pathway.
• The conversion of fructose-1,6-bisphosphate back to fructose-6-phosphate is catalysed by fructose-1,6-bisphosphatase and belongs to gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors, not to respiration.
• The step from 1,3-diphosphoglycerate to 3-phosphoglycerate, catalysed by phosphoglycerate kinase, is an example of substrate-level phosphorylation — ATP made by direct transfer of a phosphate group from a substrate, without the electron transport chain.
• After glycolysis, pyruvate is converted in the mitochondrion to acetyl coenzyme A in the link reaction, oxidised in the Krebs cycle, and the reduced coenzymes are oxidised by the electron transport chain, where oxygen is the final electron acceptor and is reduced to water.

• Accepting a reversed arrow as a step of the pathway, when a reversal of an irreversible glycolytic step belongs to gluconeogenesis and is catalysed by a different enzyme
• Missing the negation in the stem, and choosing a step that is perfectly correct in a question that asks for the one that is not
• Confusing substrate-level phosphorylation, which needs no oxygen and no electron transport chain, with oxidative phosphorylation at the inner mitochondrial membrane
• Assuming glycolysis occurs inside the mitochondrion, when it takes place in the cytoplasm and only the later stages are mitochondrial

MPSC asks respiration as location questions, as yield questions, as sequence questions of this kind, and as comparisons between aerobic and anaerobic pathways. A recurring device is to name a compound by an older synonym — fructose-1,6-diphosphate for the bisphosphate, PGA for phosphoglyceric acid — so a candidate should recognise a compound under more than one name. The Commission also likes the negative form of the sequence question, in which one of four arrows has been reversed or one intermediate replaced by a compound from a related pathway, and the safest preparation is to be able to write the glycolytic chain out on paper with its arrows pointing one way. Beyond the pathway itself, expect questions on which stage produces carbon dioxide, which requires oxygen, what the final electron acceptor is, how many ATP a glucose molecule yields in all, and what happens in a muscle cell that has run short of oxygen.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which enzyme catalyses the committed and rate-limiting step of glycolysis, the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate ?

  • (a)Hexokinase
  • (b)Phosphofructokinase
  • (c)Aldolase
  • (d)Pyruvate kinase

  Answer(b) Phosphofructokinase — it transfers a phosphate group from ATP to fructose-6-phosphate, and once that reaction has occurred the sugar is committed to the glycolytic pathway because the step is effectively irreversible. The enzyme is the principal control point of glycolysis, inhibited by ATP and citrate when the cell is energy-rich and stimulated by AMP and ADP when it is not. Hexokinase catalyses the first step, glucose to glucose-6-phosphate; aldolase splits the six-carbon sugar into two three-carbon fragments; pyruvate kinase catalyses the last step of the pathway.

• practice — not a real PYQ

  Glycolysis occurs in which part of the cell, and what is its net yield of ATP per molecule of glucose ?

  • (a)In the mitochondrial matrix, with a net yield of four ATP
  • (b)In the cytoplasm, with a net yield of two ATP
  • (c)On the inner mitochondrial membrane, with a net yield of thirty-four ATP
  • (d)In the nucleus, with no net yield of ATP

  Answer(b) In the cytoplasm, with a net yield of two ATP — four molecules of ATP are generated in the pay-off phase by substrate-level phosphorylation, but two are spent earlier in the investment phase, leaving a net gain of two, together with two molecules of NADH. Glycolysis needs no oxygen and no membrane-bound machinery, which is why it happens in the cytoplasm and is common to nearly all living cells. The link reaction and the Krebs cycle occur in the mitochondrial matrix and the electron transport chain on the inner mitochondrial membrane.