Which one of the following is not an intermediate step in Respiration ?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2021 Q64 — Science & Technology PYQ with Answer

State PCS 2021 Q64 — Science & Technology PYQ with Answer