How many Carbon atoms are present in a Pyruvate molecule?

Correct — B, 3. Pyruvate is the anion of pyruvic acid, and its skeleton is CH3-CO-COO-, giving the formula C3H3O3-. Count the carbons along the chain: a methyl carbon, a carbonyl carbon in the middle, and the carboxylate carbon at the end. Three. A candidate who has forgotten the structure can still recover the number from the arithmetic of glycolysis, and that is the more useful route. Glycolysis takes one molecule of glucose, which has six carbons, and splits it in the cytoplasm into two molecules of pyruvate, with a net yield of two ATP and two NADH and no oxygen required. Six carbons in, two products out, so each product must carry three. What happens next fixes the number from the other side. In the presence of oxygen, pyruvate crosses into the mitochondrion and is stripped of one carbon as carbon dioxide by the pyruvate dehydrogenase complex, leaving a two-carbon acetyl group carried on coenzyme A. Three minus one is two, which is exactly the acetyl unit that then joins four-carbon oxaloacetate to form six-carbon citrate and open the Krebs cycle.

• (a)2 — The size of the acetyl group in acetyl-CoA, which is what is left after pyruvate loses one carbon as carbon dioxide. It is the count one step after pyruvate, not pyruvate itself.
• (c)4 — Oxaloacetate, the four-carbon acceptor that acetyl-CoA condenses with at the start of the Krebs cycle. Succinate, fumarate and malate are also four-carbon.
• (d)5 — Alpha-ketoglutarate, the five-carbon intermediate formed when six-carbon isocitrate loses a carbon dioxide inside the Krebs cycle.

Glycolysis is the first stage of the breakdown of glucose and the one stage common to aerobic and anaerobic respiration alike. It runs in the cytoplasm, needs no oxygen, and converts one six-carbon glucose into two three-carbon pyruvate molecules with a net gain of two ATP and two NADH. What follows depends on oxygen. With oxygen, pyruvate enters the mitochondrion, is decarboxylated to a two-carbon acetyl group on coenzyme A, and that acetyl unit is oxidised in the Krebs cycle. Without oxygen, pyruvate is converted to lactate in animal muscle — still three carbons — or to ethanol and carbon dioxide in yeast.

This item is really a test of carbon bookkeeping, and the wrong options are not random: 2, 4 and 5 are the carbon counts of acetyl-CoA, oxaloacetate and alpha-ketoglutarate, the three molecules that sit immediately around pyruvate in the same pathway. Anyone who has memorised the pathway as a sequence of names without attaching a number to each will find all four options plausible. The fix is to carry the numbers with the names — glucose 6, pyruvate 3, acetyl 2, oxaloacetate 4, citrate 6, alpha-ketoglutarate 5 — because half the questions asked on respiration are answerable from that count alone.

• Pyruvate is CH3-CO-COO-, three carbons; the parent acid, pyruvic acid, is C3H4O3.
• Glycolysis splits one six-carbon glucose into two three-carbon pyruvates in the cytoplasm, with a net gain of two ATP and two NADH.
• The pyruvate dehydrogenase complex removes one carbon as carbon dioxide, leaving the two-carbon acetyl group of acetyl-CoA.
• Acetyl-CoA condenses with four-carbon oxaloacetate to give six-carbon citrate, starting the Krebs cycle.
• In oxygen-starved muscle, pyruvate becomes lactate, which also has three carbons; in yeast it becomes ethanol plus carbon dioxide.

• Answering 2 by confusing pyruvate with the acetyl group it becomes after decarboxylation.
• Forgetting that glycolysis produces two pyruvates per glucose, which is what forces the count to three.
• Assuming glycolysis needs oxygen. It runs in the cytoplasm and is the shared first stage of both respiratory routes.

As a carbon-count item like this one, as 'the end product of glycolysis is', or as a question on where in the cell each stage of respiration takes place.

• practice — not a real PYQ

  In which part of the cell does glycolysis take place?

  • (a)Mitochondrial matrix
  • (b)Cytoplasm
  • (c)Nucleus
  • (d)Ribosome

  Answer(b) Cytoplasm — glycolysis runs outside the mitochondrion and needs no oxygen, which is why it is common to aerobic and anaerobic respiration alike.

• practice — not a real PYQ

  Acetyl-CoA condenses with which one of the following to form citrate at the start of the Krebs cycle?

  • (a)Pyruvate
  • (b)Oxaloacetate
  • (c)Alpha-ketoglutarate
  • (d)Succinate

  Answer(b) Oxaloacetate — the two-carbon acetyl group plus four-carbon oxaloacetate gives six-carbon citrate.