CDS (I) 2023 General Knowledge · Question 1 of 120

Correct — A, lactic acid. A sprint burns ATP faster than the blood can carry oxygen to the thigh, so glycolysis keeps splitting glucose into pyruvate while the mitochondria fall behind. The muscle then reduces that pyruvate to lactic acid out in the cytoplasm. The point of that single step is to hand back the NAD+ glycolysis needs to keep turning, which is how the runner buys a few more seconds of power with no oxygen. What piles up in the fibre is the lactate and the hydrogen ions released along with it, and that build-up is what the paper is calling cramp and pain.

• (b)CO2 — Carbon dioxide is the end product of the aerobic route, and the body clears it fast — it dissolves into the blood, reaches the lungs and is breathed out. Hard breathing after a sprint is that clearance happening, so CO2 does not sit in the thigh.
• (c)pyruvic acid — Pyruvate is the raw material of this step, not its product. In an oxygen-short human muscle it is consumed as fast as it appears, being reduced to lactic acid, so naming it stops one reaction short of the answer.
• (d)ethanol — Turning pyruvate into ethanol and carbon dioxide is the yeast route, used in brewing and baking. Human muscle lacks the decarboxylating enzyme for it and cannot make ethanol at all.

Respiration in a muscle fibre starts the same way whether oxygen is present or not: glycolysis in the cytoplasm splits one glucose into two molecules of pyruvate and nets two ATP. What happens next is what differs. Given enough oxygen the pyruvate enters the mitochondrion and is oxidised all the way to carbon dioxide and water, for a far larger ATP yield. Starved of oxygen, the cell must still turn NADH back into NAD+ or glycolysis halts — human muscle does that by reducing pyruvate to lactic acid, yeast by splitting it into ethanol and carbon dioxide.

Every wrong option here is a genuine product of some respiratory pathway, which is what makes the item worth a second reading: the real question is which pathway a human thigh muscle runs when short of oxygen. Sort the four by organism and by position in the pathway and only lactic acid survives. One honest present-day caveat belongs on this card. School biology, and this key, put the sprinter's pain down to lactic acid. Exercise physiologists now separate three different things — the burning during hard effort, which does track lactate and the hydrogen ions that accompany it; the soreness that arrives a day later, which is micro-damage and inflammation and has nothing to do with lactate; and true cramp, which is increasingly read as runaway motor-nerve firing. The paper is testing the school account, and lactic acid is the keyed answer.

• Glycolysis turns one glucose molecule into two of pyruvate in the cytoplasm and nets two ATP, with no oxygen required.
• In oxygen-short human muscle the pyruvate is reduced to lactic acid, and the purpose of that reduction is to regenerate NAD+ so glycolysis can carry on.
• Yeast meets the same shortage differently, converting pyruvate to ethanol and carbon dioxide — the reaction behind bread and brewing.
• Lactate made in muscle travels in the blood to the liver, where it is rebuilt into glucose; that shuttle is the Cori cycle.

• Picking pyruvic acid because it sounds more advanced — pyruvate is used up at this step, not accumulated.
• Assuming ethanol forms in humans because fermentation was first taught with yeast.
• Treating carbon dioxide as something that piles up in muscle; it leaves through the lungs.

Usually a one-line item on the end product of anaerobic respiration in human muscle, or a pairing item that puts muscle with lactic acid and yeast with ethanol.

• practice — not a real PYQ

  During anaerobic respiration in yeast, pyruvate is converted into which one of the following?

  • (a)Lactic acid
  • (b)Ethanol and carbon dioxide
  • (c)Acetic acid
  • (d)Glucose and water

  Answer(b) Ethanol and carbon dioxide — yeast decarboxylates pyruvate, the reaction used in brewing and in leavening bread.

• practice — not a real PYQ

  Glycolysis, the first stage of respiration, takes place in which one of the following parts of the cell?

  • (a)Mitochondrial matrix
  • (b)Cytoplasm
  • (c)Nucleus
  • (d)Golgi apparatus

  Answer(b) Cytoplasm — glycolysis is cytoplasmic and needs no oxygen; only the later stages move into the mitochondrion.