In which part of the cell, the glucose is converted into pyruvate?

Correct — C, Cytoplasm. The breakdown of glucose to pyruvate is glycolysis, and its ten enzyme-catalysed steps all take place in the cytoplasm, outside any organelle. Glycolysis needs no oxygen and happens in every living cell, prokaryotic and eukaryotic alike. What happens to the pyruvate afterwards is what varies — in the presence of oxygen it moves into the mitochondrion for complete oxidation, and without oxygen it is converted to lactate or to ethanol.

• (a)Mitochondria — The mitochondrion handles the later stages — the link reaction, the Krebs cycle and oxidative phosphorylation. Pyruvate enters it already formed.
• (b)Nucleus — The nucleus stores and transcribes DNA. Respiratory enzymes do not operate there.
• (d)Endoplasmic reticulum — The endoplasmic reticulum is a transport and synthesis network, handling proteins on its rough face and lipids on its smooth face. It plays no part in glycolysis.

Cellular respiration divides into stages by location. Glycolysis, in the cytoplasm, splits one six-carbon glucose into two three-carbon pyruvates with a small net yield of two ATP and two NADH. The link reaction and the Krebs cycle then run in the mitochondrial matrix, and the electron transport chain sits on the inner mitochondrial membrane, where the great bulk of the ATP is made.

That glycolysis is cytoplasmic and oxygen-independent is the key to several other facts. Bacteria have no mitochondria yet respire, mature red blood cells have lost theirs and depend entirely on glycolysis, and a muscle working beyond its oxygen supply falls back on the same pathway with lactate as the end product. The evolutionary reading is that glycolysis is the oldest part of the machinery, in place before the mitochondrion was acquired.

• Glycolysis converts one glucose molecule into two molecules of pyruvate in the cytoplasm.
• It requires no oxygen and occurs in all living cells, including prokaryotes that have no mitochondria.
• The net yield of glycolysis is two ATP and two NADH per glucose molecule.
• Pyruvate enters the mitochondrion for the link reaction and the Krebs cycle when oxygen is available.
• The electron transport chain on the inner mitochondrial membrane produces most of the ATP, which is why the mitochondrion is called the powerhouse of the cell.

• Placing the whole of respiration inside the mitochondrion.
• Assuming glycolysis needs oxygen because respiration usually does.
• Forgetting that prokaryotes respire without any mitochondria at all.

A location item. Fixing the boundary between what happens in the cytoplasm and what happens inside the mitochondrion answers most questions on this pathway.

• practice — not a real PYQ

  The number of ATP molecules produced by the complete aerobic oxidation of one molecule of glucose is closest to

  • (a)2
  • (b)4
  • (c)12
  • (d)30 to 32

  Answer(d) 30 to 32 — glycolysis alone gives a net two, and the overwhelming majority comes from the mitochondrial stages.

• practice — not a real PYQ

  In yeast, the anaerobic breakdown of pyruvate yields

  • (a)lactic acid and water
  • (b)ethanol and carbon dioxide
  • (c)carbon dioxide and water
  • (d)acetic acid only

  Answer(b) ethanol and carbon dioxide — the reaction behind both brewing and the rising of bread dough.