Respiration: active recall

It occurs in the cytoplasm. One glucose is converted to two pyruvate molecules, with a net gain of two ATP and reduction of NAD. Glycolysis itself does not directly consume oxygen.

ATP is used in early phosphorylation steps and produced in later steps. The total ATP produced exceeds the amount used by two per glucose in the standard pathway. Reporting only the production ignores the initial investment.

In a eukaryotic cell, the reaction occurs in the mitochondrial matrix. Pyruvate loses carbon dioxide and is oxidised while NAD is reduced. The remaining two-carbon acetyl group combines with coenzyme A to form acetyl-CoA.

It combines with a four-carbon acceptor to form a six-carbon compound. Subsequent reactions release carbon dioxide, reduce NAD and FAD, produce a small amount of ATP or an equivalent energy carrier, and regenerate the four-carbon acceptor.

They transfer electrons derived from earlier oxidation reactions to the electron-transport system. Oxidation of these coenzymes enables electron transfer coupled to proton movement. They are recycled rather than being permanently used up.

It is in the inner mitochondrial membrane. That membrane separates the matrix from the intermembrane space, allowing an electrochemical proton gradient to be maintained. Its folds increase membrane area for the associated machinery.

Electron transfer is coupled to proton movement from the matrix towards the intermembrane space. Protons return down their electrochemical gradient through ATP synthase, driving ATP formation from ADP and inorganic phosphate. This couples oxidation to phosphorylation.

Oxygen accepts electrons at the terminal stage and combines with protons to form water. Without a suitable terminal acceptor, continued electron flow and recycling of reduced coenzymes are restricted. Oxygen is not directly consumed in glycolysis.

ATP is formed by transferring a phosphate group from a phosphorylated substrate to ADP. It does not require a proton gradient across a membrane. It occurs in glycolysis and in a reaction of the Krebs cycle.

Pyruvate is reduced to lactate while reduced NAD is oxidised back to NAD. Regenerated NAD can accept electrons in glycolysis. Lactate formation does not itself supply the large ATP yield associated with aerobic oxidative phosphorylation.

Pyruvate is decarboxylated to form an intermediate that is reduced to ethanol. Carbon dioxide is released, and reduced NAD is oxidised back to NAD during ethanol formation. This allows glycolysis to continue supplying a small net ATP yield.

RQ is carbon dioxide produced divided by oxygen consumed over a comparable interval and under comparable conditions. It can help interpret metabolism, but mixed substrates, physiological state and experimental conditions can affect the value. It does not alone prove that one fuel was used exclusively.