The link reaction, Krebs cycle and oxidative phosphorylation

Last time

Before we start: last time, in Anaerobic respiration, RQ and rice, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The power station

After glycolysis, the pyruvate has only released a small part of the energy in glucose. The rest is unlocked inside the mitochondrion, in three more stages. The link reaction and the Krebs cycle strip off hydrogen, and oxidative phosphorylation turns that hydrogen into most of the A T P. The structure of the mitochondrion fits each stage.

The mitochondrion

A mitochondrion has a double membrane. The outer membrane is smooth and permeable to small molecules. The inner membrane is folded into cristae, which increase surface area. The inner membrane holds the electron transport chain and A T P synthase. The space between the membranes holds protons that build up during respiration. The matrix is the fluid inside. It contains enzymes for the link reaction and the Krebs cycle, and small circular D N A and 70S ribosomes.

The link reaction

Pyruvate enters the matrix by active transport. There, it is decarboxylated: carbon dioxide is removed. It is also dehydrogenated: hydrogen is removed and given to N A D. What remains is a two carbon acetyl group, which combines with coenzyme A to make acetyl coenzyme A. For each glucose, the link reaction happens twice, producing two carbon dioxide and two reduced N A D.

The Krebs cycle

The two carbon acetyl group joins a four carbon compound to make a six carbon compound. This is then broken down in a series of steps back to the four carbon compound. On the way, two carbon dioxide molecules are lost, and hydrogen is removed and passed to N A D and to F A D. One A T P is made by substrate level phosphorylation. Each turn of the cycle makes three reduced N A D, one reduced F A D and one A T P.

Totals per glucose

One glucose gives two pyruvate, so there are two link reactions and two turns of the cycle. Carbon dioxide: two from the link plus four from the cycle, six in all, matching the six carbons of glucose. Reduced N A D: two from glycolysis, two from the link, six from the cycle, ten in all. Reduced F A D: two. A T P by substrate level phosphorylation: two in glycolysis and two in the cycle.

A past-paper example

May June 2025, Paper 43, question 1 part b. Describe the link reaction. Four marks. Pyruvate is decarboxylated, losing carbon dioxide, and dehydrogenated, with hydrogen passed to N A D. The two carbon acetyl group joins coenzyme A. The enzymes are in the matrix. Pyruvate enters by active transport through the carrier.