The energy in the pyruvate molecules produced in glycolysis can only be released using oxygen in a process called the Krebs Cycle.

Before pyruvate can enter the Krebs Cycle, it is oxidised in the Link Reaction. Pyruvate molecules are actively transported into the mitochondrial matrix.

The Link Reaction Carbon dioxide is removed from the pyruvate Hydrogen atoms are removed and transferred to NAD. The reactions are carried out by a collection of enzymes known as the Pyruvate Dehydrogenase Complex. A 2 carbon compound called an acetyl group is formed which is attached to a cofactor called coenzyme A. The resulting acetyl CoA can enter the Krebs Cycle. Pyruvate (3C) Acetyl CoA (2C) CO 2 NAD NADH/H + Extension – the Pyruvate Dehydrogenase Complex

The Krebs Cycle Acetyl CoA from the Link Reaction is combined with a 4C compound called oxaloacetate, forming 6C citrate. Citrate undergoes a series of decarboxylation and dehydrogenation reactions which result in the regeneration of oxaloacetate.

The Krebs Cycle Krebs Cycle animation Krebs Cycle animation

Products of the Krebs Cycle Each turn of the Krebs Cycle produces: 3 molecules of reduced NAD 1 molecule of reduced FAD 1 molecule of ATP 2 molecules of CO 2 The Krebs Cycle turns twice for every glucose molecule broken down, so per glucose molecule the yield is doubled: 6 NAD; 2 FAD; 2ATP and 4 CO 2