Glycolysis, the link reaction and the Krebs cycle

What you’ll learn

• Locate the main stages of aerobic respiration.
• Track carbon loss and reduced coenzyme production.
• Distinguish substrate-level phosphorylation from later ATP production.

Understand the idea

Respiration is an enzyme-controlled series of reactions that transfers energy from organic substrates to ATP. In glycolysis, in the cytoplasm, a hexose is phosphorylated using ATP and split into two three-carbon pathways. Oxidation reduces NAD, and substrate-level phosphorylation generates ATP. For one glucose, the net glycolytic yield is two ATP, two reduced NAD and two pyruvate molecules; oxygen is not a direct reactant in glycolysis.

In aerobic respiration, pyruvate enters the mitochondrial matrix. The link reaction removes carbon dioxide, reduces NAD and forms a two-carbon acetyl group carried by coenzyme A. This enters the Krebs cycle and combines with a four-carbon acceptor. A sequence of decarboxylations and dehydrogenations releases carbon dioxide and produces reduced NAD, reduced FAD and a small amount of ATP while regenerating the acceptor.

Per glucose, the link reaction and two turns of the Krebs cycle release six carbon dioxide molecules. Reduced coenzymes carry electrons to oxidative phosphorylation; most ATP does not arise directly in the Krebs cycle. Carbon is progressively oxidised and energy is transferred in controlled stages rather than all being released in a single combustion reaction. Learn the roles and locations without inventing an obligatory fixed total ATP yield for every cell.

Key formulae

$$\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}\longrightarrow6\mathrm{CO_2}+6\mathrm{H_2O}$$

$$\text{one glucose}\xrightarrow{\text{glycolysis}}2\text{ pyruvate}+2\text{ net ATP}+2\text{ reduced NAD}$$

Worked example

Account for the net carbon dioxide released per glucose during complete aerobic respiration through glycolysis, the link reaction and the Krebs cycle.

1. Glycolysis produces two three-carbon pyruvate molecules without releasing carbon dioxide.
2. The two link reactions each release one carbon dioxide, leaving two two-carbon acetyl groups.
3. Two Krebs-cycle turns release four carbon dioxide molecules in total while regenerating the cycle’s acceptor. This is overall carbon accounting: newly entering acetyl carbon atoms need not leave in their first turn.

Answer: Two carbon dioxide molecules are associated with the link reactions and four with the Krebs cycles per glucose completely oxidised.