NAD participates in all stages of respiration, while FAD only accepts hydrogens in the Krebs cycle.

1. Breaking down macromolecules into smaller ones
2. Producing reduced NAD and reduced FAD, which carry protons and electrons into oxidative phosphorylation
3. Regenerating the four-carbon molecule to combine with acetyl coenzyme A
4. Providing intermediate compounds for the production of other important substances

For each molecule of acetyl coenzyme A that enters the Krebs cycle, it produces reduced NAD, reduced FAD, one molecule of ATP, and two molecules of carbon dioxide.

1. Acetyl coenzyme A combines with a 4C compound to form a 6C compound
2. The 6C compound is decarboxylated, releasing two molecules of carbon dioxide
3. It is also dehydrogenated, releasing hydrogens that reduce NAD and FAD
4. ATP is synthesised from ADP and a phosphate group
5. The 4C compound is regenerated for the next cycle

Substrate-level phosphorylation in the Krebs cycle leads to the direct production of ATP.

The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix.

The coenzymes NAD and FAD pick up the hydrogens released during the Krebs cycle, forming reduced NAD and reduced FAD.

The reduced NAD and reduced FAD produced in the Krebs cycle are used in the final step of aerobic respiration to produce large quantities of ATP.

The ATP produced in the Krebs cycle is used by energy-requiring processes within the cell.

The carbon dioxide will either diffuse away and be removed from the organism as a metabolic waste, or it may be used as a raw material in photosynthesis.