Krebs cycle

The Krebs cycle (or citric acid cycle) occurs in the mitochondrial matrix and is the second stage of aerobic respiration (BioNinja). Before entering the cycle, pyruvate is converted into acetyl-CoA.

Within the Krebs cycle, acetyl-CoA is oxidised, producing: 2 ATP (per glucose), NADH, FADH2, Carbon dioxide (Monash University, 2025a).

Although only a small amount of ATP is produced, the Krebs cycle is critical to ATP production, as it generates high-energy electron carriers (NADH and FADH2). These molecules are vital for the electron transport chain, where most ATP is produced. 

This stage highlights the dependance mitochondrial density has on aerobic energy production. Athletes can improve their capacity for sustained ATP production through endurance training which increases mitochondrial number and size (Cleveland Clinic, 2023) .