The Citric Acid (Krebs) Cycle

Glycolysis only releases about 25% of glucose's chemical energy, with most energy remaining locked in pyruvate molecules. When oxygen is available, pyruvate enters the mitochondrion where the real energy extraction begins. Inside, carbon dioxide is removed from pyruvate, and the remaining two-carbon fragment forms acetyl coenzyme A (acetyl CoA), which feeds into the Krebs cycle.

The Krebs cycle functions as a cellular "metabolic furnace" with eight key steps. The cycle begins when a two-carbon acetyl group enters and progresses through oxidation reactions that transfer electrons to NAD+ (forming NADH) and FAD (forming FADH₂). One complete turn produces one ATP through substrate-level phosphorylation, but the cycle's main purpose is generating these electron carriers (NADH and FADH₂).

For each glucose molecule that enters cellular respiration, the Krebs cycle runs twice (once for each pyruvate). The complete breakdown results in three CO₂ molecules released per pyruvate, with one CO₂ released during conversion to acetyl CoA and two more during the cycle itself. The total yield from processing one glucose molecule is 6 NADH, 2 FADH₂, and 2 ATP molecules.

Energy Transfer Tip: Think of the Krebs cycle as an energy conversion system rather than an energy production system. It's transferring energy from food molecules to electron carriers (NADH and FADH₂) that will later generate much more ATP in the electron transport chain.