Summary
Highlights
Cellular respiration is the process where cells break down macromolecules like glucose to produce ATP, the energy currency of the cell. It involves three main steps: glycolysis, the Krebs or citric acid cycle, and the electron transport chain (oxidative phosphorylation).
Glycolysis takes place in the cytosol and is the first step in extracting energy from glucose. It consumes one glucose and two ATP, producing four ATP, two NADH, and two pyruvate molecules. The net gain from glycolysis is two ATP and two NADH.
The two pyruvate molecules from glycolysis undergo oxidation to produce two acetyl CoA and two more NADH. These two acetyl CoA then enter the Krebs cycle, which occurs in the mitochondria. Each acetyl CoA yields three NADH, one FADH2, and one ATP (or GTP). For a single glucose molecule, the Krebs cycle collectively produces six NADH, two FADH2, and two ATP.
The final step, oxidative phosphorylation and the ETC, also takes place in the mitochondria. It uses the NADH and FADH2 to create an electrochemical gradient of hydrogen ions across a membrane. This gradient powers ATP synthase to produce ATP. Each NADH yields approximately 2.5 ATP, and each FADH2 yields approximately 1.5 ATP.
From a single glucose molecule, the total ATP production is approximately 32 ATP. This comes from 10 NADH (yielding 25 ATP), 2 FADH2 (yielding 3 ATP), and 4 direct ATP (2 from glycolysis and 2 from the Krebs cycle). While the theoretical yield is 38 ATP, actual cellular conditions result in 30-32 ATP.