All systems ↗Acetyl-CoA enters
A two-carbon acetyl group combines with oxaloacetate to form citrate. Acetyl-CoA can derive from carbohydrate, fat, ketones, or amino acids.
CELLULAR ENERGY / 03
Carbon feeds an electrochemical gradient.
The citric-acid cycle transfers energy from acetyl-CoA to NADH and FADH₂. The respiratory chain uses their electrons to pump protons, and ATP synthase converts proton motive force into chemical work.
Oxidation releases CO₂
Cycle reactions regenerate oxaloacetate while reducing NAD+ and FAD and producing a guanine nucleotide equivalent.
Intermediates serve two roles
Citrate, alpha-ketoglutarate, succinyl-CoA, fumarate, malate, and oxaloacetate also connect to biosynthesis and signaling.
Anaplerosis refills
When intermediates leave for synthesis, reactions such as pyruvate carboxylation and amino-acid metabolism replenish the pool.
Complexes accept electrons
NADH supplies complex I; FAD-linked pathways can enter through complex II or other carriers. CoQ and cytochrome c shuttle electrons.
Oxygen closes the chain
Complex IV transfers electrons to molecular oxygen and forms water. Inadequate oxygen constrains sustained respiratory flux.
Protons store potential
Complexes I, III, and IV pump protons from matrix to intermembrane space, creating electrical and chemical gradients.
ATP synthase couples return
Proton flow through ATP synthase drives phosphorylation of ADP. Proton leak and transport costs mean ATP yield is conditional, not one fixed number.