Adenosine triphosphate (ATP) is the cell's energy currency. Hydrolyzing its terminal phosphate bond releases about 30.5 kJ/mol under standard conditions and 50–60 kJ/mol inside the cell, powering muscle contraction, ion pumping, biosynthesis, and signaling. Cells turn over their entire ATP pool every 1–2 minutes.

Modern textbook values put the net aerobic yield at 30–32 ATP per glucose, not the older 36–38 figure. The lower number reflects updated P/O ratios — about 2.5 ATP per NADH and 1.5 ATP per FADH2 — plus the energy cost of shuttling cytosolic NADH into the mitochondrion.

Without oxygen, glycolysis alone produces a net 2 ATP per glucose, with pyruvate reduced to lactate to regenerate NAD+. That is roughly 15 times less ATP than aerobic respiration, but glycolysis runs about 100 times faster, which is why sprinting muscle relies on it heavily.

Creatine phosphate donates a phosphate to ADP via creatine kinase, regenerating ATP in milliseconds. Muscle stores enough phosphocreatine to fuel maximal effort for about 8–10 seconds — the system that powers a 60 m sprint, a heavy lift, or a vertical jump before glycolysis takes over.

Phosphocreatine resynthesis is biphasic: about 70% returns within 30 seconds and 95–100% within 3–5 minutes of rest, depending on aerobic fitness. NSCA and ACSM strength guidelines recommend 2–5 minute rest intervals between maximal sets for this reason.

The tool uses 2 + 2 + 28 = 32 ATP, matching the upper bound of the 30–32 ATP consensus range in Lehninger Principles of Biochemistry. Lowering the efficiency slider models real mitochondria, which lose 20–30% of the proton gradient to leak, heat, and transport work.

About 88% of aerobic ATP is generated in the electron transport chain through chemiosmosis: NADH and FADH2 donate electrons to Complexes I–IV, protons are pumped across the inner mitochondrial membrane, and the resulting gradient drives ATP synthase to phosphorylate ADP.