Bioenergetics and Metabolism
Bioenergetics studies energy flow in living cells, especially ATP production and coupled reactions. This domain tests glycolysis, the Krebs cycle, electron transport, oxidative phosphorylation and anaerobic metabolism.
Core rules
- ATP is the energy currency; hydrolysis of ATP to ADP + Pi releases about 7.3 kcal mol−1 under standard conditions.
- Glycolysis occurs in the cytosol, needs no oxygen, and converts glucose to two pyruvate with a net gain of 2 ATP and 2 NADH.
- Pyruvate is converted to acetyl-CoA, which enters the Krebs cycle in the mitochondrial matrix.
- The Krebs cycle per glucose yields 2 ATP/GTP, 6 NADH and 2 FADH2.
- The electron transport chain is on the inner mitochondrial membrane; oxygen is the final electron acceptor, and ATP synthase makes ATP by oxidative phosphorylation.
- In anaerobic conditions, pyruvate is reduced to lactate in animals or to ethanol and CO2 in yeast.
Worked example
Calculate the net ATP gain in glycolysis. Two ATP are invested in the preparatory phase, and four ATP are produced in the payoff phase: 4 − 2 = 2 ATP net. Two NADH are also produced. The net gain is not four ATP, because the investment must be subtracted.
Traps that cost marks
Glycolysis does not require oxygen even though it is part of aerobic metabolism. The Krebs cycle occurs in the mitochondrial matrix, not the cytosol. ATP is not a long-term energy store. Do not say the electron transport chain makes ATP directly-the proton gradient drives ATP synthase.