Bioenergetics and Biological Oxidation: ATP, Electron Transport Chain and Oxidative Phosphorylation
What this domain tests
Bioenergetics questions ask how redox energy becomes ATP, how the electron transport chain is organized, and how inhibitors or uncouplers change oxygen use and phosphorylation.
Core rules
- ATP is the main energy currency. Hydrolysis of its terminal phosphoanhydride bond is strongly exergonic; ATP synthesis is coupled to proton flow.
- Glycolysis and the TCA cycle produce NADH and FADH2. NADH donates electrons at complex I; FADH2 at complex II. Electrons pass via ubiquinone and cytochrome c to complex IV, where oxygen is the final acceptor and water is formed.
- Complexes I, III, and IV pump protons. The resulting electrochemical gradient drives complex V (ATP synthase). This is chemiosmotic coupling.
- P/O ratios express ATP made per pair of electrons. Modern values are about 2.5 ATP per NADH and 1.5 per FADH2; some school texts use 3 and 2. Follow the convention stated in your textbook.
- Uncouplers increase electron transport and oxygen consumption but decrease ATP synthesis. ETC inhibitors block electron flow and also reduce ATP.
Worked example
One NADH and one FADH2 are oxidized. Using modern P/O values, ATP yield = 2.5 + 1.5 = 4 ATP. Using older values, yield = 3 + 2 = 5 ATP. The difference comes from the number of protons required per ATP and from proton leak; the chemistry of electron transfer is unchanged.
Traps
Oxygen is not a substrate for ATP synthase; it is the terminal electron acceptor. Cyanide inhibits complex IV, not ATP synthase. Oligomycin inhibits ATP synthase. An uncoupler makes oxygen consumption rise while ATP falls, so it does not simply inhibit respiration.