The core idea is that electrons move through protein complexes in the inner mitochondrial membrane, releasing energy that pumps H+ into the intermembrane space. This creates a proton gradient, and H+ flows back through ATP synthase to produce ATP from ADP and phosphate. Oxygen is the final electron acceptor and combines with electrons and H+ to form water.
Overall, oxidative phosphorylation usually produces about 26 to 28 ATP per glucose molecule in eukaryotic cells.
Key Facts
- The electron transport chain is located in the inner mitochondrial membrane in eukaryotic cells.
- NADH donates high-energy electrons to Complex I, while FADH2 donates electrons to Complex II.
- Electrons move through the chain in the order Complex I or II, coenzyme Q, Complex III, cytochrome c, Complex IV, and then oxygen.
- Oxygen is the final electron acceptor, and the reaction at the end is 1/2 O2 + 2 e- + 2 H+ -> H2O.
- Complexes I, III, and IV pump H+ from the mitochondrial matrix into the intermembrane space.
- The proton gradient stores potential energy called the proton motive force.
- ATP synthase uses the flow of H+ back into the matrix to catalyze ADP + Pi -> ATP.
- Each NADH yields about 2.5 ATP, and each FADH2 yields about 1.5 ATP in many eukaryotic cells.
Vocabulary
- Electron transport chain
- A series of membrane proteins and carriers that pass electrons from NADH and FADH2 to oxygen.
- Oxidative phosphorylation
- The process that uses electron transfer and oxygen to drive ATP production through a proton gradient.
- Chemiosmosis
- The movement of H+ down its concentration gradient through ATP synthase to make ATP.
- Proton motive force
- The stored energy from a difference in H+ concentration and charge across a membrane.
- ATP synthase
- An enzyme complex that makes ATP as H+ ions flow through it.
- Final electron acceptor
- The molecule that receives electrons at the end of an electron transport chain, which is oxygen in aerobic respiration.
Common Mistakes to Avoid
- Saying ATP is made directly by the electron transport chain is wrong because the chain mainly pumps H+ and builds the proton gradient.
- Forgetting oxygen is the final electron acceptor is wrong because electron flow stops without oxygen, which stops most ATP production.
- Thinking FADH2 makes the same ATP as NADH is wrong because FADH2 enters at Complex II and skips one proton-pumping step.
- Mixing up H+ movement is wrong because H+ is pumped from the matrix to the intermembrane space, then flows back into the matrix through ATP synthase.
- Calling oxidative phosphorylation the same as glycolysis is wrong because glycolysis happens in the cytoplasm and does not require the mitochondrial electron transport chain.
Practice Questions
- 1 If 6 NADH molecules enter the electron transport chain and each produces about 2.5 ATP, about how many ATP are produced?
- 2 If 4 FADH2 molecules enter the electron transport chain and each produces about 1.5 ATP, about how many ATP are produced?
- 3 A cell produces 10 NADH and 2 FADH2 from one glucose during earlier stages of respiration. Using 2.5 ATP per NADH and 1.5 ATP per FADH2, estimate the ATP made by oxidative phosphorylation.
- 4 Explain why ATP production decreases sharply if oxygen is not available, even if NADH and FADH2 are still present.