The electron transport chain is the final stage of aerobic cellular respiration and is where most ATP is made. It takes place in the inner mitochondrial membrane, where protein complexes pass electrons from NADH and FADH2 to oxygen. This process matters because ATP supplies usable energy for muscle contraction, active transport, biosynthesis, and many other cell activities.
Without a working electron transport chain, cells cannot efficiently harvest the energy stored in food molecules.
As electrons move through the chain, their energy is used to pump H+ ions from the mitochondrial matrix into the intermembrane space. This creates a proton gradient, with high H+ concentration outside the matrix and low H+ concentration inside the matrix. H+ ions then flow back through ATP synthase by chemiosmosis, and that flow powers the formation of ATP from ADP and phosphate.
Oxygen is the final electron acceptor, combining with electrons and H+ to form water.
Key Facts
- The electron transport chain is located in the inner mitochondrial membrane.
- NADH donates electrons to Complex I, while FADH2 donates electrons to Complex II.
- Electron flow releases energy that pumps H+ from the matrix to the intermembrane space.
- O2 + 4e- + 4H+ = 2H2O, so oxygen is reduced to water at the end of the chain.
- ADP + Pi + energy = ATP is the overall energy-storing reaction powered by ATP synthase.
- One NADH typically supports production of about 2.5 ATP, while one FADH2 supports about 1.5 ATP.
Vocabulary
- Electron transport chain
- A series of protein complexes and carriers in the inner mitochondrial membrane that transfer electrons and help create a proton gradient.
- Proton gradient
- A difference in H+ concentration and charge across a membrane that stores potential energy.
- Chemiosmosis
- The movement of H+ ions through ATP synthase that drives ATP production.
- ATP synthase
- A membrane enzyme that uses the flow of H+ ions to make ATP from ADP and phosphate.
- 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
- Thinking ATP is made directly by the electron transport chain complexes is wrong because Complexes I, III, and IV mainly pump H+ ions, while ATP synthase makes ATP.
- Putting the electron transport chain in the cytoplasm is wrong because in eukaryotic cells it is located in the inner mitochondrial membrane.
- Forgetting oxygen is the final electron acceptor is wrong because without oxygen, electrons cannot keep flowing through the chain and ATP production slows or stops.
- Reversing the proton gradient is wrong because H+ is pumped into the intermembrane space, making it higher in H+ than the mitochondrial matrix.
Practice Questions
- 1 A cell sends 8 NADH molecules into the electron transport chain. Using 2.5 ATP per NADH, how many ATP molecules can be produced?
- 2 A cell sends 6 FADH2 molecules into the electron transport chain. Using 1.5 ATP per FADH2, how many ATP molecules can be produced?
- 3 Explain why blocking oxygen from accepting electrons would also stop most ATP production in mitochondria.