Electron Transport Chains

The electron transport chain consists of four multi-subunit protein complexes labeled I through IV, embedded in the inner mitochondrial membrane. Complex I accepts electrons from NADH, while Complex II accepts electrons from FADH2. These complexes work together to transfer electrons in an energetically downhill sequence toward oxygen, the terminal electron acceptor, releasing energy used to pump protons across the membrane.

NADH and FADH2 are reduced electron carriers produced during earlier stages of cellular respiration. NADH donates its electrons directly to Complex I, while FADH2 donates electrons to Complex II. Upon donating electrons, these carriers are converted back to their oxidized forms, NAD+ and FAD, which can be recycled back to electron carriers nad and fad for use in glycolysis and the citric acid cycle.

Mobile electron carriers such as Q and cytochrome c facilitate electron transfer between the protein complexes of the electron transport chain. These carriers shuttle electrons from one complex to the next in an energetically downhill sequence, enabling the continuous flow of electrons toward oxygen. Their mobility allows efficient electron transfer across the inner mitochondrial membrane.

As electrons transfer between complexes through redox reactions, energy is released and used to pump protons from the mitochondrial matrix into the intermembrane space. This active transport creates an electrochemical gradient of protons across the inner membrane. The accumulated protons in the intermembrane space drive ATP synthesis when they flow back through ATP synthase and chemiosmosis.

The inner mitochondrial membrane contains numerous folds that increase its surface area, allowing it to accommodate many copies of the electron transport chain protein complexes. This structural adaptation maximizes the number of ETC complexes available for electron transfer and proton pumping, significantly increasing the efficiency of ATP production during oxidative phosphorylation.

After traveling through Complexes I through IV in an energetically downhill sequence, electrons reach oxygen, the terminal electron acceptor. Oxygen combines with electrons and protons to form water, completing the electron transport chain. This final step is essential for regenerating NAD+ and FAD and maintaining the continuous flow of electrons through the chain.

The electron transport chain establishes a proton gradient by pumping H+ ions across the inner membrane. This gradient stores potential energy that drives ATP synthesis as protons flow back into the matrix through ATP synthase. The coupling of electron transport to proton pumping and subsequent ATP synthesis is called oxidative phosphorylation, the final stage of cellular respiration.