8.6:
Electron Transport Chains
The inner mitochondrial membrane houses a series of proteins which participate in the electron transport chain or ETC.
In fact, the multiple folds in the inner membrane help to accommodate numerous copies of these proteins.
The ETC is mainly a series of four multi-subunit protein complexes labeled I to IV and the associated mobile electron carriers.
Cellular respiration starts with the breakdown of organic molecules like glucose to produce high-energy carrier molecules— NADH and FADH2 in addition to a few ATPs.
The NADH donates its electrons to ETC at Complex I, while FADH2 donates its electrons at Complex II.
After entering the ETC, the electrons travel from one complex to another in an energetically downhill sequence to reach oxygen, the terminal electron acceptor.
This transfer of electrons is aided by the mobile electron carriers, such as Q and cytochrome c.
The energy released during electron transfer is used to pump protons from the mitochondrial matrix into the intermembrane space, generating a proton gradient that can be further used by ATP synthase to generate ATP.
8.6:
Electron Transport Chains
The final stage of cellular respiration is oxidative phosphorylation that consists of two steps: the electron transport chain and chemiosmosis. The electron transport chain is a set of proteins found in the inner mitochondrial membrane in eukaryotic cells. Its primary function is to establish a proton gradient that can be used during chemiosmosis to produce ATP and generate electron carriers, such as NAD+ and FAD, that are used in glycolysis and the citric acid cycle.
The ETC is comprised of protein complex I, II, III, and IV. NADH and FADH2 are reduced electron carriers that donate electrons to the ETC complexes. NADH can directly donate electrons into complex I, while FADH2 donates electrons into complex II. Upon donation of electrons, NADH and FADH2 are converted back to their oxidized forms NAD+ and FAD, respectively.
These ETC complexes pass electrons to one another through multiple redox reactions in an energetically downhill sequence. These reactions release energy that is used to pump H+ across the inner membrane from the matrix into the intermembrane space, establishing a proton gradient across the inner membrane. The flow of H+ ions down their electrochemical gradient back into the matrix through ATP synthase enables the conversion of ADP to ATP.
Suggested Reading
- Reece, J.B.et al. Campbell Biology. 10th ed. Pearson, London, UK (2014).
- Guo, R., Zong, S., Wu, M., Gu, J., Yang, M. Architecture of Human Mitochondrial Respiratory Megacomplex I2III2IV2. Cell. 170 (6), 1247-1257 (2017).
- Clark, M. A., Douglas, M., Choi, J. Section 7.4: Oxidative Phosphorylation. In Biology 2e. OpenStax. Houston, TX (2018).