AP Syllabus focus:
‘Electrons from NADH and FADH2 pass through the mitochondrial electron transport chain, creating a proton gradient across the inner membrane.’
The mitochondrial electron transport chain transfers high-energy electrons to oxygen in controlled steps. Energy released during these redox reactions is captured by pumping protons, building a gradient that stores potential energy across the inner membrane.
Core Concepts: Electron Flow Coupled to Proton Pumping
Electron transport chain (ETC) overview
Electron transport chain (ETC): A series of membrane-bound protein complexes and mobile carriers that pass electrons through redox reactions, releasing energy in small, usable increments.
Practice Questions
FAQ
FADH$_2$ donates electrons at a later point in the chain (commonly Complex II), bypassing an early proton-pumping complex.
Fewer energy-releasing steps remain to power H⁺ pumping, so fewer protons are moved per electron pair.
Cyanide inhibits the terminal complex that transfers electrons to oxygen.
Electron flow halts, upstream carriers remain reduced, and proton pumping stops because the energy from electron transfer is no longer released stepwise.
Coenzyme Q (ubiquinone) is lipid-soluble and moves within the inner membrane, carrying electrons between large complexes.
Cytochrome c is water-soluble and carries electrons along the outer surface of the inner membrane between specific complexes.
If electrons escape prematurely from ETC carriers, they can partially reduce oxygen to form superoxide and other ROS.
This is more likely when carriers are highly reduced (electron “traffic jam”) or when the chain is disrupted.
Common approaches include:
Tracking pH changes in compartments using pH-sensitive dyes or probes
Measuring membrane potential with voltage-sensitive dyes
Monitoring oxygen consumption rates as a proxy for electron flow linked to pumping
