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.
The ETC is embedded in the inner mitochondrial membrane and is functionally oriented so that H⁺ (protons) are moved from the matrix to the intermembrane space.
Proton gradient as stored energy
Proton gradient: An electrochemical difference in H⁺ concentration and charge across a membrane, created by proton pumping and capable of driving cellular work.
The gradient includes both a pH difference (ΔpH) and a voltage difference (membrane potential) because moving H⁺ also separates charge.
Key Inputs and Final Electron Acceptor
Electron donors: NADH and FADH₂
NADH donates electrons to the ETC at an early entry point, so its electrons can drive more proton pumping.
FADH₂ donates electrons at a later entry point, bypassing an initial pumping step, so it typically contributes to a smaller proton gradient per pair of electrons.
These carriers are produced by earlier stages of respiration; in this subtopic, focus is on how their electrons are handled by the ETC itself.
Oxygen as the terminal electron acceptor
O₂ is the final electron acceptor in aerobic respiration.
At the end of the chain, electrons and protons combine with oxygen to form water (H₂O).
Without oxygen, electron flow backs up, and proton pumping cannot be sustained.
ETC Components and Electron Pathway (Mitochondria)
Major complexes and mobile carriers
The ETC is often described as:
Overview diagram of the mitochondrial electron transport chain (Complexes I–IV) embedded in the inner mitochondrial membrane, including the mobile carriers coenzyme Q and cytochrome c. It highlights where NADH and FADH₂ enter the chain and where protons are pumped to build the electrochemical gradient that ultimately powers ATP production. The figure also shows O₂ as the terminal electron acceptor, forming H₂O at the end of the pathway. Source
Complex I (accepts electrons from NADH; pumps H⁺)
Complex II (accepts electrons from FADH₂; does not pump H⁺)
Ubiquinone (CoQ): lipid-soluble mobile carrier within the membrane
Complex III (passes electrons onward; pumps H⁺)
Cytochrome c: small, water-soluble mobile carrier on the outer surface of the inner membrane
Complex IV (transfers electrons to O₂; pumps H⁺)
Stepwise electron transfer
Electron movement is directional because carriers are arranged by increasing electron affinity (tendency to accept electrons). In simplified sequence:
NADH → Complex I → CoQ → Complex III → cytochrome c → Complex IV → O₂
FADH₂ → Complex II → CoQ → Complex III → cytochrome c → Complex IV → O₂
Because transfers occur in small steps, less energy is lost as heat and more can be captured by proton pumping.
Building the Proton Gradient Across the Inner Membrane
Proton pumping and compartmentalization
As electrons pass through specific complexes, the released energy drives active transport of protons:
Annotated schematic of the mitochondrial ETC emphasizing compartmentalization (matrix vs intermembrane space) and the direction of proton pumping across the inner mitochondrial membrane. By pairing electron flow with H⁺ translocation, it visually explains how redox energy is converted into an electrochemical gradient (proton-motive force). This is the mechanistic link between electron transfer steps and gradient formation described in the notes. Source
Matrix → intermembrane space proton pumping occurs primarily at Complexes I, III, and IV
The inner membrane is highly selective and largely impermeable to H⁺, allowing the gradient to accumulate
The intermembrane space becomes relatively more acidic and positively charged
The matrix becomes relatively more basic and negatively charged
Why the gradient matters (without detailing the next step)
The proton gradient represents stored potential energy. Cells harness this stored energy when protons move back across the inner membrane through specific protein pathways, linking the gradient to ATP production in the next stage.
Factors that influence gradient strength (conceptual)
Availability of NADH and FADH₂: more electron donors can increase electron flow and pumping (until limited elsewhere).
Oxygen availability: limits terminal electron acceptance and therefore the entire chain.
Inner membrane integrity: if the membrane becomes “leaky” to H⁺, the gradient weakens even if electron transport continues.
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
Practice Questions
Describe how electrons from NADH contribute to forming a proton gradient in mitochondria. (2 marks)
Electrons from NADH pass along a series of carriers in the inner mitochondrial membrane (1).
Energy released is used to pump H⁺ from the matrix to the intermembrane space, creating a gradient (1).
Explain how electron flow from NADH and FADH through the mitochondrial electron transport chain results in a proton gradient, including the role of oxygen. (6 marks)
NADH donates electrons to the ETC at an earlier entry point than FADH (1).
FADH donates electrons later (e.g. via Complex II), so fewer pumping steps are used (1).
Electrons are transferred via membrane complexes and mobile carriers (e.g. CoQ and cytochrome c) by redox reactions (1).
Energy released during electron transfer powers active transport of H⁺ across the inner membrane (1).
H⁺ accumulate in the intermembrane space, leaving the matrix relatively negative/alkaline, forming an electrochemical gradient (1).
Oxygen is the terminal electron acceptor; it accepts electrons (and H⁺) to form water, allowing continued electron flow (1).
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