7.4 Oxidative Phosphorylation

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) and oxidative phosphorylation are where cells produce the bulk of their ATP. Everything that happened in glycolysis and the citric acid cycle was largely about generating electron carriers (NADH and FADH₂). Now those carriers cash in their electrons here, driving the production of roughly 30–34 ATP per glucose molecule.

Movement of Electrons in the Transport Chain

Electrons enter the ETC from NADH and FADH₂, the electron carriers built up during glycolysis and the citric acid cycle. These electrons pass through a series of protein complexes (I–IV) embedded in the inner mitochondrial membrane, losing energy at each step. That released energy is what powers proton pumping.

Here's the path electrons take:

• Complex I (NADH dehydrogenase) accepts electrons from NADH and passes them to ubiquinone (Q), a mobile carrier.
• Complex II (succinate dehydrogenase) accepts electrons from FADH₂ and also passes them to ubiquinone. Notice that Complex II does not pump protons, which is why FADH₂ yields less ATP than NADH.
• Ubiquinone shuttles electrons to Complex III (cytochrome bc1 complex).
• Complex III passes electrons to cytochrome c, another mobile carrier.
• Cytochrome c delivers electrons to Complex IV (cytochrome c oxidase).
• At Complex IV, electrons are finally transferred to $O_2$, the terminal electron acceptor, forming $H_2O$.

Every transfer in this chain is a redox reaction: one molecule is oxidized (loses electrons) while the next is reduced (gains them). The electrons move toward increasingly electronegative carriers, with oxygen being the most electronegative of all. That's why oxygen must be present for this pathway to run.

Creation of the Proton Gradient

As electrons move through the chain and release energy, Complexes I, III, and IV use that energy to pump protons ($H^+$) from the mitochondrial matrix into the intermembrane space. This builds up a concentration difference across the inner membrane.

The approximate pumping numbers per pair of electrons:

• Complex I: 4 $H^+$
• Complex III: 4 $H^+$
• Complex IV: 2 $H^+$

So for each NADH (which enters at Complex I), about 10 $H^+$ are pumped total. For each FADH₂ (which enters at Complex II, skipping Complex I), only about 6 $H^+$ are pumped. This difference is exactly why NADH generates more ATP than FADH₂.

The inner mitochondrial membrane is impermeable to protons, so they can't just leak back across. This traps them in the intermembrane space, creating what's called the proton motive force. It has two components:

• A chemical gradient: higher $H^+$ concentration in the intermembrane space than in the matrix
• An electrical gradient: the intermembrane space becomes positively charged relative to the matrix

Together, these create a strong driving force for protons to flow back into the matrix, but they can only do so through one specific channel: ATP synthase.

ATP Production Through Chemiosmosis

ATP synthase is the enzyme that converts the energy stored in the proton gradient into ATP. It sits in the inner mitochondrial membrane and has two main parts:

• $F_0$ subunit (the base, embedded in the membrane): acts as a proton channel
• $F_1$ subunit (the knob, projecting into the matrix): catalyzes the synthesis of ATP from ADP + $P_i$

Here's how it works, step by step:

1. Protons flow down their electrochemical gradient through the $F_0$ channel, from the intermembrane space back into the matrix.

2. This flow spins the c-ring in $F_0$, which is physically connected to the γ-subunit (a central stalk) of $F_1$.

3. The rotating γ-subunit causes conformational changes in the three β-subunits of $F_1$. Each β-subunit cycles through three states:

   • Open: ADP and $P_i$ bind
   • Loose: substrates are held in place
   • Tight: ADP and $P_i$ are pressed together to form ATP

4. With the next rotation step, the tight site opens and ATP is released.

5. One full 360° rotation of the γ-subunit produces 3 ATP molecules (one from each β-subunit).

This entire process of using a proton gradient to drive ATP synthesis is called chemiosmosis.

Chemiosmotic Theory and Coupling

Peter Mitchell proposed the chemiosmotic theory, which was controversial at first but is now well established. The core idea is that electron transport and ATP synthesis are not directly linked by a shared chemical intermediate. Instead, they're linked indirectly through the proton gradient.

This means oxidative phosphorylation involves two coupled processes:

• Electron transport → proton pumping: the ETC uses electron energy to build the gradient
• Proton flow → ATP synthesis: ATP synthase uses the gradient to make ATP

If you uncouple these two processes (certain poisons and uncoupling proteins do this by letting protons leak back across the membrane without passing through ATP synthase), electron transport still runs and oxygen is still consumed, but no ATP is made. The energy dissipates as heat instead. This is actually how brown fat generates body heat in newborns and hibernating animals.