Course 10.2Lesson 3 of 3

Oxidative phosphorylation

Electrons passing along the chain drive protons across the inner membrane into the intermembrane space. The protons flow back through ATP synthase, whose F1 part makes ATP and whose F0 part is a channel. Two protons pass for each ATP. This is oxidative phosphorylation. Without oxygen the chain stops and ATP is not made.

about 25 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

Making ATP from a gradient

The electrons do not make ATP directly. They do something else first. As they pass along the chain, the energy that they release is used to pump protons (H $^+$ ) from the matrix across the inner membrane into the intermembrane space.

The protons pile up in the intermembrane space. The two sides of the membrane now differ in proton concentration and in electric charge. This is a proton gradient, and it holds energy, like water held behind a dam.

The protons can return to the matrix only through one gate: ATP synthase. As they flow through it, they drive the making of ATP. The idea that ATP is made by the flow of protons down a gradient is the chemiosmotic hypothesis of Peter Mitchell.

ATP synthase

A drawing of a mitochondrion as an oval. The outer membrane is orange, and inside it a purple inner membrane makes a second oval, with a narrow intermembrane space between the two. Four folds of the inner membrane, the cristae, point into the middle. Small green knobs along the cristae are the F0F1 particles of ATP synthase. The space inside the inner membrane is the matrix. A caption says that the link reaction and the Krebs cycle take place in the matrix, and the electron transport system and ATP synthase are in the inner membrane.

ATP synthase is complex V. It has two parts.

F $_1$ is the head. It is a peripheral protein, it lies on the matrix side, and it holds the site for the making of ATP from ADP and inorganic phosphate.
F $_0$ is an integral protein of the membrane. It forms the channel through which the protons cross the membrane.

For each ATP made, 2 protons pass through F $_0$ , from the intermembrane space to the matrix, down the gradient.

Oxidative phosphorylation

The making of ATP in this way is oxidative phosphorylation. The name tells where the energy comes from: the oxidation and reduction of the carriers. Compare it with two other ways of making ATP.

Substrate level phosphorylation. Where: the cytoplasm (glycolysis) and the matrix (Krebs cycle). Energy from: a reaction of the substrate. No membrane is needed.
Oxidative phosphorylation. Where: the inner mitochondrial membrane. Energy from: oxidation and reduction in the ETS.
Photophosphorylation. Where: the thylakoid membrane of the chloroplast. Energy from: light.

In oxidative phosphorylation and in photophosphorylation, a proton gradient across a membrane and an ATP synthase make the ATP. The difference lies in what drives the pumping of the protons.

Why oxygen is vital

Suppose that there is no oxygen. Complex IV cannot pass its electrons on. All the carriers stay full of electrons, and the whole chain stops. No protons are pumped, and no ATP is made by ATP synthase. NADH is no longer changed to NAD $^+$ , so the Krebs cycle runs out of NAD $^+$ and stops too.

So oxygen is used only at the very end, but the loss of oxygen stops the whole of aerobic respiration. A poison such as cyanide stops the chain in this way. It blocks cytochrome c oxidase.

You should find that the ATP is half of the protons that pass through F $_0$ , and that without oxygen both drop to zero.

Try it in Python

The program works out the ATP from the protons, with and without oxygen.

Practise with new numbers

Common mistakes

Saying that electrons make ATP directly. They pump protons. The protons flowing back make ATP.
Swapping F $_0$ and F $_1$ . F $_0$ is the channel in the membrane. F $_1$ is the head that makes ATP.
Saying that the protons flow from the matrix to the intermembrane space through ATP synthase. They flow the other way, from the intermembrane space to the matrix.
Calling all ATP making substrate level phosphorylation. The ATP of the ETS is made by oxidative phosphorylation.
Thinking that the Krebs cycle can run without oxygen. It stops, because NAD $^+$ and FAD are not made again.

Remember these

Electrons in the chain release energy that pumps protons into the intermembrane space.
The protons return to the matrix through ATP synthase (complex V): F $_1$ makes ATP, F $_0$ is the channel.
Two protons pass through F $_0$ for each ATP made.
This is oxidative phosphorylation, driven by oxidation and reduction. Photophosphorylation is driven by light.
Without oxygen the chain, and with it the cycle, stops.