Chemiosmosis: how a proton gradient makes ATP

A dam and a turbine

Think of water held behind a dam. When it is let out through a turbine, it spins the turbine and makes electricity. A proton gradient across a membrane works in the same way. Protons held on one side drive the enzyme ATP synthase as they flow back.

This idea is the chemiosmotic hypothesis. It says that ATP is made when protons flow down a gradient across a membrane. It was put forward by Peter Mitchell. Chemiosmosis needs four things:

• a membrane,
• a proton pump,
• a proton gradient,
• ATP synthase.

Building the gradient

Protons end up in the lumen, and the stroma loses them. Three things cause this.

1. Water is split inside the lumen. Each time, protons are set free in the lumen.
2. A carrier moves protons across. The first acceptor of PS II lies on the outer, stroma side of the membrane. It does not pass its electron to an ordinary electron carrier. It passes it to a carrier that also takes a proton from the stroma. When this carrier gives the electron to the next carrier on the inner side, it sets the proton free in the lumen.
3. NADP reductase takes protons from the stroma. This enzyme lies on the stroma side. When it makes NADPH from NADP+, it uses up a proton from the stroma.

So the lumen collects protons and becomes acid. The stroma loses protons. The result is a gradient across the thylakoid membrane, with more protons inside.

ATP synthase

ATP synthase has two parts.

• CF0 is embedded in the thylakoid membrane. It forms a channel across it, through which protons can flow.
• CF1 sticks out on the stroma side of the membrane. It is the part that makes ATP.

When protons flow back from the lumen to the stroma through CF0, the gradient is lost. The flow causes a change in the shape of CF1, and CF1 joins ADP and phosphate to make ATP. So the ATP is made on the stroma side, where it is needed for the sugar-making reactions.

Where the products go

The ATP and NADPH made in the thylakoids are released into the stroma. They drive the biosynthetic phase. When they have been used, ADP and NADP+ return to the thylakoids to be made again. This cycle goes on as long as there is light.

You should find that a lumen of pH $5$ and a stroma of pH $8$ differ by $3$ units. This makes the proton concentration $1000$ times higher in the lumen. If the lumen pH rises to $7$, the gradient is only tenfold.

Try it in Python

The program turns two pH values into the steepness of the gradient.

You should see a ratio of $1000$ with the first values, and $100$ when the lumen pH is $6$. A smaller difference means a weaker drive for ATP synthase.

Practise with new numbers

Common mistakes

• Putting the protons in the stroma. They collect in the lumen.
• Saying that NADP reductase adds protons to the lumen. It uses protons from the stroma, which helps the gradient.
• Giving CF1 the channel. CF0 is the channel. CF1 makes the ATP.
• Placing the ATP on the lumen side. The ATP is made on the stroma side, where CF1 lies.
• Forgetting that the membrane must be intact. A leaky membrane cannot hold a gradient.

Remember these

• Chemiosmosis needs a membrane, a proton pump, a proton gradient and ATP synthase.
• The lumen gets protons from splitting of water and from the carrier. NADP reductase removes them from the stroma.
• CF0 is the channel. CF1 faces the stroma and makes ATP.
• The ATP and NADPH go to the stroma for the biosynthetic phase.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four things that chemiosmosis needs.

2. In which space do protons collect, and in which are they removed?

3. Name two ways in which the lumen gets its protons.

4. What do CF0 and CF1 do?

5. On which side of the thylakoid membrane is the ATP made, and why is that useful?