IB Biology Topic C1.3.12: ATP Production by Chemiosmosis in Thylakoids Notes

A Proton Gradient Across the Thylakoid Membrane Drives ATP Synthesis

1. The light-dependent reactions build a store of protons inside the thylakoid.

2. Chemiosmosis uses this proton gradient to make ATP, the same way it happens in mitochondria.

3. Making ATP this way in the light is called photophosphorylation.

The Thylakoid Membrane Separates Two Compartments

1. The thylakoid membrane divides two spaces.

   1. Thylakoid lumen: the space inside the thylakoid where protons build up.
   2. Stroma: the fluid around the thylakoid where protons stay low.

2. This difference in proton concentration across the membrane is the gradient that powers ATP synthesis.

Step 1: The Electron Transport Chain Pumps Protons Into the Lumen

1. A chain of electron carriers sits in the thylakoid membrane.

2. Excited electrons pass along the chain, releasing energy at each carrier.

3. That energy is used to pump protons from the stroma into the thylakoid lumen.

4. The lumen becomes crowded with H⁺ while the stroma stays low, building the gradient.

Step 2: Protons Flow Back Through ATP Synthase

1. Protons move down their gradient, from the high-concentration lumen back to the stroma.

2. They can only cross the membrane through ATP synthase.

3. The flow of protons makes part of ATP synthase rotate, like a turbine.

Step 3: ATP Synthase Joins ADP and Phosphate

1. The rotation changes the shape of the enzyme's active sites.

2. This brings ADP and inorganic phosphate (Pi) together to form ATP.

3. ATP is made continuously as long as protons keep flowing.

Two Electron Routes Feed the Same Chemiosmosis

1. The electrons driving the chain come from photosystems that capture light.

2. They can take one of two routes, both of which pump protons and drive ATP synthesis.

3. Non-cyclic photophosphorylation

   1. It uses both PSII and PSI.
   2. Electrons flow from PSII through the chain to PSI, pumping protons on the way.
   3. The electrons end up reducing NADP, covered in C1.3.13.
   4. Electrons lost from PSII are replaced by photolysis of water.
   5. Products: ATP, NADPH, and O₂.

1. Cyclic photophosphorylation

   1. It uses PSI only.
   2. Electrons from PSI pass through the chain, pump protons, then return to PSI in a loop.
   3. No water is split, no NADPH is made, and no oxygen is released.
   4. Product: ATP only.

Comparing the Two Routes at a Glance