C1.3.12 ATP production by chemiosmosis in thylakoids (HL) Notes

Converting Light Energy into Chemical Energy

1. Light excites electrons in photosystems, but this energy needs to be converted into a usable form (ATP).
2. The cell accomplishes this conversion through chemiosmosis, which uses a proton gradient to power ATP synthesis, similar to how it works in mitochondria during cellular respiration.
3. This process links electron flow (from light energy) to ATP production.

Thylakoid Membrane Separates Two Compartments

1. The thylakoid membrane divides the chloroplast into two regions:
   1. Thylakoid space (lumen): The interior compartment of the thylakoid.
   2. Stroma: The fluid surrounding the thylakoid.
2. This physical separation is what allows protons to accumulate on one side (thylakoid space) while remaining low on the other side (stroma).
3. The concentration difference creates the gradient that powers ATP synthesis.

Step-by-Step: How Chemiosmosis Produces ATP in Thylakoids

Step 1: Creating the Proton Gradient

The proton gradient forms through two processes working together:

1. Proton Pumping by the Electron Transport Chain (ETC)
   1. A chain of electron carrier proteins is embedded in the thylakoid membrane.
   2. As electrons move through the ETC, they transfer from carrier to carrier, losing energy at each step.
   3. This released energy is used to actively pump protons (H⁺) from the stroma into the thylakoid space.
   4. Over time, this creates a high concentration of H⁺ in the thylakoid space and a low concentration in the stroma.
2. Photolysis of Water (in Photosystem II)
   1. Light energy splits water molecules in photosystem II: H₂O → ½O₂ + 2H⁺ + 2e⁻
   2. The protons (H⁺) released are deposited directly into the thylakoid space, adding to the concentration.
   3. The electrons replace those lost by chlorophyll in photosystem II when light excites them.
   4. The oxygen is released as a waste product.

Step 2: Protons Flow Through ATP Synthase

1. Protons naturally move down their concentration gradient, from high concentration (thylakoid space) to low concentration (stroma).
2. The only way they can cross the membrane is through ATP synthase, a protein complex embedded in the thylakoid membrane.
3. As protons flow through ATP synthase, they cause part of the enzyme to rotate mechanically, like a turbine.

Step 3: ATP Synthesis

1. The mechanical rotation of ATP synthase drives conformational changes in the enzyme's catalytic sites.
2. These changes bring ADP + Pi (inorganic phosphate) together and compress them, forcing the formation of ATP.
3. As long as protons continue flowing through ATP synthase, ATP production continues.

Where Do the Electrons Come From?

1. The electrons that move through the ETC (driving proton pumping) originate from photosystems: large protein complexes that capture light energy and energize electrons.
2. There are two pathways electrons can take:

Non-Cyclic Photophosphorylation

1. Both photosystem II (PSII) and photosystem I (PSI) are involved.
2. Electron flow:
   1. Light energizes electrons in PSII, exciting them to a higher energy level.
   2. These high-energy electrons move through the ETC, pumping protons into the thylakoid space as they transfer energy.
   3. Electrons reach PSI, where they are re-energized by another photon of light.
   4. Finally, energized electrons from PSI are transferred to NADP⁺, reducing it to NADPH (needed for the Calvin cycle).
   5. Electrons lost from PSII are replaced by electrons from photolysis of water.
3. Products: ATP (from chemiosmosis), NADPH, O₂ (from water splitting)

Cyclic Photophosphorylation

1. Only photosystem I (PSI) is involved.
2. Electron flow:
   1. Light energizes electrons in PSI, exciting them to a higher energy level.
   2. These electrons move through the ETC, pumping protons into the thylakoid space.
   3. Instead of being transferred to NADP⁺, electrons return to PSI, forming a closed loop (cycle).
3. No water is split, no NADPH is produced, and no oxygen is released.
4. Products: ATP only (from chemiosmosis)

Comparing Cyclic and Non-Cyclic Photophosphorylation