7.9 Chemiosmosis powers ATP synthesis in the light reactions

How does phosphorylation compare with oxidative phosphorylation?

In cellular respiration, the high-energy electrons passed down the electron transport chain come from the oxidation of food molecules. In photosynthesis, light energy is used to drive electrons to the top of the transport chain. Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP. Notice that in the light-driven flow of electrons through the two photosystems, the final electron acceptor is NADP+, not O2 as in cellular respiration. Electrons do not end up at a low energy level in H2O, as they do in cellular respiration. Instead, they are stored at a high state of potential energy in NADPH.

In summary, what do the light reactions do?

Provide chemical energy (ATP) and reducing power (NADPH) for the next stage of photosynthesis, the Calvin Cycle.

What is the advantage of the light reactions producing NADPH and ATP on the stroma side of the thylakoid membrane?

The Calvin Cycle, which consumes the NADPH and ATP, occurs in the stroma.

Chemiosmosis

The mechanism that generates ATP in a chloroplast. The process of chemiosmosis dries ATP synthesis using the potential energy of a concentration gradient of hydrogen ions (H+) across a membrane. The gradient is created when an electron transport chain uses the energy released as it passes electronsdown the chain to pump hydrogen ions across a membrane.

What happens in chemiosmosis?

The two photosystems and electron transport chains are all located within the thylakoid membrane of a chloroplast. Here you can see that as photo-excited electrons are passed down the electron transport chain connecting the two photosystems, hydrogen ions are pumped across the membrane from the stroma into the thylakoid space. This generates a concentration gradient across the membrane. The flask-shaped structure represents an ATP synthase complex, which is like the one in the mitochondrion. The energy of the concentration gradient drives H+ across the membrane through ATP synthase. ATP synthase couples the flow of H+ to the phosphorylation of ADP. In photosynthesis, this chemiosmotic production of ATP is called phosphorylation because the initial energy input is light energy.