The light-dependent reactions take place on the thylakoid membranes of the chloroplast. They use light energy to make ATP and reduced NADP for the light-independent reactions, and they release oxygen.The main events:
1. Light absorption: light energy absorbed by the photosystems excites electrons in the reaction centre chlorophyll, which leave and pass to an electron acceptor.
2. Electron transport: the electrons pass along a chain of electron carriers in the thylakoid membrane, losing energy in a series of redox reactions.
3. Chemiosmosis: this energy pumps protons into the thylakoid space. Protons flow back into the stroma through ATP synthase, and ADP + Pi → ATP. Making ATP using light energy is called photophosphorylation.
4. Reduction of NADP: electrons, together with protons, reduce NADP to form reduced NADP.
5. Photolysis: light energy is used, with an enzyme at photosystem II, to split water:2H2O → 4H+ + 4e− + O2 The electrons replace those lost by chlorophyll, the protons are used to reduce NADP and add to the gradient, and the oxygen diffuses out of the leaf or is used in respiration.
So the products are ATP and reduced NADP (used in the stroma) and oxygen (a by-product).There are two pathways of electron flow. In non-cyclic photophosphorylation both photosystems are used and all three products are made; in cyclic photophosphorylation only photosystem I is used and only ATP is made.
1. Light absorption: light energy absorbed by the photosystems excites electrons in the reaction centre chlorophyll, which leave and pass to an electron acceptor.
2. Electron transport: the electrons pass along a chain of electron carriers in the thylakoid membrane, losing energy in a series of redox reactions.
3. Chemiosmosis: this energy pumps protons into the thylakoid space. Protons flow back into the stroma through ATP synthase, and ADP + Pi → ATP. Making ATP using light energy is called photophosphorylation.
4. Reduction of NADP: electrons, together with protons, reduce NADP to form reduced NADP.
5. Photolysis: light energy is used, with an enzyme at photosystem II, to split water:
So the products are ATP and reduced NADP (used in the stroma) and oxygen (a by-product).There are two pathways of electron flow. In non-cyclic photophosphorylation both photosystems are used and all three products are made; in cyclic photophosphorylation only photosystem I is used and only ATP is made.