Explain the light-dependent reaction of photosynthesis
The light-dependent reaction along with the Calvin cycle (light-independent) make up the process of photosynthesis in organisms such as plants, algae and cyanobacteria. In the light-dependent reaction light energy is converted into chemical energy in the form of ATP and reduced NADP (NADPH). These compounds provide energy for carrying out the subsequent light-independent reaction. The light-dependent reaction takes place on the thylakoid membranes of the chloroplasts. The chlorophyll in photosystem II absorbs light energy (photoactivation) and two of its electrons move to one plastoquinone that is now reduced. The loss of electrons makes chlorophyll a strong oxidising agent, resulting in the photolysis of water as shown below, replacing the chlorophyll's lost electrons:
2H2O -> O2 +4H+ + 4e-
The movement of the reduced plastoquinone initiates the electron transport chain which passes along the thylakoid membrane. Movement of the excited electrons releases energy which is used for pumping protons to the inside of the thylakoid, building a proton gradient that stores potential energy (build up also aided by photolysis). Protons can move back out of the thylakoid region through the enzyme ATP synthase, which uses this energy to produce ATP (chemiosmosis). At the end of the transport chain, electrons pass to plastocyanin. Chlorophyll in photosystem I is photoactivated producing reduced ferredoxin. Two molecules of reduced ferredoxin are required for the reduction of one NADP. Electrons of the photosystem I chlorophyll are replaced with those carried by plastocyanin. Finally, when there is no supply of NADP, electrons released by photosystem I return to the start of the electron transport chain to produce ATP in the process called cyclic photophosphorylation.
