9.4: Photosystem II
Photosystem II is a multi-protein complex embedded within the thylakoid membrane where it harvests light energy. Chlorophyll molecules transfer energy to a specific pair of chlorophyll a molecules in the reaction center of Photosystem II. Here, the chlorophyll a molecules lose an electron (oxidation), transferring it to a primary electron acceptor. The donated electrons pass through the electron transport chain into Photosystem I. Splitting a water molecule releases one oxygen atom, two protons (H+) and two electrons. The electrons replace the donated electrons of the two chlorophyll a molecules in the reaction center. The oxygen atom immediately reacts with another oxygen atom, producing O2 that is released into the atmosphere. The protons accumulate and create a concentration gradient across the thylakoid membrane that drives ATP synthesis in a process called chemiosmosis.
Light Harvesting in Photosystem II
The multi-protein complex Photosystem II harvests photons and transfers energy through its bound pigments chlorophyll a and b, and carotenoids. Carotenoids have a protective function as they help dissipate the vast amount of energy taken in that could otherwise damage the plant tissue.
Energy travels from chlorophyll molecule to chlorophyll molecule until it reaches a pair of specialized chlorophyll a molecules in a region called the reaction center. The reaction center is also known as P680 since it absorbs light at a wavelength of 680 nm. The energy is strong enough to break an electron from a chlorophyll a molecule (oxidation). The free electron transfers to a primary electron acceptor molecule, in a process called photoact. The electron of chlorophyll a in the reaction center is replaced by one of two electrons that are released in the splitting of a water molecule.
Proton Gradient and Generation of ATP
The splitting of water in Photosystem II also generates an oxygen atom that combines with a second oxygen atom. The resulting O2 escapes into the atmosphere. The reaction also makes two protons ( H+) that build up and create a concentration gradient to power a specialized, semi-permeable protein channel called ATP synthase. The process of the protons moving from a high concentration in the thylakoid, through the channel to a lower area of concentration in the stroma is termed chemiosmosis. Chemiosmosis creates energy that allows the ATP synthase to attach a third phosphate group to ADP to form the energy molecule ATP.