Photosystem 1 vs Photosystem 2
Chlorophyll molecules and other accessory pigment molecules together with proteins and other small organic compounds form photosystem I and photosystem II. Generally, photosystem I is referred to as PS I and photosystem II is referred to as PS II. Photosystems are located in the thylakoid membranes. A photosystem contains an antenna complex or light harvesting complex of pigment molecules and a reaction centre. There are about 200-300 pigment molecules in a light harvesting complex. Different pigment molecules found in the photosystem collect lights of different wavelengths. The light collected by the pigment molecules is transferred from one to another and finally to a specialized chlorophyll a molecule, which is known as the reaction centre in the photosystem. The reaction centre must operate 4 times in order to produce one oxygen molecule. Plants essentially need both photosystems I & II. This is because stripping electrons from water requires more energy than light-activated photosystem I can supply. Plants possess a second photosystem (PS II), which is able to absorb shorter wavelength (higher energy) light and links in tandem to PS I, enabling non-cyclic electron flow. Together PS I and PS II are geared towards production of energy.
Light energy absorbed by the pigment molecules is transferred to P 680 chlorophyll a molecules in the reaction centre of photosystem II. When energy is transferred to P 680, its electrons are boosted to high energy levels. These electrons are picked up by primary electron acceptor molecules and finally to photosystem I through a series of carrier molecules like cytochrome. When electrons are transferred through electron carriers of low energy levels, some of the energy released is used in the synthesis of ATP from ADP. This process is called photophosphorylation. At the same time, water molecules are split by the light energy and this process is called photolysis of water. As a result of photolysis of 4 water molecules, 2 oxygen molecules, 4 protons, and 4 electrons are produced. Electrons produced replace electrons lost from chlorophyll a molecule of PS II. Oxygen is evolved as a biproduct.
At PS I also light energy is absorbed when P 700 chlorophyll a molecules of photosystem I are excited. Then its electrons are boosted to higher energy levels and are accepted by primary electron acceptors. And through acceptor molecules, it is finally transferred to NADP molecules, which is reduced to NADPH2 using protons produced in photolysis. In PS I the electron that is excited may be an electron from chlorophyll a or the electron that comes from PS II.
What is the difference between Photosystem I and Photosystem II?
• Photosystem I preferentially absorbs wavelengths greater than 680 nm. Reaction centre chlorophyll a molecule is P 700 and gives a maximum absorption peak at 700 nm, whereas photosystem II absorbs best at 680 nm. Reaction centre chlorophyll a molecule is P 680.
• In photosystem II, the electron that is boosted up to higher energy levels is replaced by electrons released from the photolysis of water and, in photosystem I, the electron boosted up to higher energy levels is replaced from the electron released from the photosystem II.