Describe in detail the function of the following components

Describe in detail the function of the following components of Photosystem I: Light harvesting complex I, light harvesting complex II, D1 and D2 proteins, P700 reaction center.

Solution

Light harvesting complexes (LHC) consist of pigment molecules like chlorophyll a, b and carotenoids such as beta carotene. LHCs are present in the thylakoid membrane. These molecules accept photons and further excite the electrons to a higher energy level. LH-I is tightly associated with the reaction centers while LH-II is not directly associated with the reaction centers. LH-II transfers energy to the reaction center through LH-I. The main function of LHCs is to collect light energy and transfer it to the reaction centers for the purpose of photo-induced redox processes.

There are about 30 subunits in the photosystem II. Most of these proteins are homologous to one another like D1 or PsbA and D2 or PsbD proteins. Most of the PS II proteins are unrelated and exhibit a huge diversification. The D1 and D2 polypeptides are the major redox components that are available in the heterodimer reactive center core. These polypeptides are found to be binding with the beta-carotene, chlorophyll-a and iron. These pigments are further involved in the transfer of energy from the proximal antennae complexes of CP43 and 47 to the chromophores of the reaction center.

P700 is a chlorophyll a molecule that belongs to the photosystem I. The absorption spectrum of P700 occurs at 700nm. When the photosystem I absorbs light energy, an electron is promoted to its higher levels in the P700 chlorophyll. P700 is considered as the strongest reducing agent in the animals. The electron given by PS-I is accepted the primary electron acceptor. PS-I makes use of iron-sulfur proteins as terminal electron acceptors.