The mechanism of light energy utilization in photosystem
How is the light energy captured? In thylakoid membrane, chlorophyll and other pigments unite to form a photo system structure. In photosynthesis, chlorophyll a, chlorophyll b, and carotene function to capture light energy (photon). The energy is, then, released from one pigment molecule to others until reaching reaction center.
The reaction center consists of I chlorophyll a molecule and l primary electron acceptor molecule. The molecule of primary electron acceptor will capture the exited electron from chlorophyll molecule of reaction center and release it to electron transport chain.
There are two types of photo system, photo system I and photo system. ll Each photo system has specific characteristic. in photo system I, the chlorophyll of reaction center is called P700 because chlorophyll a can absorb light energy on 700 nm wave length. Photosystem I consists of 12 different protein subunits. Chlorophyll a has 96 molecules consisting of 2 molecules as the reaction center 4 additional molecules, and 90 molecules as antenna pigment. The an
tenna pigment is necessary to capture the light.
In photo system II, the reaction center of chlorophyll a is called P680 because chlorophyll a can absorb light energy in 680 nm wavelength. Photosystem II consists of 20 different protein subunits. Fifty or more chlorophyll a molecules consist of 2 chlorophyll molecules as the reaction center, 2 additional molecules, 2 pheophytin molecules (chlorophyll without Mg2+), and other chlorophyll a molecules that function as antenna pigment. About 6 carotene molecules function as antenna pigment and 2 plastoquinone molecule.
A.Cyclic electron flow
B.Non-Cyclic electron Flow
In light reaction, there occur 2 kinds of electron flow, cyclic and non-cyclic electron flows. It is called cyclic electron flow because the electron that has been charged flows to the origin reaction center, while the other is called non-cyclic electron flow because the energized electron is transferred to NADP+ instead of to reaction center. Cyclic electron flow only generates ATP while non-cyclic electron flow produces ATP and NADPH. The ATP formation in these processes occurs through photophosphorylation. Photophosphorylation is art energy transformation process from exited electron into pyrophosphate derived from ADP molecule. Look at Figure!
NADPH formed in photosystem II functions to bring the energy required in dark reaction. Electron in photosystem II will replace the exited electron in P700 molecule in photosystem I. That is why there is electron flow from water to NADH' Next, NADPH and ATP will be used to form carbohydrate in dark reaction.
Source: Biology, 2011