fotosystemów

Photosystems are protein complexes found in the thylakoid membranes of chloroplasts in plants, algae, and cyanobacteria. They are responsible for the initial steps of photosynthesis, the process by which light energy is converted into chemical energy in the form of ATP and NADPH, which are used to power the Calvin cycle and produce glucose. There are two types of photosystems: Photosystem I (PSI) and Photosystem II (PSII).

Photosystem II is the first photosystem to be activated by light. It absorbs light at wavelengths of 680 nanometers (nm) and uses this energy to split water molecules, releasing oxygen as a byproduct. The electrons released from water are then passed to an electron transport chain, which ultimately reduces NADP+ to NADPH. The energy from the light absorbed by PSII is also used to pump protons across the thylakoid membrane, creating a proton gradient that drives the synthesis of ATP.

Photosystem I is the second photosystem to be activated. It absorbs light at wavelengths of 700 nm and uses this energy to reduce ferredoxin, which is then used to reduce NADP+ to NADPH. The electrons for PSI come from the electron transport chain of PSII. The energy from the light absorbed by PSI is also used to pump protons across the thylakoid membrane, contributing to the synthesis of ATP.

The two photosystems work together in a process called cyclic electron flow, which allows for the continuous production of ATP and NADPH. In this process, electrons from PSI are passed back to PSII, bypassing the production of NADPH. This process is particularly important under low light conditions, as it allows for the production of ATP without the need for NADPH.