Fotosysteemissä

Fotosysteemissä, or photosystem, refers to a complex of proteins and pigments embedded in the thylakoid membranes of chloroplasts and cyanobacteria. These complexes are crucial for the light-dependent reactions of photosynthesis, the process by which light energy is converted into chemical energy. There are two main types of photosystems: Photosystem II (PSII) and Photosystem I (PSI).

Photosystem II is the first photosystem in the photosynthetic electron transport chain. Its primary function is to absorb light energy and use it to split water molecules, a process known as photolysis. This splitting releases electrons, protons, and oxygen as a byproduct. The electrons are then passed along an electron transport chain, eventually reaching Photosystem I.

Photosystem I receives the electrons from PSII and uses further light energy to energize them. These high-energy electrons are then used to reduce NADP+ to NADPH, another key energy-carrying molecule in photosynthesis. Both PSII and PSI contain chlorophyll pigments, primarily chlorophyll a and chlorophyll b, which are responsible for absorbing light energy. They also contain accessory pigments like carotenoids, which broaden the spectrum of light that can be absorbed. The arrangement of these pigments within the photosystem allows for efficient light harvesting and energy transfer.