Photosystems

Photosynthetic organisms capture sunlight through the pigment-protein complexes called photosystems, embedded within the chloroplast's thylakoid membrane.

These complexes are categorized into photosystem I or PSI and photosystem II or PSII.

Inside the chloroplast, PSI complexes are predominantly located in the unstacked regions, called the stromal lamellae, while PSII complexes are present within the stacked granal lamellae.

Each photosystem is a collection of about 200 chlorophyll and 50 carotenoid pigment molecules, distributed across two different domains of the photosystem─the core domain called the reaction center and a peripheral domain called the antenna complex.

Although all pigment molecules absorb photons, only a few chlorophyll molecules associated with the reaction center can convert absorbed light energy to chemical energy.

The pigments in the antenna complex only funnel the absorbed energy to the reaction center.

The photosystems also have associated cofactors essential for their functioning.

For instance, PSI has a ferredoxin cofactor, a key junction in the electron transport chain, while PSII contains an oxygen-evolution complex that catalyzes water oxidation, a step crucial for photosynthesis.