Photosynthetic Apparatus

Photosynthetic pigments are well organized in an energy-efficient manner in the thylakoid membrane. In photosynthetic membrane of chloroplasts, chlorophyll a, b, carotenoids, other pigments, various proteins and redox carriers are arranged in an array of photosynthetic units so-called photosystems. Earlier it was described as quantasomes.

Quantasome

Roderic B. Park in 1962 first identified the quantasomes. Quantasomes are particles embedded in the thylakoid membrane. These are consisting of photosynthetic pigments, proteins, lipids and electron carriers. These are considered as photosynthetic units. Quantasomes are of two sizes; smaller and larger, which are currently identified as photosystem I and II respectively.

Photosystem I (PSI)

PSI is the multisubunit protein complex found in the thylakoid membranes of green plants and algae. PSI consists of an array of light harvesting complex (LHC) including antenna pigments and reaction center. The antenna pigments contain several molecules of carotenoida and chlorophyll b. The reaction center has single chlorophyll a molecule with absorption maxima of 700 nm.
PSI produces strong reductant, which reduces the NADP⁺ into NADPH.

Photosystem II (PSII)

PSII is similar to the PSI but for the reaction center. In PSII, reaction center chlorophyll a molecule has absorption maxima of 680 nm.
PSII produces strong oxidant, which oxidizes water molecule into e^-, H⁺ and O₂.

The numbering of PSI and PSII are in the order of their discovery. In electron transport process (Z-scheme) of photosynthesis, PSII comes first.

Structural organization of photosystems I and II

Photosystems harvest light energy and start redox process)

The antenna pigments (carotenoids and chlorophyll b) harvest light energy and upon excitation by photons, funnel and transfer their energy to the neighboring molecules through the phenomenon of inductive resonance. This energy is ultimately transferred to the respective reaction center molecule. The reaction center chlorophyll then emits the electron to the electron acceptor by the phenomenon of photoinduced charge separation. In this manner light induced redox process is started.