Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Molecular mechanisms of photosynthesis
Two photosystems in plant chloroplasts

Photosynthesis in plants is driven by Two Photosystems, PSI and PSII (Figure 184).

PSI is excited by light with wavelengths shorter than 700 nm, whereas PSII is excited by light with wavelengths shorter than 680 nm. Similarly to green and purple Bacteria, each chloroplast photosystem contains a dimer of specialized chlorophyll $a$ molecules capable of initiating photoinduced electron transfer.

The absorption spectra of chlorophylls in the reaction centers of PSI and PSII differ due to their distinct protein environments. Therefore, these chlorophylls are commonly designated as P₆₈₀ (in photosystem PSII) and P₇₀₀ (in photosystem PSI).

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Figure 184 - Linear electron transport chain in plants

Analogously to bacterial reaction centers, each reaction center in the chloroplast is associated with numerous Light-Harvesting Complexes (LHCs), with the protein composition of LHCs linked to PSI differing from that linked to PSII.

The spatial arrangement of the Two Types of photosystems within the thylakoid membranes also differs: PSII is localized predominantly in the stacked Regions of the thylakoid grana, whereas PSI is situated in the non-stacked (stroma-exposed) areas of the thylakoid membranes.

And finally, the most important distinction between the two types of chloroplast photosystems is functional:

✵ Water splitting and molecular oxygen formation occur exclusively at PSII,

✵ electron transfer to NADP⁺ is carried out exclusively by PSI.

Photosynthesis in METABOLISM/14.html">Chloroplasts, much like in green and purple bacteria, can proceed via both unidirectional (linear) and Cyclic electron transport:

✵ the linear Electron Transport Chain drives carbon fixation and ATP synthesis,

✵ the cyclic chain Supports ATP synthesis alone without initiating carbon fixation, because electron cycling does not yield the NADPH required for the carbon fixation process.

Photosynthetic Algae and cyanobacteria also possess two types of photosystems analogous to those of plants.