PSII is called P680 because its primary light-absorbing pigment, a special form of chlorophyll a, absorbs light most strongly at a wavelength of 680 nanometers (nm).
In more detail:
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P680 as the Reaction Center: P680 is the reaction center chlorophyll of Photosystem II (PSII). This means it's the specific chlorophyll molecule where the initial light energy is converted into chemical energy.
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Light Absorption and Excitation: When P680 absorbs a photon of light at or near 680 nm, it becomes excited. This excitation pushes an electron to a higher energy level, denoted as P680*.
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Electron Transfer and Photosynthesis: This excited electron (P680*) is then passed to an electron acceptor molecule, initiating the electron transport chain of photosynthesis. This process ultimately leads to the splitting of water, the release of oxygen, and the generation of ATP and NADPH, which are used to power the Calvin cycle.
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Significance of the Name: The name "P680" directly reflects the pigment's peak light absorption wavelength, providing a quick reference to its spectral properties and its role in photosynthesis. It differentiates it from other chlorophylls with different absorption peaks (e.g., P700 in Photosystem I).
In summary, P680's name directly reflects the wavelength of light that it absorbs most efficiently, making it the key player in the light-dependent reactions of photosynthesis within PSII.
