Photosystems are complex protein complexes found in the thylakoid membranes of chloroplasts in plants and algae. They are responsible for capturing light energy during photosynthesis. The specific wavelengths of light that photosystems absorb are determined by the pigments present in the photosystems, primarily chlorophyll molecules.
In photosystem I (PSI) and photosystem II (PSII), the main pigments involved are chlorophyll a and chlorophyll b. These pigments have specific absorption spectra, meaning they absorb light most efficiently at certain wavelengths.
The reason violet and blue wavelengths are not absorbed as efficiently by photosystems compared to longer wavelengths like 680 and 700 nm is primarily due to the specific molecular structure of chlorophyll pigments and the energy levels of their electrons.
Chlorophyll pigments have a porphyrin ring structure with a magnesium atom at the center. The electrons within this structure can be excited by absorbing photons of specific energies. In the case of chlorophyll a, it absorbs light most efficiently in the red and blue regions of the spectrum, with peak absorption occurring around 430 nm (blue) and 662 nm (red). Chlorophyll b has similar absorption properties, with its peak absorption occurring around 453 nm (blue) and 642 nm (red).
The absorption of specific wavelengths of light by photosystems is important for the process of photosynthesis. The absorbed light energy is used to drive the transfer of electrons through a series of reactions, ultimately leading to the production of energy-rich molecules like ATP and NADPH, which are used in the synthesis of carbohydrates.
The specific wavelengths of 680 and 700 nm that you mentioned correspond to the maximum absorption peaks of chlorophyll a in PSII (P680) and PSI (P700), respectively. These wavelengths are particularly effective in exciting the electrons in chlorophyll pigments, leading to the initiation of the electron transfer chain in the photosystems.
In summary, photosystems selectively absorb certain wavelengths of light based on the properties of the chlorophyll pigments involved. While violet and blue wavelengths are absorbed to a lesser extent, the absorption peaks of chlorophyll a in the red and blue regions of the spectrum (such as 680 and 700 nm) play a crucial role in driving the process of photosynthesis.