Explain why the chlorophylls in light-harvesting complexes must absorb light of shorter wavelength than the light that would directly ex-cite the special pair of PSI or PSII.

Photosystems are the protein molecules that are optimized to harvest light. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).

PSI is called P700 and PSII is called P680. It is called P700 because it absorbs the light at around 700 nm wavelength. P680 because it absorbs the light at around 680 nm wavelength.

In non-cyclic photophosphorylation, the photons of light is first transferred to P680, boosting an electron to a high energy level. The high-energy electron is passed to an acceptor molecule and replaced with an electron from water. This splitting of water releases the O2 we breathe.

In cyclic photophosphorylation, the light of wavelength 700 nm is absorbed by P700 and the electrons are the passes to the acceptor molecule by several carriers.