Question

why photosynthesis takes place in the blue and red regions

Answer 1

that's an interesting question.

you know that light is energy, right? and the wavelength of a photon is indirectly proportional to its energy. this means, that blue photons with a shorter wave length have got more energy than red ones (longer wavelength).

now, the purpose of the photon is to bring an electron of the chlorophyll into an excited state. absorption occurs if the energy of the photon goes into the electron which is then begin excited. as you see, a photon of the wavelength 688nm has exactly the right amount of energy to do that.

now why isn't the same effect triggered by ALL smaller wavelength photons? because the energy is quantized - it can only take certain values. a photon can't give just parts of its energy to the chlorophyll electron and then fly on, say, with a longer wavelength. it either gets absorbed or flies by without interaction. this effect causes the "green gap". also, electrons can't just be "half excited" - either whole or not. this is stated in quantum physics.

now, the second part isn't as easily answered. to hold it simple: there is another state the electron can be put into by inserting more energy (like a 442nm photon). this causes another absorption maximum.

if you don't want to be confused, stop reading here. =)

okay, now here comes the more complex explaination. through absorption at 688nm, the chlorophyll will be put into a singulett state (one unpaired electron). through absorption at 442nm, it will be put into a triplett state (three unpaired electrons). the additional energy of the 442nm photon is used to excite a paired electron, not an unpaired one (needs more energy to do that, obviously). this is a dangerous state, since the chlorophyll now can power radicalic reactions. normally, assistant molecules like the xanthophylls will take this state away instantly, or two chlorophylls will bump into each other, giving away the energy as heat. in seldom cases, only parts of the energy is given away, resulting in the "falling back" of the electron into singulett state, which enables the energy to be transferred through the light harvesting complex.