I am getting slightly confused about how the above relate to each other. My current understanding is that P680 and P700 refer to the primary pigment reaction centres in Photosystems 2 and 1 respectively, with the numbers giving the peak wavelength of absorption. Now I initially thought that one of these primary pigment reaction centres must be composed of chlorophyll a and the other of chlorophyll b, hence the slightly differning absorption wavelengths. Instead, here it seems to suggest that the primary pigment reaction centre is always chlorophyll a. Most sources I have looked at do not specify what forms the primary pigment reaction centre (the Wikipedia article on photosynthetic reaction centre, for example, does not state for the general case of for photosystem 2; however it does mention that photosystem 1 has a special pair of chlorophyll a molecules at the centre).
So my specific questions are:
- Is the primary pigment reaction centre in both photosystem 1 and 2 (i.e. P700 and P680) a pair of chlorophyll a molecules?
- If so, what is chlorophyll b? Is it simply an accessory pigment?
- Finally, if it is the case that the primary pigment reaction centre is chlorophyll a in both cases, how can it be that it absorbs at a different wavelength in the two photosystems if it is the same molecule?