molecular biology - Photosystem 1 and 2; P680/P700; Chlorophyll a/b

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?

Answer

Regarding your questions #1 ("Is the primary pigment reaction centre in both photosystems a pair of chlorophyll a molecules?") and #3 ("How can it be that it absorbs at a different wavelength in the two photosystems if it is the same molecule?"):

Both reaction centers in Photosystem I and Photosystem II contain only chlorophyll a. According to Lodish (Molecular Cell Biology):

As in the bacterial reaction center, at the center of each chloroplast photosystem is a pair of specialized reactioncenter chlorophyll a molecules, which are capable of undergoing light-driven electron transfer. The chlorophylls in the two reaction centers differ in their light-absorption maxima because of differences in their protein environment. For this reason, the reaction-center chlorophylls are often denoted P680 (PSII) and P700 (PSI). (emphases mine)

Regarding your question #2 ("If so, what is chlorophyll b? Is it simply an accessory pigment?"):

Chlorophyll b has a different molecular structure (and, therefore, a different absorption spectrum) and it's less abundant in the chloroplast than chlorophyll a. Here is an image depicting their molecular structures and absorption spectra:

_{(a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum. Source: "OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013."}

Both chlorophyll a and b are present in the antenna complex. Still according to Lodish:

As in photosynthetic bacteria, each reaction center has an associated antenna that consists of a group of light-harvesting complexes (LHCs); the LHCs associated with PSII and PSI contain different proteins.

However, most of chlorophyll b is associated to PSII antenna complex, not PSI, which is one of the main differences between PSII and PSI.

Source: Lodish, H. (2000). Molecular cell biology. 4th ed. New York: W.H. Freeman.