Re: What does Chlorophyll a do, comparred to chlorphyll b?

I found what might be a useful website for you: http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html

Basically, the two chlorophylls are very similar.

Chl a seems more ancient: it's the only one found in the reaction
 centres of photosynthesis and the only one in cyanobacteteria.
Chl b is found in light-harvesting complexes.

Chl b is, technically, an accessory pigment - its job is to absorb
 light and transfer it to the reaction centres.

Some chl a is directly involved in the transfer of light energy to
 chemicals in the reaction centre. Other chl a is also just
 accessory pigment.

The purpose of slight differences in absorbed wavelengths of the
 two chlorophylls can be explained by considering that different
 photosynthetic organisms will have different wavelengths of light
 available in their ecological niches. Even a slightly better
 pigment for capturing light at one wavelength or another would
 give an appreciable advantage to an organism that used it.