Let's start with the most well known molecule involved in photosynthesis: Chlorophyll. Chlorophyll A, shown to the left, is common to all eukaryotic photosynthetic organisms. Other types of chlorophyll, listed below, all have a very similar structure, consisting of a porphyrin ring with a central magnesium ion ( green), a long phytol tail (), and various side chains.

is an accessory pigment, mainly found in land plants. The difference between chlorophyll A and B is that chlorophyll B has a aldehyde group substituted in place of a methyl group ().

was originally thought to be used by red algae, although it is now known that chlorophyll D is actually from an epiphytic cyanobacterium (Acaryochloris marina) that lives on the red algae. It differs from chlorophyll A in having a ketone in place of a CHCH2 group ().

is used by purple bacteria, and absorbs infrared light between 800nm - 1000nm wavelength. It is most similar to chlorophyll D, although the ketone group is orientated differently (). The green sulphur bacteria use chlorobium chlorophyll instead (not shown).

We shall further explore the role of chlorophylls, and other molecules, in light capture. Recall that there are four phases to light harvesting:

  • Light harvesting complexes (or antennae complexes) collect light and pass excitation energy to reaction centres.
  • Reaction centres containing a special pair of chlorophylls start photochemistry, and e- transfer.
  • Electrons transfer from donor to acceptor and donor to reaction centre.
  • A proton gradient is formed across the thylakoids that enables the synthesis of ATP.