Drive towards
complexity
It is of
interest to compare anoxygenic light harvesting mechanisms to oxygenic ones.
Purple bacteria, which perform anoxygenic photosynthesis, feature the oldest
known light harvesting apparatus, while the oxygenic cyanobacteria, plants and
green algea are evolutionarily more recent. In a purple bacterial
photosynthetic unit pigments form highly symmetric ring-like structures (see
figure). In contrast, the pigment network of photosystem I from oxygenic
species forms a rather random looking array with a higher packing density of
chlorophyll per unit mass. In fact, photosystem I and its associated complexes
display a greater complexity in plants as compared to cyanobacteria as
discussed in the tale of two
photosystems.
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