Friday, 17 September 2010

Black plants

Interesting new theory, gleaned from the latest New Scientist:
Why aren't plants black?
Plants are green because they don't absorb green light. The question is: why? Why let these wavelengths go to waste? No one can say for sure, but the most intriguing explanation was proposed by Andrew Goldsworthy of Imperial College London (New Scientist, 10 December 1987, p 48).

When photosynthesis evolved, Goldsworthy suggests, the oceans were full of a purple pigment called bacteriorhodopsin. Some simple cells make this so they can exploit light energy in a primitive way, and it looks purple because it absorbs green light. In fact, chlorophyll absorbs precisely the wavelengths that bacteriorhodopsin does not. So plants might be green because photosynthesis evolved in bacteria that had to make do with leftover light.

Because photosynthesis is so complex, by the time these cyanobacteria started to dominate the oceans, it was impossible to make major changes to chlorophyll without breaking the system. Some plants, particularly marine algae, have evolved extra pigments that can capture other wavelengths, but most remain stuck with the wavelengths chlorophyll can absorb.
It looks like our green world is a fluke; and that science fiction stories, to be consistent, really ought to fill their distant jungle planets with purple and black vegetation. I know I shall, in future books.

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