Wednesday, April 11, 2007

Why are plants green?

My last post was about light and fortuitously today a couple of news items fit right in. Both items relate to a question I ask my students when we discuss photosynthesis, namely why are plants green. The simple answer is that the chlorophylls, the main photosynthetic pigments reflect green light and absorb red and blue light. The reflected light is of course what our eyes detect and our nervous system perceives as color and most professors seem to stop here. But I then turn the question on its head and ask why aren't plants black? After all, if the name of the game in photosynthesis is light absorption then shouldn't plants be black?

The usual response is that maybe plants can't do any better. The problem is plants that have other pigments, such as carotenes and xanthophylls that absorb light in the middle of the visible spectrum at those wave lengths that chlorophyll cannot. Another hypothesis my students often raise is that if plants absorb all the light available to them, they might overheat, much like wearing a black shirt on a hot day. Seems reasonable until you consider that plants in the shade where they ought not overheat are still typically green.

Getting to the news items, the first one from Live Science suggests that photosynthesis arose in a group of bacteria that used a simpler pigment called retinal found today in a group of bacteria (Actually Archeans) called Halobacteria . Shil DasSarma, from the University of Maryland notes that retinal absorbs best in the middle of the spectrum where chlorophyll cannot. Indeed the absorption spectra of the two pigments are eerily complementary as the accompanying figure from American Scientist shows.

DasSarma hypothesizes that retinal was used by the first photosynthetic organisms and that chlorophyll based photosynthetic organisms were able to survive because they could use wavelengths that retinal based photosynthetic organisms cannot. Since chlorophyll is more efficient than retinal, bacteria that used chlorophyll eventually out competed the earlier reddish purple hued retinal using bacteria. If so the early Earth may have been the Purple Planet and looked more like the Halobacteria rich landscape in the image to the left from NASA.


The second article from Scientific American looks at the sorts of colors photosynthetic organisms might be on other planets. Nancy Kiang from the Goddard Space Center analysed the spectra from other stars to attempt to predict what color photosynthetic organisms might be. This seems like an elaboration of a question I ask my students where I present them with a hypothetical planet Munimula (for those who remember Ruff and Ready Cartoons) orbiting a sun that produces light of the wavelengths we perceive as green. I ask the student to predict the color of plants that might evolve on such a planet. They are supposed to get the idea that the plants must be able to absorb the available light so were you to view plants from Munimula under white light, they would definitely not be green because they must have evolved to absorb the wave lengths we perceive as green.

So why are plants green? Maybe the good enough hypothesis combined with DasSarma's hypothesis is on the right track. But plants are still able to fine tune pigments. Indeed algae and other photosynthetic organisms in the ocean where short wavelengths of light penetrate further than long wavelengths, perceived as red, absorb short wavelengths and appear black or red under white light. So it seems terrestrial photosynthetic organisms ought to better fine tune their pigment systems.

I leave you with another idea that I have been thinking about. Maybe for all intents and purposes plants ARE black in the sense of absorbing as much light as possible and that it is our visual systems that have been fine tuned by evolution to be sensitive to what little light that is reflected from the leaves. What ever idea or combination of ideas is involved, sometimes the best and most interesting questions in science are what at first glance are the simplest to state.

One more little tidbit. Retinal is found not just in the Halobacteria, but is found as part of the light detection systems of the eye. So is this convergence in the use of retinal independently hit upon by the early photosynthetic organisms and animals or do we see in the visual pigments of the eye the remnants of the first invention of photosynthesis by early cells maintained in the context of light detection and image formation rather than photosynthesis.

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