Jon Webb's Blog

Sunday, March 27, 2005

Plants and Sight

All seeing things on the planet -- everything from mosquitoes to people (with the remarkable exception of some dragonfish) -- sense light by virtue of closely-related chemicals called rhodopsins. Three slightly different versions of rhodopsin give us the ability to perceive light as red, blue, and green components.
Rhodopsin originated in a bacterium. Salt-loving escendants of the bacterium still exist in the Red Sea. The bacterium's version of rhodopsin preferentially absorbed green light, like our eyes. That is why color digital cameras use the following mask to break light down into its red, green, and blue components:

R-G-R-G-R-G...
G-B-G-B-G-B...
R-G-R-G-R-G...
G-B-G-B-G-B...
That is, every other pixel is masked by a filter that lets through green light, and red and blue components are each half as frequent as green. The higher spatial sensitivity to green makes the color image appear to have higher resolution than it would if red, green, and blue were equally frequent, since our eyes have more retinal cells sensitive to green light than red or blue.
The bacteria's preference for green light and our eyes greater sensitivity to it have the same origin. The rhodopsin in bacteria was more efficient if it captured green light because that was what was left over in the pools of water it was swimming about it. At that time, the competitor for that light was algae floating on the surface of the pond. The chlorophyll in algae leave behind the green light -- hence their color. The bacteria therefore absorb green light, and look red as a result.
Our eyes, too, had to deal with green light in the forest environment that we evolved in. The forests and grasslands where we hunted gave good protection to our prey--but by having eyes that had high spatial sensitivity in this region of the spectrum, we could improve our chances of survival. So chlorophyll once again guided our evolution.
Algae evolved beyond the form of chlorophyll that absorbs just green light. More adept forms of algae absorb a wide spectrum of light, so that algae now comes in all colors. But the descendants of algae, which are the multicellular plants, were stuck with the early form of chlorophyll. So they are still green, and we, the descendants of the bacteria, see them with green-sensitive eyes.

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