Newton Kid Knowledge on Nature
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Why are leaves green? Wouldn't black absorb light better?
Dec. 4, 2006 [Boston Globe]: Like most questions to do with the origins of things, history is everything. There's a tendency to think that evolution makes things perfect, but in reality it works with what's already there to get things to work just well enough.
Here's at least one idea of how leaves wound up being green: Photosynthesis is the process whereby a living thing gathers energy from sunlight and stores it as chemical energy. Long before there were plants, sometime just after life started maybe around 4 billion years ago, some bacteria had managed to get an early form of photosynthesis going.
These bacteria had no chlorophyll and instead used a purple pigment called bacteriorhodopsin. These bacteria were pretty good at what they did; their descendants -- still using bacteriorhodopsin -- are doing fine today.
At that time there was also a lot of metabolism going on that produced carbon dioxide, and the time was ripe for something to come along that could use energy from the sun to stick carbon dioxide and water together to form carbohydrates -- in other words, to do modern photosynthesis.
Purple bacteria absorb green light (they reflect blue and red, which is why they look purple), so that means there was a lot of green light going to waste, and that made the scene ripe for chlorophyll and green plants.
On some other world where purple bacteria didn't get a hold first, one might see something like plants of some other color.
Black, as you rightly point out, would be the best, but, evolution doesn't always give the best of all possible results and, in any event, is still going on. Maybe black plants are in our future.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.