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Name: A.
Status: Student
Grade:  9-12
Location: AK
Country: United States
Date: July 2008


Question:
Why is Chlorophyll Green?



Replies:
This is a good question for the chemists. Chlorophyll is a complex biomolecule containing magnesium. The molecule contains special ring shaped structures that capture preferred wavelengths of light. Green is not "captured" so it is reflected back to our eye. I do know that plants may contain modified chlorophyll and other pigments to take advantage of the type of light available to them. One example are sea plants where only certain wavelengths of light may reach specific depths and the plants have evolved to capture this light for energy.

Michael B Lomonaco.


We can also look at this from another angle. Why does chlorophyll reflect ("throw away") green light, which is the most abundant color in sunlight, and utilize instead the weaker reds and blue? Scientists theorize that it may have been because competing organisms were absorbing much of the green wavelengths billions of years ago, so algae (the earliest plants) reflected the green away and instead absorbed the red and blue hues that remained.

Early in Earth's history, the oceans were dominated by archaea, bacteria-like organisms that are often purple in color, due to a pigment used to create energy from the sun in a process analogous to photosynthesis (but completely differently at the chemical level). As algae came along, they would have found a beneficial niche by utilizing the unused red and blue wavelengths (and reflecting the green). If you compare the absorption spectra of chlorophyll (plants) and retinal (the pigment in archaea), they are mirrors of each other, which supports this theory.

Why archaea never evolved into complex organisms like algae did into plants and trees is not known (to me, at least), but another roll of the evolutionary dice might have led to large, purple archaea-trees that could outcompete plants (since plants use only the weaker red/blue wavelengths). Today, archaea ancestors remain as microorganisms that tend to inhabit extreme environments (geysers, salt ponds, etc.) where their purple (and red) colors can still be seen.

For more info, see:
"Extreme Microbes", S. DasSarma,

www.americanscientist.org/issues/feature/2007/3/extreme-microbes

"Early Earth was Purple, Study Suggests", Ker Than,

www.livescience.com/environment/070410_purple_earth.html

Paul Bridges



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