The media, Senator Obama, and why plants are green
It’s apparently news that the media have acknowledged that many in their ranks are in the tank for Barack Obama (discussions here, here, and here, for example). What a revelation! Who would have suspected that the media are biased? Well, perhaps it is news that the liberal media have now apparently discovered this about themselves, though it’s been hidden in plain sight for a long time now (eg, here and here).
In any event, we thought we’d discuss something you actually might not know already — why plants are green. Nancy Y. Kiang in Scientific American:
The energy spectrum of sunlight at Earth’s surface peaks in the blue-green, so scientists have long scratched their heads about why plants reflect green, thereby wasting what appears to be the best available light. The answer is that photosynthesis does not depend on the total amount of light energy but on the energy per photon and the number of photons that make up the light.
Whereas blue photons carry more energy than red ones, the sun emits more of the red kind. Plants use blue photons for their quality and red photons for their quantity. The green photons that lie in between have neither the energy nor the numbers, so plants have adapted to absorb fewer of them.
The basic photosynthetic process, which fixes one carbon atom (obtained from carbon dioxide, CO2) into a simple sugar molecule, requires a minimum of eight photons. It takes one photon to split an oxygen-hydrogen bond in water (H2O) and thereby to obtain an electron for biochemical reactions. A total of four such bonds must be broken to create an oxygen molecule (O2). Each of those photons is matched by at least one additional photon for a second type of reaction to form the sugar. Each photon must have a minimum amount of energy to drive the reactions.
The way plants harvest sunlight is a marvel of nature. Photosynthetic pigments such as chlorophyll are not isolated molecules. They operate in a network like an array of antennas, each tuned to pick out photons of particular wavelengths. Chlorophyll preferentially absorbs red and blue light, and carotenoid pigments (which produce the vibrant reds and yellows of fall foliage) pick up a slightly different shade of blue. All this energy gets funneled to a special chlorophyll molecule at a chemical reaction center, which splits water and releases oxygen.
The funneling process is the key to which colors the pigments select. The complex of molecules at the reaction center can perform chemical reactions only if it receives a red photon or the equivalent amount of energy in some other form. To take advantage of blue photons, the antenna pigments work in concert to convert the high energy (from blue photons) to a lower energy (redder), like a series of step-down transformers that reduces the 100,000 volts of electric power lines to the 120 or 240 volts of a wall outlet. The process begins when a blue photon hits a blue-absorbing pigment and energizes one of the electrons in the molecule. When that electron drops back down to its original state, it releases this energy—but because of energy losses to heat and vibrations, it releases less energy than it absorbed.
The pigment molecule releases its energy not in the form of another photon but in the form of an electrical interaction with another pigment molecule that is able to absorb energy at that lower level. This pigment, in turn, releases an even lower amount of energy, and so the process continues until the original blue photon energy has been downgraded to red. The array of pigments can also convert cyan, green or yellow to red. The reaction center, as the receiving end of the cascade, adapts to absorb the lowest-energy available photons. On our planet’s surface, red photons are both the most abundant and the lowest energy within the visible spectrum.
Bonus piece of information: the sky is actually violet, not blue, but we don’t see violet that well.
April 20th, 2008 at 10:10 am
Thomas Mann in his novel “Doctor Faustus” writes about the color of the sky. His protagonist is facinated by the fact that there is no blue in the sky and it is only a sort of optical illusion that we perceive it as blue. This is a common theme in Mann’s writings: that the artist is a con man and illusionist and we can find the same characteristics in nature. Mann would have liked the fact that the green isn’t really in the plant, rather it’s band width that makes it to our eyes. That’s all I’m going to say about this. I will spare myself and anyone else who reads this a disquisition on how all of this relates to Plato’s allegory of The Cave. But it does.
April 21st, 2008 at 12:12 pm
A number of years ago, New Scientist had another theory of why the earth is green. The theory is that the first photosynthetic organisms were the purple bacteria, which preempted the green band (reflecting blue and red = purple) leaving the cyanobacteria (evolving later) only the red and blue bands.
April 21st, 2008 at 12:15 pm
Several years ago, New Scientist proposed that the earth is green because the purple bacteria, which evolved photosynthesis first, absorbed the green band leaving only the red and blue for the later-evolving cyanobacteria. I like the new theory, too.