How Photosynthesis Builds Energy Against Entropy

No law of physics is more basic than the law of entropy, the tendency of the universe to move toward disorder and death. But life bucks the tide. using available free energy wherever it can get it, and hereabouts the most abundant source of energy is sunlight. The mayflower constructs its tiny oasis of order by drawing upon a corresponding increase of disorder at the center of the Sun, where hydrogen is fused into helium. There, deep at the heart of our planet's star, is the source of the energy that drove the Ames shovel shops and drives the flower too. The Ameses turned radiant solar energy into a family fortune, by harnessing waterpower and the energy of coal; the Canada mayflower does it too, amassing its own Midas wealth of carbohydrates. In summer, about a hundred-millionth of an ounce of the Sun's depleted mass (multiplied by the speed of light squared) falls each second onto these woodlands; in winter less than half as much. A fraction of a millionth of an ounce of matter turned into energy is all it takes to tip the balance of the season from winter toward summer. A fraction of a millionth of an ounce of fused hydrogen is all it takes to rocket the Canada mayflower up out of the ground. The leaves of the plant push aside the detritus of ruinous winter, thrusting toward light, its fuse lit by a match scratched at the Sun's core. Each pair of green leaves soaks up sunlight, using the radiant energy to make sugar through the magic of photosynthesis.

Biologists are not sure how photosynthesis evolved, although they have likely scenarios; that it happened early in the history of life is certain. Every high school student learns the basic equation: Carbon dioxide plus water plus sunlight yields sugar and oxygen (with the Cs, Hs, and Os appropriately balanced). The equation doesn't nearly convey the complex chemical reactions that connect one side of the reaction arrow to the other. Crucial to the process is a boxy molecule with a magnesium nitrogen heart and a long carbon-hydrogen tail: chlorophyll. Atomic electrons in the molecule absorb solar photons and are bumped up in energy. As they return their bounty, they energize reactions that create intermediate products called ATP and NADPH, which then move along the assembly line. When all is said and done, it is sugar that appears at the factory door. I love to think of all this molecular activity going on in the splayed leaves of the Canada mayflower. The only hint of this humming manufactory is the color of the leaves.

Chlorophyll absorbs energy from the blue and red parts of the solar spectrum—the ends of the rainbow. The green middle of the spectrum is reflected, and that's the light that enters our eyes. Green is not the color of photosynthesis, as one might assume. Rather, green is the leftovers of the solar feast. If the absorption of sunlight were more efficient, and the entire solar spectrum were used by the plant, the leaves of plants would be a nonreflecting black. It's as if nature limited its consumption, gobbling the bulk of sugar-building light but generously tossing us scraps of green.

Storing sugars, reflecting green light, and being consumed to power predators.