The Garden Explored by Mia Amato

Author:Mia Amato
Language: eng
Format: epub
Publisher: Henry Holt and Co.

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Explaining Photosynthesis

Just how a leaf captures light is a matter of quantum and wave mechanics, and is easily explained. (Oh, yes it is—stay with me here.) In our post-Einstein world, scientists accept that light has a dual nature: it travels as a continuous wave and also as individual particles, called photons.

The white light of sunshine is made up of light of different colors—a color spectrum. We can see those colors when sunlight shines through raindrops to make a rainbow. You can think of the colors of the rainbow as light waves where each different color is vibrating at a different frequency. Or you can think of those colors as groups of photons, where each different color is made up of photons of a different energy. When you are talking about photosynthesis, it’s easiest to talk about light as photons.

When the sun shines on a green leaf, certain photons (particularly those from the red and blue parts of the spectrum) get absorbed by the molecules of chlorophyll. When a molecule of chlorophyll absorbs a photon, the radiant energy of the photon causes the outermost electrons of the chlorophyll molecule to become “excited” and rev up to a higher energy level. The chlorophyll molecule passes its excited electrons along to another nearby molecule. That molecule, in turn, passes the excited electrons to still other molecules, in what’s called an “electronic transport train.”

Every time the electrons move from one molecule to another, a little of their energy is converted to chemical energy. Then that chemical energy is used to rearrange carbon dioxide and water to make simple sugars and oxygen.

The simple sugars created by photosynthesis may remain in the leaf, to be broken down again to release energy for other chemical reactions as needed. This is the plant equivalent of grabbing a candy bar when you need a little energy boost in the afternoon. Sugar molecules also get carried around the plant through its vascular system, a water-based transport network. (See Chapter 2.) The simple sugars may also bond together to form larger and more complex molecules, such as disaccharides (including sucrose, the sugar in your sugar bowl) and polysaccharides like starches and cellulose.

Plants tend to squirrel away their complex carbohydrates into certain plant parts—seeds, for example, because the seeds will need the stored sun energy for their first growth spurt. Some plants store their energy reserves, in the form of starches, in underground tubers. This is the plant equivalent of keeping a candy bar in your pocket for that late afternoon energy boost. Every time you bite into an ear of sweet corn, or spoon into a sweet potato, you are eating the stored energy of the sun.

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