2004-09-17 / Columnists

Drawing On Science

One of the most fascinating displays of nature occurs during the fall season. It is the flaming color change in green leaves. Photosynthetic pigments called chlorophyll A and chlorophyll B cause the green color of leaves . Both pigments absorb the red and blue part of the visible spectrum (light), but the green of the spectrum is reflected back to our eyes. (Light will be a future topic in this column.) However, there are other photosynthetic pigments found in plants: Xanthophylls (ZAN-thuh-fils) and carotenes (KA-ruh-teens). They also absorb light energy, but from parts of the spectrum different from chlorophyll. Because of this, plants absorb and use light from a wide region of the spectrum. Xanthophyll reflects the yellow part of the spectrum while carotene reflects the orange part of the spectrum. But the overwhelming abundance of chlorophyll results in its green color masking the yellow and orange of the other two pigments. All of the photosynthetic pigments are found in chloroplasts (KLOR-uh-plasts). It is only when the chlorophyll begins to break down in the fall that the other colors are exposed. The combination of the various colors gives us a delightful eye-full.

The chemistry preceding this change in color is even more fascinating. The process is called photosynthesis— perhaps the most important chemical process for virtually the whole of the living world. Photosynthesis may be viewed as a series of step-by-step chemical reactions in which the elements hydrogen, carbon, and oxygen, with the energy from light, are joined to form a simple carbohydrate (sugar) called glucose . To best understand the process, let’s take a look at a cross section of a typical leaf (Fig. 1). The lower epidermis contains many, small openings called stomata or stomates (STO-mah-ta, STO-mates). The size of the opening is determined by two guard cells (Fig. 2). These cells, adjust the opening and closing of the stomates. This controls the amount of gases to enter or exit the leaf. The upper epidermis has fewer stomates. Between the upper and lower epidermis is a tissue called mesophyll (MEH-zo-feel, meso=middle + phyll=leaf), which consists of two types of cells: palisade layer and spongy mesophyll . Cells of the palisade layer contain many chloroplasts. The spongy mesophyll or layer, which has fewer chloroplasts than palisade cells, is so-called because of the many air spaces between the cells. These spaces contain oxygen, carbon dioxide, and water vapor. Also within the mesophyll layer may be found the vascular bundles or veins of the plant. These veins are composed of two types of transport tubes: xylem (ZY-lum), which transports water and phloem (FLO-um), which transports sugar. A waxy cuticle covers both the upper and lower structures of the leaf.

Photosynthesis takes place in the chloroplasts and occurs in two parts. Now, without getting too technical here’s what happens. Electrons within the chlorophyll absorb light energy. This added energy excites the electrons. This means that the electrons move to a higher energy level and leave the chlorophyll. Then they follow a path along which a series of chemical reactions occur. Along this path the extra energy is transferred to special molecules, which function as energy storage molecules. The electrons are then passed to different energy storage molecules. Now remember we said that the chlorophyll molecules absorb the light energy and lose their electrons. Well the chlorophyll molecule, then, has to replace its lost electrons. They do this by taking them from water. When the water molecule loses its electrons it splits into hydrogen and oxygen. This is the first part of photosynthesis called photolysis or the Light Reaction, because light energy is required.

While the light reaction is taking place some of the stored energy in those special molecules is being used for the second part of photosynthesis. This energy is used to combine the hydrogen (from the water) with carbon dioxide. The chemical reaction forms glucose, a simple sugar. This part of photosynthesis is called carbon fixation or the Dark Reaction. It doesn’t mean that it takes place in the dark; it’s called the Dark Reaction because light is not required. Through the whole process of photosynthesis, certain enzymes are required to help the chemical reactions along. Photosynthesis, however, is not the only way living things make food. Some bacteria use inorganic (non-living) substances to produce food. They use the chemical energy stored in the substance. This is called chemosynthesis , which follows, basically, the same process discussed here.

Photosynthesis may be summarized as follows: Light energy + carbon dioxide + water + enzymes + chlorophyll Ä sugar + oxygen. The importance of photosynthesis can easily been seen. Green plants remove carbon dioxide from the atmosphere. They use it to make food (glucose), which is passed from animal to animal. The glucose produced by photosynthesis, then, is the primary food source for the majority of life on earth. What happens to the oxygen released from the splitting of water? Simple…you’re breathing it. So the next time you see a blade of grass, get down on your knees, pucker-up your lips and give it a kiss saying, “Thank you.”

Then look up and take in the miraculous fall foliage.

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