chlorophyll, green pigment that gives most plants their color and enables them to carry on the process of photosynthesis. Chemically, chlorophyll has several similar forms, each containing a complex ring structure and a long hydrocarbon tail. The molecular structure of the chlorophylls is similar to that of the heme portion of hemoglobin, except that the latter contains iron in place of magnesium. Within the photosynthetic cells of plants the chlorophyll is in the chloroplasts—small, roundish, dense protoplasmic bodies that contain the grana, or disks, where the chlorophyll molecules are located. Chlorophyll absorbs light in the red and blue-violet portions of the visible spectrum; the green portion is not absorbed and, reflected, gives chlorophyll its characteristic color. Chlorophyll tends to mask the presence of colors in plants from other substances, such as the carotenoids. When the amount of chlorophyll decreases, the other colors become apparent. This effect can be seen most dramatically every autumn when the leaves of trees "turn color."

Any member of one of the most important classes of pigment molecules involved in photosynthesis. Found in almost all photosynthetic organisms, it consists of a central magnesium atom surrounded by a nitrogen-containing structure called a porphyrin ring, to which is attached a long carbon-hydrogen side chain, known as a phytol chain. In structure it is remarkably similar to hemoglobin. Chlorophyll uses energy that it absorbs from light to convert carbon dioxide to carbohydrates. In higher plants it is found in chloroplasts.

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