Photosynthesis

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Photosynthesis
Photosynthesis is the process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose. Photosynthesis provides the basic energy source for just about all organisms. Oxygen, which is a necessity of life for just about every organism, is a byproduct of photosynthesis.

Photosynthesis occurs in green plants, seaweeds, algae, and certain bacteria. These organisms produce millions of new glucose molecules a second. Glucose is a carbohydrate which plants use as an energy source to build leaves, flowers, fruits, and seeds. They also convert glucose to cellulose, which is the structural material used in their cell walls. Excess glucose is stored in the form of starch in roots, stems, and leaves. These reserves can be used for extra energy or building materials.

Plant photosynthesis occurs in leaves and green stems in specialized cell structures called chloroplasts. One leaf is composed of tens of thousands of cells containing 40 to 50 chloroplasts. The chloroplast is an oval-shaped structure divided by membranes into many disk-shaped compartments. These disklike compartments are called thylakoids. They are arranged vertically in the chloroplast like a stack of coins or pancakes. These stacks of thylakoids are called grana.The grana lies in a fluid called stroma.

Chlorophyll is a light-trapping pigment needed for photosynthesis. There are hundreds of chlorophyll molecules in the membranes of thykaloids. Additional light-trapping pigments, enzymes, which are organic substances that speed up chemical reactions, and other molecules needed for photosynthesis are also located within the thylakoid membranes. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, e...