Photosynthesis and Respiration

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All organisms on earth depend on photosynthesis, in which light energy is used to make sugar and other food molecules from carbon dioxide and water. For example, plants and other photo-synthesizers need only energy from sunlight, carbon dioxide from air, and water from the soil to make the sugar glucose. Photosynthesis is the most important chemical process on earth because it provides food for virtually all organisms, not only for photo-synthesizers but for the organisms that eat them. Plants can capture the energy of the sun by a chemical process called photosynthesis. This chemical reaction can be described by the following simple equation: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2

The product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere. All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight. This chemical reaction is catalyzed by chlorophyll acting in concert with other pigment, lipid, sugars, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, or it can be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids.

Light is central to the life of a plant, and its role goes beyond photosynthesis. The sensing of light is as important for plants as vision for animals. Light receptors in plant cells convert light into chemical signals that bring about important stages in the plant's life cycle, from the germination of seeds to the production of flowers. However, light is essential to plant reproduction and growth, too much sunlight can be harmful. Sunlight can overheat a plant or overload its photosynthetic machinery....
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