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Biology: Key Area 7 Summary Sheet The overall word equation for photosynthesis is: Carbon dioxide + water Raw materials Light energy Chlorophyll glucose + oxygen Products Photosynthesis is a two stage process. The first stage of photosynthesis is called the Light Reaction and depends on light energy. During the Light Reaction, light energy from the sun is trapped by chlorophyll (the green pigment in chloroplasts) and is converted into chemical energy in the form of ATP. ATP is used to split water into hydrogen and oxygen. Excess oxygen diffuses out of the cell. Hydrogen attaches to hydrogen acceptor molecules and is carried to stage 2. ATP made in the Light Reaction is also used in stage 2. The second stage of photosynthesis is called carbon fixation. The second stage is called carbon fixation and is a series of enzyme controlled reactions. Hydrogen and ATP produced by the Light Reaction (stage 1) are used. ATP is used as the energy source. The hydrogen combines with carbon dioxide from the atmosphere to produce a sugar called glucose. The chemical energy in the glucose is available to use in a process called respiration. The glucose can also be converted into a starch or cellulose. Starch is a large and insoluble storage carbohydrate. Cellulose is a structural carbohydrate that is used to build the plant cell wall. Limiting factors of photosynthesis are carbon dioxide concentration, light intensity and temperature. If in short supply, limiting factors limit the rate of photosynthesis and therefore, plant growth. To measure the rate of photosynthesis in a green plant, count the number of oxygen bubbles released per unit of time.