Photosynthesis is the process in which light energy is absorbed by chlorophyll and used to make organic molecules from carbon dioxide and water. It converts light energy into chemical energy stored in carbohydrate. The overall equation is:6CO2 + 6H2O → C6H12O6 + 6O2 Light energy and chlorophyll are needed, and oxygen is released as a waste product. The equation is the reverse of aerobic respiration, but the pathway is completely different.Photosynthesis happens in chloroplasts in two linked stages:
• The light-dependent reactions take place on the thylakoid membranes. Light energy is absorbed, water is split (photolysis) releasing oxygen, and the energy is used to make ATP and reduced NADP.
• The light-independent reactions (the Calvin cycle) take place in the stroma. ATP and reduced NADP are used to reduce carbon dioxide to carbohydrate.
The oxygen released comes from water, not from carbon dioxide. This was shown by supplying plants with water containing the heavy isotope 18O: the oxygen gas given off contained 18O. When the label was in the CO2 instead, the oxygen gas was unlabelled.Glucose is usually written as the product, but the first carbohydrate made is a 3-carbon sugar phosphate, which is then used to build glucose and other molecules.Worked example: to make 3 molecules of glucose, a plant must fix 3 × 6 = 18 molecules of CO2 and release 18 molecules of O2.
• The light-dependent reactions take place on the thylakoid membranes. Light energy is absorbed, water is split (photolysis) releasing oxygen, and the energy is used to make ATP and reduced NADP.
• The light-independent reactions (the Calvin cycle) take place in the stroma. ATP and reduced NADP are used to reduce carbon dioxide to carbohydrate.
The oxygen released comes from water, not from carbon dioxide. This was shown by supplying plants with water containing the heavy isotope 18O: the oxygen gas given off contained 18O. When the label was in the CO2 instead, the oxygen gas was unlabelled.Glucose is usually written as the product, but the first carbohydrate made is a 3-carbon sugar phosphate, which is then used to build glucose and other molecules.Worked example: to make 3 molecules of glucose, a plant must fix 3 × 6 = 18 molecules of CO2 and release 18 molecules of O2.