| Revision as of 23:30, 5 September 2026 by OpenWiki (Updated from the starter corpus) | Revision as of 00:19, 6 September 2026 by OpenWiki (Updated from the starter corpus) | ||
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| === Light-dependent reactions === | === Light-dependent reactions === | ||
| − | In the thylakoid membranes of the chloroplast, chlorophyll absorbs light, mostly in the blue and red parts of the | + | In the thylakoid membranes of the chloroplast, chlorophyll absorbs light, mostly in the blue and red parts of the spectrum, which is why leaves reflect green and appear that colour. The absorbed energy excites electrons, which pass down a chain of carriers. |
| The electrons lost by chlorophyll are replaced by stripping them from water, splitting it and releasing oxygen as a by-product. Meanwhile the electron transport chain pumps protons across the membrane, and the resulting gradient drives ATP synthase, which manufactures ATP. The stage yields ATP and NADPH, and no sugar at all. | The electrons lost by chlorophyll are replaced by stripping them from water, splitting it and releasing oxygen as a by-product. Meanwhile the electron transport chain pumps protons across the membrane, and the resulting gradient drives ATP synthase, which manufactures ATP. The stage yields ATP and NADPH, and no sugar at all. | ||
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| In the stroma, the enzyme '''RuBisCO''' attaches carbon dioxide to a five-carbon sugar, and the ATP and NADPH from the first stage are spent reducing the product to carbohydrate. Three turns of the cycle fix three CO<sub>2</sub> molecules and yield one three-carbon sugar; the rest of the material is recycled to keep the cycle running. | In the stroma, the enzyme '''RuBisCO''' attaches carbon dioxide to a five-carbon sugar, and the ATP and NADPH from the first stage are spent reducing the product to carbohydrate. Three turns of the cycle fix three CO<sub>2</sub> molecules and yield one three-carbon sugar; the rest of the material is recycled to keep the cycle running. | ||
| − | RuBisCO is probably the most abundant protein on Earth and is also strikingly inefficient. It fixes only a few molecules of CO<sub>2</sub> per second, and it confuses carbon dioxide with | + | RuBisCO is probably the most abundant protein on Earth and is also strikingly inefficient. It fixes only a few molecules of CO<sub>2</sub> per second, and it confuses carbon dioxide with oxygen. When it takes O<sub>2</sub> instead, the plant must spend energy clearing up the useless product, a process called photorespiration. Plants compensate by making enormous quantities of the enzyme. |
| == Variants == | == Variants == | ||
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| Oxygenic photosynthesis evolved in cyanobacteria at least 2.4 billion years ago and produced the '''Great Oxidation Event''', filling the atmosphere with a gas that was toxic to most life then existing. It was among the largest changes any organism has made to the planet, and it made possible aerobic respiration, which yields far more energy per unit of food than the alternatives, and with it large, active animals such as the [[Octopus|octopus]]. | Oxygenic photosynthesis evolved in cyanobacteria at least 2.4 billion years ago and produced the '''Great Oxidation Event''', filling the atmosphere with a gas that was toxic to most life then existing. It was among the largest changes any organism has made to the planet, and it made possible aerobic respiration, which yields far more energy per unit of food than the alternatives, and with it large, active animals such as the [[Octopus|octopus]]. | ||
| − | Chloroplasts are descended from free-living cyanobacteria taken up by another cell and | + | Chloroplasts are descended from free-living cyanobacteria taken up by another cell and retained; they keep their own DNA and divide independently. Photosynthesis also produced, over hundreds of millions of years, the buried carbon now burned as fossil fuel. |
| == See also == | == See also == | ||
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