Dark reactions don't require light, but they aren't inhibited by it, either. For most plants, the dark reactions take place during daytime. The dark reaction occurs in the stroma of the chloroplast. This reaction is called carbon fixation or the Calvin cycle. In this reaction, carbon dioxide is converted to sugar using ATP and NADPH. Carbon dioxide is combined with a 5-carbon sugar to form a 6-carbon sugar. The 6-carbon sugar is broken into two sugar molecules, glucose and fructose, which can be used to make sucrose. The reaction requires 72 photons of light.
The efficiency of photosynthesis is limited by environmental factors, including light, water, and carbon dioxide. In hot or dry weather, plants may close their stomata to conserve water. When the stomata are closed, the plants may start photorespiration. Plants called C4 plants maintain high levels of carbon dioxide inside cells that make glucose, to help avoid photorespiration. C4 plants produce carbohydrates more efficiently than normal C3 plants, provided the carbon dioxide is limiting and sufficient light is available to support the reaction. In moderate temperatures, too much of an energy burden is placed on the plants to make the C4 strategy worthwhile (named 3 and 4 because of the number of carbons in the intermediate reaction). C4 plants thrive in hot, dry climates.
