Plants can make energy even when there's no sun, but only for so long. At night, plants regularly make energy through the dark reaction. These dark reactions are why plants don't look wilted and brown when the sun rises. Dark reactions use complex chemical reactions that vary depending on the plant species.
Dark reactions occur in the dark as long as the energy carriers created by the light reactions are still inside the plant. Energy carriers are the compounds created through photosynthesis. Dark reaction is a misnomer, since a major enzyme is indirectly simulated by light.
The CO2 needed to initiate the dark reaction enters the plant through the stomata. However, plants submerged in water do not need stomata, since the carbon dioxide diffuses into the plant. The chemical ribulose biphosphate captures the carbon dioxide.
The dark reaction in the plant takes place in the stroma, inside the chloroplast. This reaction converts CO2 into sugar. The dark reaction needs no direct light, but does need the ATP created from photosynthesis and NADPH. The ATP and the CO2 form sugar through the Calvin cycle. The first organic compound created through the Calvin cycle is glyceraldehyde 3-phosphate, which contains three carbon atoms. The three carbon compound is called phosphoglyceraldehyde (PGAL). Two molecules of PGAL combine to create a 6-carbon glucose molecule and other carbohydrates for the plant.
The Calvin cycle uses six molecules of carbon dioxide to create one molecule of glucose. Two atoms of hydrogen join with the NADP, breaking apart water and releasing oxygen. The whole process creates C-C covalent bonds of carbohydrates.
Plants cannot perform the dark reaction when hot temperatures cause water to evaporate from the plant. Plants close their stomata to keep water from escaping, but closing the stomata prevents CO2 entry. Fortunately, some plants perform dark reactions even with very little CO2, thanks to special enzymes. The dark reaction turns the CO2 into oxaloacetate, which has four carbons. Plants living in the desert only open their stomata at night, so the dark reaction only occurs during the nighttime. These plants store the CO2 in organic compounds at night. During the day, they put the stored CO2 into the Calvin cycle.
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