explain calvin cycle
Answers
The function of the Calvin cycle is to create three-carbon sugars, which can then be used to build other sugars such as glucose, starch, and cellulose that is used by plants as a structural building material. The Calvin cycle takes molecules of carbon straight out of the air and turns them into plant matter.
This makes the Calvin cycle vital for the existence of most ecosystems, where plants form the base of the energy pyramid. Without the Calvin cycle, plants would be unable to store energy in a form that herbivores could digest. Carnivores would subsequently not have access to energy stored in the bodies of herbivores!
The carbon backbones created in the Calvin cycle are also used by plants and animals to make proteins, nucleic acids, lipids, and all the other building blocks of life.
The Calvin cycle also regulates the levels of carbon dioxide, a greenhouse gas, in the Earth’s atmosphere. Scientists have raised concerns because, in addition to putting huge amounts of CO2 back into the air by burning coal, oil, and gasoline, humans have also cut down about half of all Earth’s forests, which play an important role in removing CO2 from the air.
Carbon dioxide enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle.
The Calvin cycle reactions can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end.
RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is fixed from its inorganic form into organic molecules.
ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.
One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). The remaining G3P molecules regenerate RuBP.
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