The Basics of the Calvin Cycle


The Calvin Cycle-or also known as the C3c cycle, Calvin-Benson Bassham (CBB) cycle, dark reaction, or reductive pentose phosphate cycle- was discovered by Melvin Calvin, James Bassham, and Andrew Benson in the year of 1950. This cycle does not directly require light making it a light independent reaction of photosynthesis.

The Calvin Cycle starts out with carbon dioxide entering a plant through its leaves that contain pores called the stomata. The reactions that occur are called light-independent reactions because of their lack of use of light. Carbon atoms-which are provided by the carbon dioxide- are then incorporated into organic molecules and used to build glyceraldehyde 3-phosphate(G3P). Such process uses the help of ATP and NADPH from light reactions. Different from the light reactions, which occur in the thylakoid membrane, the cycle happens in the stroma. Then, the G3P is used as the starting point to make glucose, cellulose, starch, and other organic compounds that the plant might need

To simplify, the Calvin Cycle can be filtered down to 4 simple steps. In step one, carbon is changed. The enzyme rubisco combines a carbon dioxide molecule with the a 5-carbon sugar ribulose bisphosphate. This unstable result causes the molecule to split into 2 molecules of 3-carbon phosphoglyceric acid. The end result of this are 6 molecules of 3 carbon 3-PGA. The second step is reduction of 3-PGA molecules. Energy from the 6 molecules of ATP is used to react NADPH TO NADP+. From there, the product is six molecules of energy-rich 3-carbon sugar G3P. The third step is the releasing of a molecule of the G3P.

This leaves the remaining 5 molecules to continue the process. The final step of this cycle is to renew the RuBP. This is done through a series of chemical reactions that uses the energy of ATP to rearrange the atoms of the 5 G3P molecules, (15 carbon molecules), which forms 3 RuBP molecules (15 total carbon molecules). The end result of the last step ends with three molecules of five carbon RuBP. This is only one turn of the Calvin Cycle. This whole process will cycle again repeatedly.

To summarize, the equation of the Calvin Cycle is 3 O2+9ATP+6 NADPH+ 5H20-> glyceraldehyde-3-phosphate (G3P) + 2H+6+NADPH+9 ADP+ 8 Pi. Pi is the same as inorganic phosphate. The main reactant of the Calvin Cycle is C02, and the main product is glucose.


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