The Calvin Cycle Stages and Process in Photosynthesis

Calvin cycle- combining three “turns”

The Calvin cycle, also known as the Calvin-Benson cycle, is a series of biochemical reactions that occur in the chloroplasts of plants during the light-independent reactions of photosynthesis. It is named after Melvin Calvin, who, along with Andrew Benson and James Bassham, elucidated the cycle in the 1950s.

The Calvin cycle can be divided into three main stages: carbon fixation, reduction, and regeneration. Each stage is comprised of a series of chemical reactions that ultimately result in the production of glucose or other carbohydrates

1. Carbon Fixation:
During this stage, carbon dioxide (CO2) from the atmosphere is captured and converted into an organic molecule. The first step involves the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzing the attachment of CO2 to a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This results in the formation of two molecules of a three-carbon compound called 3-phosphoglycerate (PGA)

2. Reduction:
In the second stage, energy from ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) is used to convert the PGA molecules into a different three-carbon compound called glyceraldehyde-3-phosphate (G3P). This process involves the transfer of electrons and the addition of phosphate groups

Out of the six molecules of G3P produced, only one is used to synthesize glucose or other carbohydrates, while the remaining five molecules continue in the cycle for regeneration

3. Regeneration:
The final stage of the Calvin cycle involves the regeneration of the starting molecule, RuBP. The five remaining G3P molecules undergo a series of reactions that require ATP. These reactions rearrange and combine the carbon atoms, ultimately forming three molecules of RuBP. These RuBP molecules can then initiate a new cycle by binding with CO2 again

Overall, it takes three “turns” of the Calvin cycle to produce one molecule of glucose, consisting of six carbon atoms. This means that the carbon dioxide fixation and reduction steps must occur three times to produce enough G3P molecules that can be used to synthesize glucose

In summary, the Calvin cycle is a complex series of reactions that allows plants to convert carbon dioxide into glucose using energy from ATP and NADPH. The three stages of the cycle, carbon fixation, reduction, and regeneration, work together to produce carbohydrates needed for plant growth and metabolism

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