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ENERGY TRANSFORMATION

The Calvin Cycle

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The Calvin Cycle
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The Calvin Cycle, in which carbon is fixed into organic compounds, takes place in the stroma of the chloroplasts.

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All the reactions of photosynthesis that are not directly dependent upon light are known as the dark reactions. The dark reactions happens whether there is light present or not. The dark reactions occurs in the part of the chloroplast known as the stroma. The purpose of the dark reactions is to take the energy from ATP and energized electrons and hydrogen ions from NADPH and add them to CO2 to make glucose or sugar. 

Step 1: CO2 diffuses into the stroma from surrounding cytosol. An enzyme combines each CO2 molecule with a five-carbon molecule called ribulose bisphosphate(RuBP). The six-carbon molecules that result are very unstable, and they each immediately split into two three-carbon molecules. These three-carbon molecules are called 3-phosphoglycerate(3-PGA)
 
Step 2: Each molecule of 3-PGA is converted into another three-carbon molecule,G3P, in a two part process.
 
Step 3: One of the G3P molecules leaves the Calvin Cycle and is used to make organic compounds (carbohydrates) in which energy is stored for later use.
 
Step 4: The remaining G3P molecules are converted back into RuBP throught the addition of phosphate groups from ATP molecules. The resulting RuBP molecules then enter the Calvin Cycle again.