The Calvin cycle is a series of chemical reactions that photosynthetic organisms use to turn atmospheric carbon dioxide into sugar, which is an organic compound used for energy and growth. This cycle is the “synthesis” part of photosynthesis, building the molecules necessary for life from simple inputs and forming a foundational biological pathway for most ecosystems.
The Role of the Calvin Cycle in Photosynthesis
Photosynthesis has two components: light-dependent reactions and light-independent reactions, with the Calvin cycle being the latter. The light-dependent reactions capture solar energy to generate adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules act as energy carriers for the cell.
The energy from ATP and NADPH drives the synthesis of sugars in the Calvin cycle. While the cycle’s reactions do not directly require light, they depend entirely on the products from the light-capturing stages. Photosynthesis is housed within chloroplasts, and the Calvin cycle specifically occurs in the stroma, the fluid-filled space surrounding the thylakoids where light reactions take place.
Stage One: Carbon Fixation
The Calvin cycle begins with carbon fixation. Carbon dioxide (CO2) from the air enters the stroma of the chloroplasts and is attached to a five-carbon molecule named ribulose-1,5-bisphosphate (RuBP).
This reaction is facilitated by the enzyme RuBP carboxylase/oxygenase, known as RuBisCO, one of the most abundant proteins on Earth. The joining of CO2 and RuBP creates an unstable six-carbon compound. This molecule immediately breaks down into two identical three-carbon molecules called 3-phosphoglycerate (3-PGA), the first stable product of the cycle.
Stage Two: Reduction
In the reduction stage, the 3-PGA molecules are converted into a high-energy sugar. Each molecule of 3-PGA is first activated by receiving a phosphate group from an ATP molecule, which then becomes ADP.
Next, the activated molecule is reduced by NADPH, which donates electrons. This reaction releases a phosphate group and converts the molecule into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. For every three CO2 molecules that enter the cycle, six molecules of G3P are produced.
Stage Three: Regeneration
The final stage regenerates the starting molecule, RuBP, to ensure the cycle’s continuation. For every three CO2 molecules fixed, six molecules of G3P are produced, but only one G3P molecule exits the cycle as a net product. This single G3P is a building block for glucose, starch, and other organic molecules for the plant’s growth.
The other five G3P molecules are used to regenerate RuBP. This process involves a series of enzymatic reactions that rearrange the five three-carbon molecules into three five-carbon RuBP molecules. This regeneration phase also requires additional energy in the form of ATP, preparing the cycle to fix more carbon dioxide.