The Calvin cycle is the set of reactions that occur during the light-independent reactions of photosynthesis, also known as the dark reactions or the carbon fixation cycle. The purpose of the Calvin cycle is to convert carbon dioxide (CO2) from the atmosphere into glucose, which can be used by plants as a source of energy and building material.
The Calvin cycle occurs in the stroma of chloroplasts, which is the fluid-filled region surrounding the thylakoid membranes where light-dependent reactions occur. The Calvin cycle is a complex process that involves a series of biochemical reactions and enzyme-catalyzed steps. The cycle can be divided into three main stages: carbon fixation, reduction, and regeneration.
During the carbon fixation stage, carbon dioxide from the atmosphere is fixed into organic molecules. The enzyme responsible for this reaction is called rubisco (ribulose bisphosphate carboxylase/oxygenase), which catalyzes the addition of CO2 to ribulose bisphosphate (RuBP) to form an unstable six-carbon molecule that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
In the reduction stage, ATP and NADPH produced during the light-dependent reactions are used to reduce the 3-PGA to form glyceraldehyde 3-phosphate (G3P). This step requires energy from ATP and reducing power from NADPH.
In the regeneration stage, some of the G3P molecules are used to regenerate RuBP, which is needed for the next round of carbon fixation. This step requires ATP, which is produced during the light-dependent reactions.
The net result of the Calvin cycle is the conversion of three molecules of CO2 into one molecule of G3P. Two molecules of G3P are then used to produce one molecule of glucose, while the remaining G3P molecules are used for other metabolic processes.
Carbon fixation is a critical process for life on Earth, as it provides the basis for most food chains and supports the growth of all plant life. The Calvin cycle plays a key role in this process by converting atmospheric CO2 into organic molecules that can be used by plants for energy and growth. Understanding the mechanisms and regulation of the Calvin cycle is essential for developing strategies to improve crop productivity and for mitigating the effects of climate change.