CALVIN-BENSON CYCLE
The Calvin-Benson cycle, also known as the Calvin cycle or the carbon fixation cycle, is a series of chemical reactions that occur in the chloroplasts of green plants and algae. It is part of the process of photosynthesis, where organisms convert carbon dioxide and sunlight into glucose molecules, which can be used as an energy source for the cell.
The Calvin-Benson cycle involves three main stages:
1. Carbon Fixation: In this stage, carbon dioxide molecules from the atmosphere are incorporated into an organic molecule called ribulose-1,5-bisphosphate (RuBP), with the help of an enzyme called RuBisCO. This process forms an unstable six-carbon intermediate, which quickly breaks down into two molecules of a three-carbon compound called 3-phosphoglycerate (3PG).
2. Reduction: In this stage, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) molecules, which are produced during the light-dependent reactions of photosynthesis, are used to convert the 3PG molecules into another three-carbon compound called glyceraldehyde-3-phosphate (G3P). This step requires energy from ATP and reduces NADPH to NADP+.
3. Regeneration: In this final stage, some of the G3P molecules produced in the previous step are used to synthesize glucose, while others are used to regenerate the RuBP molecule so that the cycle can continue. The regeneration of RuBP requires ATP, which is produced during the light-dependent reactions.
Overall, the Calvin-Benson cycle uses energy from ATP and reducing power from NADPH to convert carbon dioxide into glucose, which can be stored for later use or used as a building block for other organic molecules in the plant. This cycle occurs in the stroma of the chloroplasts and is essential for the survival of plants and the production of oxygen as a byproduct.
The Calvin-Benson cycle, also known as the Calvin cycle or the light-independent reactions of photosynthesis, is a series of biochemical reactions that occur in the stroma of chloroplasts. These reactions are responsible for fixing carbon dioxide (CO2) and producing glucose, which is crucial for plant growth and survival.
Here are the general steps of the Calvin-Benson cycle:
1. Carbon Fixation: The cycle begins when CO2 from the atmosphere enters the stroma of the chloroplast. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) combines CO2 with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) to produce an unstable six-carbon compound. This process is known as carbon fixation.
2. Production of PGA: The six-carbon compound formed in the previous step quickly breaks down into two molecules of three-carbon compounds called 3-phosphoglycerate (PGA). Each molecule of CO2 fixes one molecule of RuBP, resulting in two molecules of PGA.
3. Reduction: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) produced in the light-dependent reactions of photosynthesis (in the thylakoid membrane) provide the energy and reducing power required for the conversion of PGA to glyceraldehyde-3-phosphate (G3P). In this step, PGA is reduced to G3P, with the help of ATP and NADPH. Some G3P molecules are used to regenerate RuBP, while others continue on to produce glucose.
4. Regeneration of RuBP: To keep the Calvin-Benson cycle running, some of the G3P molecules need to be converted back into RuBP. At this stage, ATP is used to convert some G3P molecules into RuBP. This step requires energy input and completes the cycle.
5. Glucose Production: Each turn of the Calvin-Benson cycle produces one molecule of glucose. Glucose is an essential carbohydrate that can be used for energy production, converted into starch for storage, or used to synthesize other organic compounds needed by the plant.
It's important to note that the Calvin-Benson cycle needs to go through several cycles to produce a net gain of glucose and regenerate RuBP. The exact number of cycles required depends on the plant species and environmental conditions.