Simon Marweyi
Jan 2
Describe the major stages of the Calvin cycle?
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Jan 2
The Calvin cycle, also known as the light-independent reactions or the dark reactions, is a series of biochemical reactions that occur in the stroma of chloroplasts during photosynthesis. It is named after Melvin Calvin, who discovered the cycle in the 1950s. The major stages of the Calvin cycle are as follows:
1. Carbon Fixation: The Calvin cycle begins with the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzing the fixation of carbon dioxide (CO2) from the atmosphere. RuBisCO combines CO2 with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon compound. This compound immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: In this stage, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) produced during the light-dependent reactions of photosynthesis are utilized. Each molecule of 3-PGA is phosphorylated by ATP and then reduced by NADPH, resulting in the formation of glyceraldehyde-3-phosphate (G3P). For every three molecules of CO2 fixed, six molecules of G3P are produced, but only one molecule exits the cycle to be used for the synthesis of glucose and other organic compounds.
3. Regeneration of RuBP: The remaining five molecules of G3P undergo a series of reactions to regenerate three molecules of RuBP, which are essential for the continuation of the Calvin cycle. This regeneration process requires ATP, as three molecules of ATP are used to convert the five molecules of G3P into three molecules of RuBP.
Overall, the Calvin cycle is a cyclic process, as the starting molecule (RuBP) is regenerated at the end of the cycle. This allows the cycle to continue as long as there is an adequate supply of ATP and NADPH from the light-dependent reactions. The net result of the Calvin cycle is the conversion of carbon dioxide into glucose and other organic molecules, which are essential for the growth and development of plants.
1. Carbon Fixation: The Calvin cycle begins with the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzing the fixation of carbon dioxide (CO2) from the atmosphere. RuBisCO combines CO2 with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon compound. This compound immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: In this stage, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) produced during the light-dependent reactions of photosynthesis are utilized. Each molecule of 3-PGA is phosphorylated by ATP and then reduced by NADPH, resulting in the formation of glyceraldehyde-3-phosphate (G3P). For every three molecules of CO2 fixed, six molecules of G3P are produced, but only one molecule exits the cycle to be used for the synthesis of glucose and other organic compounds.
3. Regeneration of RuBP: The remaining five molecules of G3P undergo a series of reactions to regenerate three molecules of RuBP, which are essential for the continuation of the Calvin cycle. This regeneration process requires ATP, as three molecules of ATP are used to convert the five molecules of G3P into three molecules of RuBP.
Overall, the Calvin cycle is a cyclic process, as the starting molecule (RuBP) is regenerated at the end of the cycle. This allows the cycle to continue as long as there is an adequate supply of ATP and NADPH from the light-dependent reactions. The net result of the Calvin cycle is the conversion of carbon dioxide into glucose and other organic molecules, which are essential for the growth and development of plants.
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