calvin cycle consists of three phase, what are they? Explain the significance of each of them.
Answers
Carbon Fixation
In carbon fixation, a CO2 molecule from the atmosphere combines with a five-carbon acceptor molecule called ribulose-1,5-bisphosphate (RuBP).
The resulting six-carbon compound is then split into two molecules of the three-carbon compound, 3-phosphoglyceric acid (3-PGA).
This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, also known as RuBisCO. Due to the key role it plays in photosynthesis, RuBisCo is probably the most abundant enzyme on Earth.
Reduction
In the second stage of the Calvin cycle, the 3-PGA molecules created through carbon fixation are converted into molecules of a simple sugar – glyceraldehyde-3 phosphate (G3P).
This stage uses energy from ATP and NADPH created in the light-dependent reactions of photosynthesis. In this way, the Calvin cycle becomes the way in which plants convert energy from sunlight into long-term storage molecules, such as sugars. The energy from the ATP and NADPH is transferred to the sugars.
This step is called “reduction” because NADPH donates electrons to the 3-phosphoglyceric acid molecules to create glyceraldehyde-3 phosphate. In chemistry, the process of donating electrons is called “reduction,” while the process of taking electrons is called “oxidation.”
Regeneration
Some glyceraldehyde-3 phosphate molecules go to make glucose, while others must be recycled to regenerate the five-carbon RuBP compound that is used to accept new carbon molecules.
The regeneration process requires ATP. It is a complex process involving many steps.
Because it takes six carbon molecules to make a glucose, this cycle must be repeated six times to make a single molecule of glucose.
To accomplish this equation, five out of six glyceraldehyde-3 phosphate molecules that are created through the Calvin cycle are regenerated to form RuBP molecules. The sixth exits the cycle to become one half of a glucose molecule.