The Calvin-Benson Cycle
The Calvin-Benson cycle is a series of biochemical reactions that take place in the stroma of chloroplasts during photosynthesis. Its main function is to use the energy from ATP and NADPH (produced in the light-dependent reactions) to convert carbon dioxide (CO2) into glucose.
The First Step: Carbon Fixation
* Reactant: CO2 (carbon dioxide)
* Enzyme: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
The Fourth Step:
The fourth step is actually a complex series of reactions. Let's break it down:
* The product of the third step: 1,3-bisphosphoglycerate (a 3-carbon compound) is reduced by NADPH to glyceraldehyde 3-phosphate (G3P)
* G3P is the key product: It's a 3-carbon sugar that is the final product of the Calvin cycle. However, the Calvin cycle needs to regenerate its starting material, ribulose bisphosphate.
* The final product (for the purpose of the cycle's regeneration): Some of the G3P molecules are used to generate ribulose 1,5-bisphosphate (RuBP), the 5-carbon molecule that accepts CO2 in the first step.
So, to answer your question directly:
* Reactant in the first step: CO2
* Final product in the fourth step (for the purpose of the cycle's continuation): RuBP (ribulose 1,5-bisphosphate)
Important Note: While G3P is a product of the fourth step and the primary product of the Calvin Cycle, it is not the final product in the sense of being the molecule used to continue the cycle. That role is played by RuBP.
