06-12-2012, 01:16 PM
Photosynthesis: Calvin Cycle
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Photosynthesis takes place in chloroplasts.
It includes light reactions and reactions that are not directly energized by light.
Light reactions:
Energy of light is conserved as
“high energy” phosphoanhydride bonds of ATP
reducing power of NADPH.
Proteins & pigments responsible for the light reactions are in thylakoid (grana disc) membranes.
Light reaction pathways will be not be presented here.
Large subunits within RuBisCO are arranged as antiparallel dimers, with the N-terminal domain of one monomer adjacent to the C-terminal domain of the other.
Each active site is at an interface between monomers within a dimer, explaining the minimal requirement for a dimeric structure.
Continuing with Calvin Cycle:
A portion of the glyceraldehyde-3-P is converted back to ribulose-1,5-bisP, the substrate for RuBisCO, via reactions catalyzed by:
Triose Phosphate Isomerase, Aldolase, Fructose Bisphosphatase, Sedoheptulose Bisphosphatase, Transketolase, Epimerase, Ribose Phosphate Isomerase, & Phosphoribulokinase.
Many of these are similar to enzymes of Glycolysis, Gluconeogenesis or Pentose Phosphate Pathway, but are separate gene products found in the chloroplast stroma. (Enzymes of the other pathways listed are in the cytosol.)
The process is similar to Pentose Phosphate Pathway run backwards.
Regulation of Calvin Cycle
Regulation prevents the Calvin Cycle from being active in the dark, when it might function in a futile cycle with Glycolysis & Pentose Phosphate Pathway, wasting ATP & NADPH.
Light activates, or dark inhibits, the Calvin Cycle (previously called the “dark reaction”) in several ways.