Question

have answered them Total: 17 marks 23. Describe the main events occurring inside each of the three phases of the Calvin cvel using the acronyms of the molecules, enzymes, and energy carriers involved. (9 marks: 3 marks for each phase) it moins energy during the light

Answer 1

Carbon fixation during Calvin cycle takes place in the three phases:

a. Carboxylation or Carbon Fixation: In this phase a carbon dioxide molecule combines with a molecule of ribulose-1,5-bisphosphate which is a five carbon compound and yields two molecules of 3 carbon compound 3-phosphoglyceric acid (3-PGA). This reaction is catalysed by an enzyme known as rubisco.

Carbon dioxide enters the Calvin cycle by reacting with ribulose1,5 bisphosphate to yield two molecules of 3-phosphoglyceric acid (3-PGA) this reaction is catalysed by an enzyme ribulose bisphosphate carboxylase/oxygenate, referred to as rubisco. The CO2 is added to carbon 2 of ribulose-1,5-bisphosphate, yielding an unstable, enzyme-bound intermediate, which is hydrolyzed to yield two molecules of the stable 3-phosphoglycerate.

b. Reduction: In this phase 3-phosphoglyceric acid undergo reduction to produce molecules of 3 carbon compound glyceraldehyde 3 phosphate (G3P). Reduction of 3-PGA utilises the input NADPH and ATP.

The 3- phosphoglycerate formed in the carboxylation stage is first phosphorylated via 3-phosphoglycerate kinase to 1,3-bisphosphoglycerate by transferring  a phosphate group from the ATP and then it is reduced to glyceraldehyde-3-phosphate through use of the NADPH and catalysed by the glyceraldehyde-3-phosphate dehyhydrogenase enzyme.

c. Regeneration: The continued uptake of CO2 requires that the CO2 acceptor, ribulose-1,5-bisphosphate be constantly regenerated. Most of the glyceraldehyde 3-phosphate is recycled to ribulose 1,5-bisphosphate. The intermediates in this pathway include three, four, five, six, and seven carbon sugars. A small fraction of the “extra” glyceraldehyde 3-phosphate may be used immediately as a source of energy, but most is converted to sucrose for transport or is stored in the chloroplast as starch. Glyceraldehyde 3-phosphate condenses with dihydroxyacetone phoshate in the stroma to form fructose 1,6-bisphosphate, a precursor of starch. In other situations the glyceraldehyde 3-phosphate is converted to dihydroxyacetone phosphate, which leaves the chloroplast via a specific transporter and, in the cytosol, can be degraded glycolytically to provide energy or used to form fructose 6-phosphate and hence sucrose.