Unit 13B

Question 1

Identify the three stages of carbon assimilation in photosynthesis.

Answer 1

1) Carbon dioxide fixation into 3-phosphoglycerate
2) 3-phosphoglycerate reduction to a phosphate triose (glyceraldehyde 3-phosphate)
3) Regeneration of ribulose 1,5-bisphosphate from glyceraldehyde 3-phosphate

Question 2

What are the two possible alternative fates for glyceraldehyde 3-phosphate after stage 2?

Answer 2

The extra glyceraldehyde 3-phosphate not converted back to ribulose 1,5-bisphosphate either:
A) condenses with dihydroxyacetone available in the chloroplast to give fructose 1,6-bisphosphate, a precursor to fructose 6-phosphate and starch storage in the plastid
B) Isomerized to dihydroxyacetone phosphate and transported out into the cytosol to be converted to fructose 6-phosphate in the same way and used as a precursor for sucrose.