Question

Draw how the chemical structure changes during the fixation phase of the Calvin Cycle (Ribulose-1,5-bisphosphate + CO2 --> 6-Carbon Intermediate --> 3-Phosphoglycerate). Include enzymes used and any byproducts formed.

What enzyme mediates this change in Q3? What does it do? Are there any byproducts other than 3-Phosphoglycerate? Why or why not?

Answer 1

The first stage of the Calvin cycle incorporates carbon from CO2 into an organic molecule, a process called carbon fixation. In plants, atmospheric CO2 enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata. It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place.

In the first step of the cycle, an enzyme nicknamed rubisco (RuBP carboxylase-oxygenase) catalyzes attachment of a five-carbon sugar called ribulose bisphosphate (RuBP). The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). Thus, for each CO2 two 3-PGA molecules are produced.

The actual molecular structures are shown below:

Ribulose-1,5-bisphosphate carboxylase/oxygenase catalyzes the addition of gaseous carbon dioxide to ribulose-1,5-bisphosphate (RuBP), generating two molecules of 3-phosphoglyceric acid (3-PGA), and is thus the key enzyme in CO2 assimilation. Rubsico is capable of a competing oxygenation reaction, which generates one molecule of 3-PGA and one of 2-phosphoglycolate from RuBP.

OH 9 + ou so over kona -O-Holloo halo CH IRUDISCO unstable 6-laerbon Compound 13- phosphoglycerate (3-PGA) P=R-P-00 60 Phosphate group