Assume that you have an illuminated suspension of Chlorella cells carrying out photosynthesis in the presence of 0.1% carbon dioxide and 20% oxygen.
What will be the short-term effects of the following changes in conditions on the levels of 3-phosphoglycerate (PGA) and ribulose-1,5-bisphosphate (RuBP)?

Carbon dioxide concentration is suddenly reduced 1000-fold.

a. PGA down, RuBP up
b. PGA up, RuBP down
c. PGA up, RuBP unchanged
d. PGA unchanged, RuBP up

a. PGA down, RuBP up

Explanation:

RuBP, also known as Ribulose-1,5-bisphosphate is a molecule that assist in he fixation of carbon dioxide in the Calvin Cycle or light independent reaction of photosynthesis.

RuBP is combined with carbon dioxide using the enzyme known as rubisco to form an short-lived intermediate product which divides to form two 3-carbon molecule structure known as 3-phosphoglycerate.

If the concentration of carbon dioxide is suddenly reduced, it thus means that the rate of production of 3-phosphoglycerate will reduce as carbon dioxide becomes a limiting factor. Hence, 3-phosphoglycerate's  concentration will reduce while more RuBP will become available as a result.

The correct option is a.