Question

The enzyme ribulose bisphosphate carboxylase (Rubisco) normally adds carbon dioxide to ribulose 1,5-bisphosphate. However, it will also catalyze a competing reaction in which O2 is added to ribulose 1,5-bisphosphate to form 3-phosphoglycerate and phosphoglycolate. Assume that phosphoglycolate is a compound that cannot be used in any further reactions. If O2 and CO2 have the same affinity for Rubisco, which of the following is the lowest ratio of CO2 to O2 at which a net synthesis of sugar can occur?

(a) 1:3
(b) 1:2
(c) 3:1
(d) 2:1

Answer 1

The lowest ratio of CO2 to O2 at which a net synthesis of sugar can occur, with equal affinities for RuBisCO, is at least 2:1. This ensures that carbon fixation is favored over photorespiration.

Step-by-step explanation:

The enzyme Ribulose bisphosphate carboxylase (RuBisCO) can add either CO2 or O2 to ribulose 1,5-bisphosphate. The addition of CO2 is essential for the synthesis of sugars via the Calvin Cycle, where CO2 is fixed to produce 3-phosphoglycerate (3-PGA), which is eventually used to form glucose, a process that also involves the utilization of ATP and NADPH. However, when RuBisCO adds O2 instead, no sugars are synthesized and a wasteful process called photorespiration occurs, yielding phosphoglycolate, a compound that, as per the question, has no further use.

In a scenario where CO2 and O2 have the same affinity for RuBisCO, the lowest ratio of CO2 to O2 at which net synthesis of sugar can occur is when there is at least as much CO2 as O2 available. Therefore, we are seeking the minimum ratio of CO2 to O2 that favors carboxylation over oxygenation.

Given the options (a) 1:3, (b) 1:2, (c) 3:1, or (d) 2:1, the correct answer is, (d) 2:1, because at this ratio, CO2 has a greater chance of being fixed by RuBisCO than O2 being added, allowing for a net synthesis of sugar to occur.