Question

practice problem

1. rubisco, the enzyme involved in the calvin cycle, can bind to both co₂ and o₂. after learning this, a

student became interested in photorespiration. after conducting research, she claimed that
photorespiration is simply a metabolic relic; early atmospheric conditions did not have oxygen, so it
never posed a problem early on. would you support or reject this claim? justify your response. (hint:
you can include evidence related to the evolutionary history of rubisco, the composition of early
earths atmosphere, and the implications of photorespiration on modern plants).
→use this space to reflect on topic 4
→textbook chapters/pages to review

Explanation:

Rubisco evolved in an oxygen - free environment. Early on, without oxygen, photorespiration (which occurs when Rubisco binds to \(O_2\)) was not a problem. However, modern plants face issues with photorespiration as it reduces the efficiency of carbon fixation (since some \(O_2\) binding instead of \(CO_2\) leads to a less productive pathway). But it's not just a "metabolic relic" because Rubisco still has a crucial role in the Calvin cycle (binding to \(CO_2\) for carbon fixation). Also, there may be some as - yet - unknown benefits or it could be a trait that is difficult to eliminate evolutionarily due to the complexity of changing the enzyme's properties.

Answer:

Reject the claim. While it's true that early atmospheric conditions (no oxygen) meant photorespiration (due to Rubisco binding \(O_2\)) was not a problem initially (as Rubisco evolved in an \(O_2\) - free environment), in modern plants, although it reduces the efficiency of carbon fixation (a negative aspect), Rubisco is still essential for the Calvin cycle (carbon fixation when it binds \(CO_2\)). Also, it's an oversimplification to call it just a "metabolic relic" as there could be unrecognized evolutionary constraints or minor benefits (or it's a trait that is hard to eliminate completely due to the enzyme's complex structure and function).