Photorespiration and the C3 vs C4 Divide
RuBisCO is the most abundant enzyme in the world, yet its active site cannot tell carbon dioxide from oxygen. When oxygen wins the competition, the plant enters photorespiration — a wasteful detour that consumes ATP, releases already-fixed CO2, and yields neither sugar nor ATP.
This quiz tests the first step of the Calvin pathway, the competitive binding at RuBisCO’s active site, the products of the oxygenase reaction, and the elegant C4 strategy that suppresses photorespiration altogether.
- Recall the carboxylation and oxygenation reactions of RuBP
- Reason about competitive binding and relative gas concentrations
- Compare C3 and C4 plants using the logic of Table 11.1
Difficulty: Medium. Answer all 15 questions, then review each explanation carefully.
Q1. In the first ##\ce{CO2}## fixation step of the Calvin pathway, ##\ce{RuBP}## combines with ##\ce{CO2}## to form which product, and which enzyme catalyses the reaction?
The first step of the Calvin pathway is the reaction in which ##\ce{RuBP}## combines with ##\ce{CO2}## to form 2 molecules of 3PGA, and this reaction is catalysed by RuBisCO.
Q2. Which statement best describes the active site of RuBisCO?
RuBisCO is characterised by the fact that its active site can bind to both ##\ce{CO2}## and ##\ce{O2}## — hence the name ribulose bisphosphate carboxylase-oxygenase.
Q3. Which enzyme is described in the lesson as the most abundant enzyme in the world?
The lesson states that RuBisCO is the most abundant enzyme in the world.
Q4. When the concentration of ##\ce{CO2}## and ##\ce{O2}## are nearly equal, how does RuBisCO behave towards the two gases?
RuBisCO has a much greater affinity for ##\ce{CO2}## when the ##\ce{CO2}##:##\ce{O2}## ratio is nearly equal.
Q5. What ultimately determines whether ##\ce{CO2}## or ##\ce{O2}## binds to the active site of RuBisCO?
The binding is competitive, and it is the relative concentration of ##\ce{O2}## and ##\ce{CO2}## that determines which of the two will bind to the enzyme.
Q6. In C3 plants, what happens when some ##\ce{O2}## binds to RuBisCO?
In C3 plants some ##\ce{O2}## does bind to RuBisCO, and hence ##\ce{CO2}## fixation is decreased.
Q7. In photorespiration, ##\ce{RuBP}## binds with ##\ce{O2}## to form which two products?
Instead of being converted to 2 molecules of PGA, ##\ce{RuBP}## binds with ##\ce{O2}## to form one molecule of phosphoglycerate and phosphoglycolate (2 carbon).
Q8. Which statement about the photorespiratory pathway is correct?
In the photorespiratory pathway there is neither synthesis of sugars nor of ATP, and there is no synthesis of ATP or NADPH.
Q9. What is the overall carbon and energy outcome of photorespiration?
Photorespiration results in the release of ##\ce{CO2}## with the utilisation of ATP — the plant loses carbon and spends energy.
Q10. What does the lesson say about the biological function of photorespiration?
The lesson explicitly states that the biological function of photorespiration is not known yet.
Q11. Why does photorespiration not occur in C4 plants?
In C4 plants photorespiration does not occur because they have a mechanism that increases the concentration of ##\ce{CO2}## at the enzyme site.
Q12. In C4 plants, how is the ##\ce{CO2}## concentration at the enzyme site raised?
When the C4 acid from the mesophyll is broken down in the bundle sheath cells, ##\ce{CO2}## is released, increasing the intracellular concentration of ##\ce{CO2}##.
Q13. What is the consequence of the raised ##\ce{CO2}## level in the bundle sheath cells of C4 plants?
The raised ##\ce{CO2}## concentration ensures that RuBisCO functions as a carboxylase, minimising the oxygenase activity.
Q14. According to the lesson, why are productivity and yields better in C4 plants?
Since C4 plants lack photorespiration, productivity and yields are better in them; in addition, these plants show tolerance to higher temperatures.
Q15. In which cells does the Calvin pathway take place in C3 plants and in C4 plants respectively?
The Calvin pathway occurs in all the mesophyll cells of C3 plants, but in C4 plants it does not take place in the mesophyll cells and occurs only in the bundle sheath cells.