Photosynthesis Photorespiration and C4/CAM Pathways

Question 1

Which enzyme is responsible for both carbon fixation in the Calvin cycle and the oxygenase reaction that initiates photorespiration?

Accepted Answer

RuBisCO

Answer

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) can bind either CO2 or O2, and when it binds O2 it triggers the photorespiration pathway.

Question 2

During photorespiration, RuBisCO catalyzes the addition of which molecule to RuBP instead of CO2?

Accepted Answer

Oxygen (O2)

Answer

Photorespiration occurs when RuBisCO fixes O2 to RuBP instead of CO2, an energetically wasteful reaction that releases previously fixed carbon.

Question 3

When RuBisCO catalyzes the oxygenase reaction with RuBP, what two products are formed?

Accepted Answer

One molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate

Answer

The oxygenase reaction splits the 5-carbon RuBP into one 3-carbon molecule (3-PGA) and one 2-carbon molecule (2-phosphoglycolate), the entry point of photorespiration.

Question 4

In C4 plants, what is the first stable product of carbon fixation in mesophyll cells?

Accepted Answer

Oxaloacetate (OAA)

Answer

In C4 plants, PEP carboxylase in mesophyll cells combines CO2 with PEP to form oxaloacetate, a 4-carbon compound — which is why these plants are called C4.

Question 5

Which enzyme fixes CO2 in the mesophyll cells of C4 plants?

Accepted Answer

PEP carboxylase (PEPcase)

Answer

PEP carboxylase fixes CO2 onto phosphoenolpyruvate (PEP) in mesophyll cells; unlike RuBisCO, it has no oxygenase activity and a very high affinity for CO2.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Photorespiration and C4/CAM Pathways