Photosynthesis Carbon Fixation 2

Question 1

Which enzyme catalyzes the carboxylation step in the Calvin cycle where CO₂ is added to RuBP?

Accepted Answer

RuBisCO

Answer

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the addition of CO₂ to RuBP, producing two molecules of 3-phosphoglycerate.

Question 2

How many carbon atoms does ribulose-1,5-bisphosphate (RuBP) contain?

Accepted Answer

5

Answer

RuBP is a 5-carbon compound that accepts one CO₂ during carbon fixation, briefly forming an unstable 6-carbon intermediate.

Question 3

In C4 photosynthesis, carbon is initially fixed by PEP carboxylase into which 4-carbon compound?

Accepted Answer

Oxaloacetate

Answer

PEP carboxylase combines CO₂ with phosphoenolpyruvate (PEP) to form oxaloacetate (OAA), the first stable 4-carbon product in C4 plants.

Question 4

What is the primary advantage of C4 carbon fixation over C3 fixation in hot, dry environments?

Accepted Answer

It concentrates CO₂ around RuBisCO, reducing photorespiration

Answer

C4 plants use a CO₂-concentrating mechanism that delivers high CO₂ to bundle sheath cells, suppressing RuBisCO's oxygenase activity and minimizing wasteful photorespiration.

Question 5

In CAM photosynthesis, when does carbon fixation by PEP carboxylase primarily occur?

Accepted Answer

At night when stomata are open

Answer

CAM plants open their stomata at night to fix CO₂ via PEP carboxylase into malic acid, which is stored in the vacuole until daytime use.

Photosynthesis Practice Test

Photosynthesis Practice Test

Photosynthesis Carbon Fixation 2