BioInteractive Photorespiration and Alternative Carbon Fixation

Question 1

What is photorespiration?

Accepted Answer

A process where RuBisCO fixes O2 instead of CO2, releasing CO2 and reducing photosynthetic efficiency

Answer

Photorespiration occurs when RuBisCO oxygenates RuBP instead of carboxylating it, wasting energy and releasing CO2 without fixing carbon.

Question 2

Under what conditions is photorespiration most likely to occur?

Accepted Answer

High O2, low CO2, and high temperature

Answer

Photorespiration is favored when O2 concentrations are high relative to CO2 and temperatures are elevated, causing RuBisCO to act as an oxygenase.

Question 3

How do C4 plants reduce photorespiration?

Accepted Answer

They concentrate CO2 around RuBisCO in bundle sheath cells using a CO2 pump

Answer

C4 plants initially fix CO2 into 4-carbon acids in mesophyll cells and transport them to bundle sheath cells, where CO2 is released in high concentrations for RuBisCO.

Question 4

What enzyme initially fixes CO2 in C4 plants?

Accepted Answer

PEP carboxylase

Answer

PEP carboxylase (PEPC) fixes CO2 into oxaloacetate in the mesophyll cells of C4 plants; it has a much higher affinity for CO2 than RuBisCO.

Question 5

What is the 4-carbon compound first produced when CO2 is fixed in C4 plants?

Accepted Answer

Oxaloacetate

Answer

PEP carboxylase combines CO2 with phosphoenolpyruvate (PEP) to form oxaloacetate, a 4-carbon compound, in the mesophyll cells.

BioInteractive - Photosynthesis BioInteractive Test Practice Test

BioInteractive - Photosynthesis BioInteractive Test Practice Test

BioInteractive Photorespiration and Alternative Carbon Fixation