BioInteractive Photorespiration and Alternative Carbon Fixation 2 — Questions and Answers
Question 1: What is CAM photosynthesis?
- A strategy where CO2 is fixed at night and stored as organic acids, then released for the Calvin cycle during the day (Correct answer)
- Photosynthesis that occurs only in cold climates
- A form of cyclic electron flow
- Carbon fixation that bypasses RuBisCO
Correct answer: A strategy where CO2 is fixed at night and stored as organic acids, then released for the Calvin cycle during the day
CAM (Crassulacean Acid Metabolism) plants open stomata at night to fix CO2 into malate, then close stomata during the day and release CO2 for the Calvin cycle.
Question 2: Which plants use CAM photosynthesis?
- Cacti and succulents (Correct answer)
- Corn and sugarcane
- Wheat and rice
- Mosses and liverworts
Correct answer: Cacti and succulents
CAM photosynthesis is common in succulent plants like cacti, agaves, and pineapples that live in arid environments.
Question 3: What is the key advantage of CAM over C3 photosynthesis in desert environments?
- Stomata open only at night, dramatically reducing water loss (Correct answer)
- CAM plants fix more CO2 per day
- CAM plants grow faster
- CAM plants require less sunlight
Correct answer: Stomata open only at night, dramatically reducing water loss
By opening stomata at night when temperatures are cooler and humidity is higher, CAM plants lose far less water through transpiration than C3 plants.
Question 4: In C3 plants, what is the first stable product of carbon fixation?
- 3-phosphoglycerate (3-PGA) (Correct answer)
- Glucose
- Oxaloacetate
- Pyruvate
Correct answer: 3-phosphoglycerate (3-PGA)
In C3 plants, RuBisCO fixes CO2 onto RuBP to produce two molecules of 3-phosphoglycerate, a 3-carbon compound.
Question 5: Why is RuBisCO considered an inefficient enzyme?
- It has a slow catalytic rate, a dual affinity for both CO2 and O2, and is easily inhibited (Correct answer)
- It cannot function above 20°C
- It only works in bundle sheath cells
- It requires light to function
Correct answer: It has a slow catalytic rate, a dual affinity for both CO2 and O2, and is easily inhibited
RuBisCO is slow (3–10 reactions/second), can react with O2 (causing photorespiration), and requires large amounts of protein to compensate, making it metabolically expensive.
Question 6: How does elevated atmospheric CO2 affect photorespiration in C3 plants?
- It suppresses photorespiration by increasing CO2/O2 ratio at RuBisCO's active site (Correct answer)
- It increases photorespiration
- It has no effect on photorespiration
- It converts C3 plants to C4 plants
Correct answer: It suppresses photorespiration by increasing CO2/O2 ratio at RuBisCO's active site
Higher CO2 levels increase the likelihood that RuBisCO binds CO2 rather than O2, thereby reducing the rate of photorespiration.
BioInteractive Photorespiration and Alternative Carbon Fixation 2
What is CAM photosynthesis?