Photosynthesis Photorespiration and C4/CAM Pathways 2 — Questions and Answers
Question 1: In C4 plants, which molecule(s) serve as the CO2 carrier transporting fixed carbon from mesophyll cells to bundle sheath cells?
- Pyruvate
- Oxaloacetate
- Malate or aspartate (Correct answer)
- Phosphoenolpyruvate (PEP)
Correct answer: Malate or aspartate
Oxaloacetate is quickly converted to malate or aspartate in mesophyll cells; these 4-carbon acids are transported to bundle sheath cells where they are decarboxylated to release CO2 for RuBisCO.
Question 2: CAM plants open their stomata primarily during which part of the day to minimize water loss?
- Midday
- Early morning only
- Night (Correct answer)
- Late afternoon
Correct answer: Night
CAM plants open stomata at night when temperatures are cooler and humidity is higher, allowing CO2 uptake with far less water loss than if stomata were open during the hot daytime.
Question 3: In CAM plants, CO2 collected at night is stored in vacuoles as which organic acid?
- Malic acid (malate) (Correct answer)
- Oxaloacetate
- Citric acid
- Aspartic acid
Correct answer: Malic acid (malate)
CO2 fixed by PEP carboxylase at night forms oxaloacetate, which is rapidly reduced to malate; malate accumulates in vacuoles overnight and is released during the day for the Calvin cycle.
Question 4: Which of the following plants is a classic example of CAM photosynthesis?
- Corn (Zea mays)
- Sugarcane (Saccharum officinarum)
- Prickly pear cactus (Opuntia) (Correct answer)
- Spinach (Spinacia oleracea)
Correct answer: Prickly pear cactus (Opuntia)
Cacti like Opuntia are classic CAM plants adapted to desert environments; they fix CO2 at night and keep stomata closed during hot days to conserve water.
Question 5: What is the immediate substrate for PEP carboxylase in C4 and CAM plants?
- RuBP (ribulose-1,5-bisphosphate)
- PEP (phosphoenolpyruvate) (Correct answer)
- 3-phosphoglycerate (3-PGA)
- NADPH
Correct answer: PEP (phosphoenolpyruvate)
PEP carboxylase catalyzes the carboxylation of phosphoenolpyruvate (PEP) using CO2 (as HCO3−) to produce the 4-carbon compound oxaloacetate.
Question 6: During the daytime in CAM plants, accumulated malate is decarboxylated to release CO2, which then enters which pathway?
- Glycolysis
- The Calvin cycle (light-independent reactions) (Correct answer)
- The light reactions
- Photorespiration
Correct answer: The Calvin cycle (light-independent reactions)
Released CO2 is fixed by RuBisCO in the Calvin cycle during the day while stomata remain closed, using light energy harvested by the light reactions.
Question 7: Compared to RuBisCO, PEP carboxylase has which key advantage in carbon fixation?
- It requires less ATP per carboxylation event
- It has a much higher affinity for CO2 and has no oxygenase activity (Correct answer)
- It produces NADPH directly during the carboxylation reaction
- It functions exclusively in bundle sheath cells
Correct answer: It has a much higher affinity for CO2 and has no oxygenase activity
PEP carboxylase does not react with O2 and has a higher affinity for CO2 (binds HCO3−), making it far more efficient at capturing CO2 even at low concentrations without triggering photorespiration.
Photosynthesis Photorespiration and C4/CAM Pathways 2
In C4 plants, which molecule(s) serve as the CO2 carrier transporting fixed carbon from mesophyll cells to bundle sheath cells?