AP Biology Questions on Photorespiration and Alternative Carbon Fixation

📚 Quick Study Guide

• ☀️ Photorespiration: 🌿 Occurs when Rubisco binds to $O_2$ instead of $CO_2$.
• 🔥 Why it Happens: 🌡️ Favored in hot, dry conditions when stomata close, leading to $O_2$ buildup and $CO_2$ depletion.
• 📉 Efficiency: 😥 Reduces photosynthetic output because it consumes ATP and releases $CO_2$ without producing sugar.
• 🌵 C4 Plants: 🌾 Spatially separate initial $CO_2$ fixation and the Calvin cycle. $CO_2$ is first fixed in mesophyll cells using PEP carboxylase, forming a 4-carbon compound (oxaloacetate).
• 🌱 CAM Plants: ⏰ Temporally separate initial $CO_2$ fixation and the Calvin cycle. Stomata open at night, allowing $CO_2$ to enter and be fixed into organic acids, which are stored until daytime.
• 🧪 PEP Carboxylase: 🔑 Has a higher affinity for $CO_2$ than Rubisco, and does not bind to $O_2$.
• 🌍 Examples: 🍍 C4 plants include corn and sugarcane; CAM plants include cacti and pineapples.

Practice Quiz

1. Which of the following conditions favors photorespiration?
   1. A. High $CO_2$ concentration and low temperature
   2. B. Low $O_2$ concentration and high humidity
   3. C. High $O_2$ concentration and high temperature
   4. D. Low $CO_2$ concentration and low temperature
2. What enzyme is responsible for the initial carbon fixation step in C4 plants?
   1. A. Rubisco
   2. B. PEP carboxylase
   3. C. Amylase
   4. D. Kinase
3. In CAM plants, when do stomata typically open to allow $CO_2$ to enter the leaves?
   1. A. During the day
   2. B. During the night
   3. C. Only when it rains
   4. D. Never
4. What is the primary disadvantage of photorespiration for plants?
   1. A. It consumes ATP and reduces photosynthetic output
   2. B. It produces excess glucose
   3. C. It increases water loss
   4. D. It enhances nitrogen fixation
5. Which of the following plants is an example of a CAM plant?
   1. A. Corn
   2. B. Sugarcane
   3. C. Cactus
   4. D. Rice
6. What is the initial carbon fixation product in C4 plants?
   1. A. Glucose
   2. B. Glyceraldehyde-3-phosphate (G3P)
   3. C. Oxaloacetate
   4. D. Ribulose-1,5-bisphosphate (RuBP)
7. Where does the Calvin cycle occur in C4 plants?
   1. A. Mesophyll cells
   2. B. Bundle sheath cells
   3. C. Epidermal cells
   4. D. Guard cells