Alternative Mechanisms of Carbon Fixation

Q: Why is water important for plants?

A: Water is a major component of plants; As a result, plants have adaptations to reduce water loss and adapt to environmental changes

Q: What adaptations help plants reduce water loss and adapt to environmental changes?

A: Cuticle and Stomata.

Q: What is the cuticle?

A: A thin, waterproof layer covering the external plant that prevents rapid loss of water.

Q: What are stomata?

A: Pores that contain guard cells controlling gas exchange with the environment.

Q: Where are stomata found?

A: On the surface of each leaf, controlled by a surrounding pair of guard cells.

Q: What happens when guard cells are open?

A:

• Open during the day (swollen)

• Allow CO₂ to enter so photosynthesis can occur

• Some water is lost

Q: What happens when guard cells are closed?

A:

• Closed during the night (deflated)

• Can also close when temperatures are high or plant risks losing too much water

• Conserves water

Q: What is Rubisco?

A: An important enzyme used in the Calvin Cycle.

Q: What are the problems with Rubisco?

A:

• It’s a very slow enzyme

• Its active site can bind to O₂ instead of CO₂

• Leads to photorespiration, producing a useless product that wastes energy

Q: When concentrations of O₂ and CO₂ are equal, which does Rubisco bind to?

A: CO₂, because its active site has a greater affinity for CO₂.

Q: How often does Rubisco bind to each gas in nature?

A:

• 75% of the time → CO₂

• 25% of the time → O₂

Q: What happens when CO₂ concentration is low?

A: Photorespiration occurs at a rapid rate and can kill the plant.

Q: How do warm temperatures affect gas solubility?

A: Warm temperatures decrease the solubility of CO₂ and O₂ (more so for CO₂).

Q: What is the result of this?

A: Photorespiration is high in warm climates.

Q: How much energy is lost to photorespiration in warm climates?

A: Approximately 50% of a plant’s energy.

Q: What are C4 plants?

A: Plants with adaptations to survive in warm climates.

Q: What special cells do C4 plants have?

A: Bundle-sheath cells that undergo the Calvin Cycle and are separated from air sacs.

Q: What do these cells do?

A:

• Reduce Rubisco’s exposure to O₂

• Reduce photorespiration

Q: What do C4 plants reduce besides oxygen exposure?

A: They also reduce carbon dioxide loss.

Q: What cycle do they use for carbon fixation?

A: The C4 Cycle.

Q: What are the steps of the C4 Cycle?

1. CO₂ reacts with PEP → Oxaloacetate

2. Catalyzed by PEP carboxylase

3. Oxaloacetate reduces to malate

4. Malate enters bundle sheath cells into the chloroplast

5. Oxidized into pyruvate

6. CO₂ released → ATP production

PEP Carboxylase Q: How does PEP carboxylase differ from Rubisco?

A: It has a greater affinity for CO₂ than O₂.

Q: Does PEP carboxylase bind to CO₂ even if O₂ levels are high?

A: Yes, it always binds to CO₂ regardless of O₂ concentration.

Q: What is a disadvantage of this method?

A: It uses more energy — each turn of the C4 Cycle requires double the amount of ATP hydrolysis.

CAM Plants - Q: Where do CAM plants typically live?

A: In regions that are hot and dry during the day but cool at night (e.g., succulents).

Q: What cycles do CAM plants undergo?

A: The Calvin Cycle and C4 Cycle at different times of the day for better CO₂ fixation efficiency.

Q: When do CAM plants open their stomata?

A: At night.

Q: What happens when stomata open at night?

A:

• CO₂ enters and O₂ leaves

• CO₂ is fixed by the C4 cycle

• Stored as malic acid in vacuoles

Q: What happens during the day?

A:

• Stomata close to reduce water loss

• Malic acid oxidizes to pyruvate

• CO₂ is released

• Favors Rubisco to bind to CO₂ → Calvin Cycle proceeds