Photorespiration and Plant Adaptation

Photorespiration and Plant Adaptation

• Photorespiration is a process where RuBisCO, an enzyme crucial for photosynthesis, mistakenly binds oxygen instead of carbon dioxide. This reduces the efficiency of photosynthesis.
• This occurs under hot and dry conditions when stomata close to conserve water.
• Oxygen builds up, and CO₂ cannot enter the leaves, leading to photorespiration, and reducing the production of glucose.
• RuBisCO competes for the same active site with both CO₂ and O₂.
• Factors such as specificity and solubility affect the enzyme's selection between CO₂ and O₂.

Photorespiration Wastage

• Photorespiration is a wasteful process, short-circuiting the Calvin cycle.
• It reverses carbon fixation.
• Two-carbon compounds are recycled instead of three-carbon compounds.
• The regeneration of RuBP consumes energy.
• This pathway cycles through three organelles: chloroplast, peroxisome, and mitochondrion.

Pathway through Organelles

• Photorespiration's process involves these three organelles.
• The byproducts are recycled.

Photorespiration Role

• In some cases, photorespiration acts protectively.
• Plants with poor photorespiration are more susceptible to light damage.
• Photorespiration neutralizes harmful byproducts from light reactions, especially when CO₂ levels are low and limit the Calvin cycle.

Plant Adaptations

• Different plants have evolved various strategies to minimize photorespiration:
  • C₃ plants (rice, wheat, barley, etc) exhibit photorespiration.
  • C₄ plants (corn, sugarcane) employ a spatial separation of CO₂ and O₂ fixation within the leaf.
  • CAM plants (pineapple, cacti) use temporal separation of CO₂ fixation to mitigate photorespiration.

C₄ Plants

• CO₂ is initially fixed in mesophyll cells using PEP carboxylase.
• 4-carbon compounds are transported to bundle-sheath cells.
• CO₂ is released in bundle-sheath cells, where RuBisCO functions in the Calvin cycle.
• This effectively concentrates CO₂ around RuBisCO, minimizing photorespiration.

CAM Plants

• CAM plants open their stomata at night to absorb CO₂.
• CO₂ is converted to organic acids and stored in vacuoles.
• During the day, stomata close, and CO₂ is released from organic acids for use in the Calvin cycle.
• This temporal separation minimizes water loss by keeping stomata closed during the day.

Comparing C₃, C₄, and CAM plants

• Photosynthesis mechanisms vary.
• Carbon dioxide acceptance, reaction location, and optimum temperature for reactions differ.
• Each strategy optimizes photosynthesis and water usage in specific environments.