Photosynthesis in C3, C4, and CAM Plants

Photosynthesis in C3, C4, and CAM Plants

• Photosynthesis is a crucial process for plants, converting carbon dioxide and water into glucose using sunlight. Different plant types have evolved various strategies for this process, adapting to varied environmental conditions.

The Calvin Cycle

• The Calvin cycle is a series of reactions that convert carbon dioxide into glucose.
• It includes three phases:
  • Carbon Fixation: CO₂ is combined with RuBP (ribulose bisphosphate) using the enzyme rubisco to form 3-PGA.
  • Reduction: ATP and NADPH are used to convert 3-PGA into G3P (glyceraldehyde 3-phosphate), a precursor to glucose.
  • Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.
• The reactants are CO2, ATP, and NADPH; the products are glucose.

Stomata and Guard Cells

• Stomata are pores on the underside of leaves.
• They allow for gas exchange, including the uptake of CO₂ for photosynthesis and the release of O₂ as a byproduct.
• Guard cells control the opening and closing of stomata, regulating water loss.
• Stomata closing conserves water during hot, dry conditions, but reduces CO₂ uptake, thereby reducing photosynthetic activity.

Photorespiration

• Photorespiration is a process that occurs when rubisco uses O₂ instead of CO₂ in the Calvin cycle. This can occur when CO₂ levels are low and O₂ levels are high (example: hot, dry weather).
• Photorespiration reduces photosynthetic efficiency by 30%.
• Oxygen competes with carbon dioxide for the rubisco active site. This affects the first step of the Calvin cycle negatively.

Competitive Inhibition of Rubisco

• At high temperatures, oxygen competes with CO2, reducing the efficiency of the Calvin cycle.
• Higher O2:CO2 ratio leads to photorespiration, reducing photosynthetic output.

Stages of Photorespiration

• Oxygen is fixed instead of carbon dioxide.
• An intermediate molecule, phosphoglycolate, forms.
• Phosphoglycolate is metabolized to release CO2.
• The net effect is reduced glucose production and loss of fixed carbon atoms.

C4 plants

• C₄ plants have evolved adaptations to reduce photorespiration.
• They concentrate CO₂ in bundle sheath cells.
• A different enzyme (PEP carboxylase) initially fixes CO₂ to form a four-carbon molecule, oxaloacetate. This molecule is then transported to the bundle sheath cells, where CO₂ is released and enters the Calvin cycle.
• C₄ plants thrive under higher temperatures and higher light intensities.

C₄ Pathway

• The four-carbon molecules and the two-carbon molecule are transported between cells.
• C₄ photosynthesis has an energy cost, compared to C₃ photosynthesis.

CAM plants

• CAM (Crassulacean Acid Metabolism) plants also minimize photorespiration.
• These plants open their stomata at night to absorb CO₂ and store it as an organic acid.
• During the day, the stomata close to prevent water loss, and the stored CO₂ is released to drive the Calvin cycle.
• CAM plants are common in arid environments.

Comparison of C3, C4, and CAM Photosynthesis

• These different photosynthetic pathways represent adaptations to different environments.
• C3 plants are most efficient in moderate temperatures and conditions.
• C4 plants are most efficient in high-light and high-temperature conditions.
• CAM plants are most efficient in arid conditions where water scarcity is limiting.