Lecture 2.6 - Photosynthesis

Learning Objectives

• Explain why photosynthesis is crucial for life on Earth.
• Describe how photosynthetic organisms harvest energy from light.
• Detail the stages of photosynthesis.
• Clarify the process and significance of photorespiration, along with adaptations allowing plants to thrive in arid climates.

Introduction to Photosynthesis

• Life relies heavily on solar energy.
• Chloroplasts are organelles in plants that capture sunlight and convert it into chemical energy through photosynthesis.

Importance of Photosynthesis

• Photosynthesis is fundamental because it provides energy for nearly all living organisms.
  • Autotrophs: Organisms that produce their own food from inorganic molecules (e.g., CO2).
    • Photoautotrophs: Use sunlight as an energy source (e.g., plants).
  • Heterotrophs: Cannot synthesize their own food and rely on other organisms for nutrients (e.g., animals).

Chloroplast Structure

• Chloroplasts contain:
  • Two membranes
  • Their own DNA and ribosomes
  • Thylakoid stacks: Where chlorophyll, the pigment essential for absorbing sunlight, is located.
• Endosymbiotic theory: Proposes that chloroplasts originated from photosynthetic prokaryotes that entered ancestral eukaryotic cells.

The Chemistry of Photosynthesis

• Photosynthesis reaction:
  $$ 6 CO2 + 6 H2O
  ightarrow C6H{12}O6 + 6 O2 $$
• Oxygen is produced from water (H2O) during the process, not from CO2.

Stages of Photosynthesis

• Light Reactions (occur in the thylakoid membranes):

  • Convert solar energy into chemical energy (ATP and NADPH).
  • Involves the absorption of photons by chlorophyll.

• Calvin Cycle (occurs in the stroma):

  • Uses ATP and NADPH to fix carbon from CO2, generating sugars like glyceraldehyde 3-phosphate (G3P).

Photosystems in Light Reactions

• Photosystem II (PSII):

  • Absorbs light at 680 nm (P680).
  • Uses light energy to split water, generating O2.

• Photosystem I (PSI):

  • Absorbs light at 700 nm (P700).
  • Accepts electrons from the electron transport chain, producing NADPH.

Cyclic Electron Flow

• In situations where ATP is needed more than NADPH, PSI can operate independently to produce ATP without reducing NADP+.

The Calvin Cycle

• The cycle involves:
  • 3 CO2: Used for carbon fixation.
  • 6 NADPH: Provides high-energy electrons.
  • 9 ATP: Powers the anabolic processes.
• RuBisCO: Enzyme responsible for fixing carbon dioxide; crucial for the Calvin cycle.

Photorespiration and Plant Adaptations

• Photorespiration: Occurs when RuBisCO incorporates O2 instead of CO2, leading to a loss of carbon and wasteful processes.
  • Can waste up to 50% of carbon fixed during the Calvin cycle.
• Stomata: Open for gas exchange but can lead to water loss, especially in hot conditions:
  • Result: O2 accumulates, leading to increased photorespiration in plants like C3 plants (e.g., rice, wheat).

Plant Adaptations to Reduce Photorespiration

1. C4 Plants:
   • Fix CO2 into a 4-carbon compound in mesophyll cells before sending it to bundle sheath cells for Calvin cycle processing (e.g., corn, sugarcane).
2. CAM Plants:
   • Take in CO2 at night, store it, and use it during the day, conserving water (e.g., succulents).

Conclusion

• Photosynthesis converts solar energy into chemical energy, supporting life on Earth.
• The light reactions and the Calvin cycle work together to produce glucose and oxygen while adapting to environmental conditions to minimize photorespiration.