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.