Lec 8 notes photosynthesis

Biology 1009: Photosynthesis Lecture Notes

Learning Objectives

• Understand how light energy is converted to chemical energy.

• Identify forms of electromagnetic radiation.

• Learn how photosynthetic pigments capture light energy through:

  • Photosystem II

  • Photosystem I

• Comprehend energy flow in the production of ATP and NADPH.

• Explore CO2 fixation and reduction for sugar formation through the Calvin cycle.

• Recognize different forms of CO2 fixation and the phenomenon of photorespiration.

Key Terms

• Chloroplast: Organelle where photosynthesis occurs.

• Bundle-sheath cells: Cells surrounding vascular tissue in plants.

• Photophosphorylation: Process of converting light energy into chemical energy.

• Stroma: Fluid-filled space in chloroplasts where the Calvin cycle occurs.

• Stomata: Pores on leaf surfaces that allow gas exchange.

• Photosynthetic pigments: Substances that absorb light; include chlorophyll.

• Thylakoids: Membrane-bound structures where light reactions occur.

• Light reactions: Initial phase of photosynthesis where light energy is absorbed.

• Visible light: Part of the electromagnetic spectrum that is visible to the human eye.

• Carbon fixation: Process of converting inorganic CO2 into organic molecules.

• Electromagnetic spectrum: Range of all types of EM radiation, including visible light.

• Photons: Light particles that carry energy.

• Cyclic and noncyclic electron flow: Pathways for electron transport in photosynthesis.

Overall Reaction of Photosynthesis

• For every 6 CO2 and 6 H2O combined:

  • Produces C6H12O6 + 6O2 (glucose + oxygen) using energy from sunlight.

Photosynthesis Process

2-Step Process

• Light Reactions:

  • Chlorophyll converts sunlight into chemical energy (NADPH and ATP).

• Dark Reactions (Calvin cycle):

  • Utilizes energy from light reactions to reduce CO2 to sugar.

Light Reactions Explained

1. Light Absorption: Involves Photosystem II (PSII) and Photosystem I (PSI).

   • PSI: P700

   • PSII: P680

2. Electron Transport:

   • Electrons reduce NADP+ to form NADPH.

   • Establishes a proton (H+) gradient across the thylakoid membrane.

3. Chemiosmotic ATP Synthesis:

   • Proton gradient drives ATP synthesis via ATP synthase (Photophosphorylation).

Paths for Electrons

• Noncyclic Electron Flow:

  • Involves both PSI and PSII.

  • Produces NADPH and ATP; oxygen is released as a by-product.

• Cyclic Electron Flow:

  • Utilizes only PSI.

  • ATP produced but no NADPH or oxygen.

Dark Reactions / Calvin Cycle

1. Carbon Fixation:

   • CO2 reacts with ribulose bisphosphate (RuBP).

2. Reduction Phase:

   • CO2 is converted to a 3-carbon sugar.

3. Regeneration of RuBP:

   • Ensures the continuity of the cycle.

Photorespiration

• Occurs under hot, arid conditions when stomata close, leading to low CO2.

• O2 is incorporated into Calvin cycle instead, resulting in:

  • No ATP generation and no sugar production.

Alternative Mechanisms in Hot Climates

• C4 Plants:

  • Carbon fixation occurs in distinct cells (mesophyll and bundle-sheath).

• CAM Plants (Crassulacean Acid Metabolism):

  • Stomata open at night to minimize water loss and fix CO2.

  • Calvin cycle occurs during the day.

Summary

• Photosynthesis involves complex interactions between light absorption, electron transport, and chemical energy production.

• Understanding both light and dark reactions is essential for grasping how plants capture and utilize energy.