15_Photosynthesis (1)

Photosynthesis Overview

• Definition: Process by which plants, some bacteria, and protistans convert sunlight energy into chemical energy, producing glucose and oxygen.

  • Word Equation: Carbon dioxide + Water -> Glucose + Oxygen.

Importance of Chlorophyll

• Chlorophyll Function: Main pigment involved in photosynthesis; absorbs sunlight and initiates the conversion of light energy into chemical energy.

• Chlorophyll Variants:

  • Chlorophyll a: Essential for all photosynthetic organisms.

  • Accessory Pigments: Include chlorophyll b, c, d, and e (found in algae and protistans), xanthophylls, carotenoids (like beta-carotene).

Leaves and Leaf Structure

• Function of Leaves: Act as solar collectors filled with photosynthetic cells.

  • Raw Material Entry: Water and carbon dioxide enter leaf cells.

  • Products Exit: Sugar and oxygen are released.

• Water Transport:

  • Transported through xylem vessels from the roots to leaves.

  • Land plants feature stomata for gas exchange, allowing the entry of CO2 and exit of O2 but risking water loss.

Structure of Chloroplasts

• Chloroplast Characteristics:

  • Thylakoids: Flattened sacs where photosynthesis occurs, stacked in grana.

  • Stroma: Space between grana.

  • Membrane System: Chloroplasts have three membranes, creating three compartments.

Stages of Photosynthesis

• Two Main Stages:

  • Light Dependent Reactions: Occur in grana; use light energy to produce ATP and NADPH.

    • Key Processes:

      • Photophosphorylation: ATP production.

      • Photolysis: Splitting of water, releasing O2 and providing electrons.

  • Light Independent Reactions (Calvin Cycle): Occur in stroma; utilize ATP and NADPH to fix carbon dioxide into carbohydrates.

    • Key Reaction: Carbon dioxide + RuBP forms unstable six-carbon intermediate, which yields GP.

Light Dependent Reactions - Details

• Photoexcitation: Light energy absorbed by chlorophyll excites electrons, leading to photoionization.

• Electron Transfer: Electrons travel through an electron transport chain, driven by Photosystem II (PSII) and Photosystem I (PSI).

• Z Scheme: Diagrammatic representation of the process indicating energy transfer.

Non-Cyclic Phosphorylation (Z Scheme)

• Processes Involved:

  • Photoionization, electron transfer to NADP+, production of ATP and NADPH.

Chemiosmosis and ATP Synthesis

• Mechanism: H+ ions concentrated in the thylakoid compartment create an electrochemical gradient, driving ATP synthesis.

Cyclic Phosphorylation

• Function: Occurs only in PSI to produce extra ATP without forming NADPH.

Light Independent Reactions - Calvin Cycle

• Process:

  • Carbon fixation: CO2 combines with RuBP to ultimately form glucose via GP and GALP.

  • ATP and NADPH from light-dependent reactions drive the cycle.

Factors Affecting Photosynthesis

• Key Factors:

  • Light Intensity: Directly correlates with the rate of photosynthesis until limited by other factors.

  • Carbon Dioxide Concentration: Increased availability boosts photosynthesis rate.

  • Temperature: Enzyme-catalyzed reactions, rising to an optimum then decreasing if temperatures exceed this.

Conclusion

• Success of photosynthesis is contingent on environmental factors and the effective functioning of chlorophyll and associated structures.