Photosynthesis: An Introduction

Definition

• Photosynthesis: Process using light energy to build carbohydrates.
  • "Photo" = Light
  • "Synthesis" = To build

Photosynthesis Equation

• Net Equation: 6 H₂O + 6 CO₂ + Light → C₆H₁₂O₆ (Glucose) + 6 O₂
  • Reactants: Water, Carbon Dioxide
  • Products: Glucose, Oxygen

Process Overview

• Water enters plant through roots.
• Carbon Dioxide enters through stomata in leaves.
• Oxygen exits through stomata.

Chloroplast vs. Mitochondria

• Chloroplast: Organelle for photosynthesis.
• Mitochondria: Organelle for cellular respiration.

Photosynthesis vs. Cellular Respiration

• Photosynthesis: CO₂ + H₂O + Light → Glucose + O₂
• Cellular Respiration: Glucose + O₂ → CO₂ + H₂O + Energy

Chlorophyll and Light Absorption

• Chlorophyll: Pigment absorbing light energy.
  • Found in thylakoids in chloroplasts.
  • Absorbs blue and red light, reflects green.

Structure of Chloroplast

• Thylakoids: Location of light-dependent reactions.
• Stroma: Site of light-independent reactions (Calvin Cycle).
• Grana: Stacks of thylakoids.

Light-Dependent Reactions

• Occur within thylakoids.
• Products: O₂, ATP, NADPH
• Reactants: H₂O, NADP⁺, ADP, Phosphate
  • Water is oxidized to oxygen.
  • ATP Synthase: Enzyme producing ATP via chemiosmosis.
  • Photosystem II and Photosystem I: Capture light energy, excite electrons.

Light-Independent Reactions (Calvin Cycle)

• Occur in stroma of chloroplasts.
• Steps:
  1. Carbon Fixation: CO₂ reacts with RuBP (Ribulose Biphosphate).
  2. Reduction: 3-Phosphoglycerate reduced to G3P (Glyceraldehyde 3-phosphate).
  3. Regeneration of RuBP
• Reactants: CO₂, ATP, NADPH
• Products: Sugars (e.g., Glucose), NADP⁺, ADP, Phosphate

Electron Transport Chain

• In thylakoid membrane.
• Photosystem II: Absorbs light, oxidizes water.
• Plastoquinone: Transfers electrons to cytochrome b6f.
• Cytochrome b6f: Pumps protons, creating gradient.
• Photosystem I: Absorbs light, re-energizes electrons.
• Ferrodoxin: Transfers electrons to NADP⁺ reductase.
• NADP⁺ Reductase: Reduces NADP⁺ to NADPH.

Calvin Cycle Summary

• 3 CO₂ → 1 G3P (requires 9 ATP, 6 NADPH)
• To make 1 glucose, 6 CO₂ → 2 G3P (requires 18 ATP, 12 NADPH)

Key Points to Remember

• Photosystem II: Converts water to oxygen.
• Calvin Cycle: Converts CO₂ to glucose.
• ATP & NADPH: Essential energy carriers in the process.

• This lecture provides a comprehensive overview of the process of photosynthesis, detailing each step and the roles of different molecules and organelles involved in the light-dependent and light-independent reactions.