Introduction to Photosynthesis

What is Photosynthesis?

• Photo: Light
• Synthesis: To build
• Uses energy from light to build carbohydrates (e.g., glucose)

Net Equation for Photosynthesis

• Reactants: 6 H₂O + 6 CO₂
• Products: C₆H₁₂O₆ (glucose) + 6 O₂

Molecule Entry and Exit in Plants

• Water enters through roots
• Carbon dioxide enters through stomata (leaves)
• Oxygen exits through stomata

Chloroplasts and Cellular Respiration

• Chloroplasts: Organelle for photosynthesis
• Mitochondria: Organelle for cellular respiration (opposite of photosynthesis)
• Photosynthesis: Converts CO₂ + H₂O → Glucose + O₂
• Cellular Respiration: Converts Glucose + O₂ → CO₂ + H₂O, releasing energy

Chlorophyll

• Pigment responsible for absorbing light
• Found in thylakoids within chloroplasts
• Granum: One stack of thylakoids
• Grana: Multiple stacks of thylakoids
• Lumen: Fluid inside thylakoid
• Stroma: Fluid inside chloroplast
• Absorbs blue and red light, reflects green light

Stages of Photosynthesis

Light Dependent Reactions

• Occur in thylakoids
• Reactants: H₂O, NADP⁺, ADP, Pi
• Products: O₂, ATP, NADPH
• Process: Light energy oxidizes H₂O to O₂ → Electrons transferred to NADP⁺ to form NADPH → ATP produced by chemiosmosis via ATP synthase

Light Independent Reactions (Calvin Cycle)

• Occur in stroma (also known as dark reactions)
• Reactants: CO₂, ATP, NADPH
• Products: Glucose, NADP⁺, ADP, Pi
• Process: CO₂ reduced to sugars (e.g., glucose) → ATP and NADPH provide energy and electrons → NADPH oxidized back to NADP⁺, ATP converted to ADP and Pi

Detailed Steps of Photosynthesis

Electron Transport Chain in Light Dependent Reactions

1. Photosystem II (P680): Absorbs light (680 nm) → Excites electrons in chlorophyll → Electrons passed to plastoquinone, replenished from H₂O (produces O₂ and H⁺ ions)
2. Cytochrome b6f Complex: Electrons transported, pumps protons creating gradient
3. Plastocyanin: Transports electrons to Photosystem I (P700)
4. Photosystem I (P700): Absorbs light (700 nm) → Excites electrons → Electrons passed to ferrodoxin
5. NADP Reductase: Reduces NADP⁺ to NADPH → Reduces H⁺ in stroma, aiding ATP production
6. ATP Synthase: Utilizes proton gradient to synthesize ATP from ADP and Pi via chemiosmosis

Calvin Cycle (Light Independent Reactions)

1. Carbon Fixation: CO₂ reacts with Ribulose 1,5-bisphosphate (RuBP) via enzyme Rubisco → Produces 3-phosphoglycerate (PGA)
2. Reduction: PGA converted to 1,3-bisphosphoglycerate using ATP → Reduced to G3P via NADPH
3. Regeneration: G3P used to regenerate RuBP → Requires ATP
4. Summary: 3 CO₂ → 1 G3P (requires 9 ATP and 6 NADPH). For 1 glucose (needs 2 G3P): 6 CO₂, 18 ATP, 12 NADPH

Final Summary

• Photosynthesis: Light Dependent + Light Independent Reactions
• Light Dependent: In thylakoid membrane, produces O₂, ATP, NADPH
• Calvin Cycle: In stroma, produces glucose