Introduction to Photosynthesis

Definition

• Photosynthesis: Process using light energy to build carbohydrates.
• "Photo" means light, "synthesis" means to build.

Net Equation

• Combines 6 water molecules and 6 carbon dioxide molecules using light to produce glucose (C6H12O6) and oxygen gas.
  • Reactants: Water and carbon dioxide.
  • Products: Glucose and oxygen gas.

Process Overview

• Water enters through roots; carbon dioxide enters through stomata.
• Oxygen exits through stomata.

Chloroplast

• Organelle where photosynthesis occurs.
• Mitochondria: Organelle for cellular respiration (opposite of photosynthesis).

Chlorophyll

• Pigment responsible for absorbing light energy.
• Found in thylakoids (one stack is a granum, multiple stacks are grana).
  • Lumen: Fluid inside thylakoid.
  • Stroma: Fluid inside chloroplast.

Light Absorption

• Chlorophyll absorbs blue and red light, reflects green light.

Stages of Photosynthesis

Light-Dependent Reactions

• Occur in the thylakoids.
• Oxidizes water into oxygen gas.
• Produces ATP and NADPH.
  • Reactants: Water, NADP+, ADP, and phosphate.
  • Products: Oxygen gas, ATP, and NADPH.

Electron Transport Chain

1. Light strikes Photosystem II, exciting electrons in chlorophyll.
2. Electrons pass through mobile carrier plastoquinone.
3. Chlorophyll replenished by electrons from water (produces oxygen).
4. Electrons carried to Cytochrome b6f complex, pumping protons into lumen.
5. Electrons transfer to peripheral protein plastocyanin.
6. Light re-excites electrons in Photosystem I.
7. Electrons transfer to ferrodoxin, then to NADP+ reductase.
8. NADP+ accepts electrons and becomes NADPH.
9. High proton concentration in lumen drives ATP synthesis via chemiosmosis through ATP synthase.

Light-Independent Reactions (Calvin Cycle)

• Occur in the stroma.
• Reduces carbon dioxide to glucose.
  • Reactants: Carbon dioxide, ATP, NADPH.
  • Products: Glucose, NADP+, ADP, and phosphate.

Calvin Cycle Steps

1. Carbon Fixation: CO2 reacts with ribulose biphosphate (RuBP), catalyzed by rubisco, forming 3-phosphoglycerate (PGA).
2. Reduction Phase: ATP and NADPH used to convert PGA to glyceraldehyde 3-phosphate (G3P).
   • Kinase enzyme transfers phosphate to molecules.
   • Dehydrogenase enzyme reduces 1,3-bisphosphoglycerate to G3P.
3. Regeneration of RuBP: Uses remaining G3P to regenerate RuBP, requiring ATP.

Calvin Cycle Summary

• Converts 3 CO2 into 1 G3P.
• Requires 9 ATP and 6 NADPH for 1 G3P.
• To form glucose, 18 ATP and 12 NADPH are needed for 2 G3P (equivalent to 6 CO2).

Key Points

• Light-dependent reactions occur in thylakoid membranes.
• Calvin cycle occurs in the stroma.
• Essential understanding of electron transport chain and Calvin cycle processes for photosynthesis.

End of summary.