Photosynthesis Lecture Notes

Introduction to Photosynthesis

• Definition: Photosynthesis is the process of using light energy to build carbohydrates.
• Word Breakdown:
  • "Photo" means light.
  • "Synthesis" means to build something.
• Net Equation:
  • Reactants: 6H₂O (water) + 6CO₂ (carbon dioxide) + light
  • Products: C₆H₁₂O₆ (glucose) + 6O₂ (oxygen gas)

Plant Anatomy for Photosynthesis

• Water: Enters through roots.
• Carbon Dioxide: Enters through stomata in leaves.
• Oxygen: Leaves the plant through stomata.

Organelles Involved

• Chloroplasts: Organelle carrying out photosynthesis.
• Mitochondria: Organelle responsible for cellular respiration.
• Chlorophyll: Pigment responsible for absorbing light, found in thylakoids.

Chloroplast Structure

• Thylakoids: Contains chlorophyll; site of light-dependent reactions.
• Grana: Stacks of thylakoids.
• Stroma: Fluid inside chloroplasts; site of Calvin cycle.
• Membranes: Chloroplasts have inner and outer membranes.

Light Absorption

• Chlorophyll: Absorbs blue and red light, reflects green light.

Photosynthesis Stages

1. Light-dependent Reactions:

   • Occur in thylakoids.
   • Use light to oxidize water into oxygen gas.
   • Products: Oxygen gas, ATP, NADPH.
   • Reactants: Water, NADP+, ADP, phosphate.

2. Light-independent Reactions (Calvin Cycle):

   • Occur in the stroma.
   • Convert CO₂ into sugars like glucose.
   • ATP and NADPH used to power the process.
   • Products: Glucose, NADP+, ADP, phosphate.
   • Reactants: Carbon dioxide, ATP, NADPH.

Electron Transport Chain

• Photosystem II (P680): Converts water into oxygen gas.
  • Excites electrons which travel through plastoquinone to cytochrome B6F, pumping protons to create a gradient.
• Photosystem I (P700): Re-energizes electrons, transferring them to ferredoxin then to NADP reductase.
• Chemiosmosis: Uses ATP synthase to convert ADP and phosphate into ATP.

The Calvin Cycle

• Stages:
  1. Carbon Fixation: CO₂ reacts with RuBP, catalyzed by rubisco.
  2. Reduction: ATP and NADPH convert 3-phosphoglycerate to G3P.
  3. Regeneration: G3P regenerates RuBP.
• Carbon Accounting: Conversion of 3 CO₂ into 1 G3P molecule.

Summary of the Calvin Cycle

• Net Result: 3 CO₂ → 1 G3P, using 9 ATP and 6 NADPH.
• Glucose Formation: Requires 6 CO₂ for 1 glucose, needing 18 ATP and 12 NADPH.

Conclusion

• Photosynthesis consists of interconnected light-dependent and independent reactions occurring in chloroplasts.
• Essential understanding for biology and plant physiology.