Photosynthesis Lecture Notes

Introduction to Photosynthesis

• Photosynthesis is the process by which plants make organic compounds, especially sugars, using light.
• This process is crucial for life on Earth since organic compounds are the building blocks for us and our food.

How Photosynthesis Works

• Key Inputs: Carbon dioxide and water.
• Key Outputs: Glucose and oxygen gas.
• Photosynthesis consists of two sets of reactions:
  1. Light-dependent reactions
  2. Calvin Cycle

Light-Dependent Reactions

• Depend on light and occur in the chloroplasts of plant cells.
• Chloroplast Structure:
  • Contains thylakoids, which are disk-like structures with pigments (e.g., chlorophyll).
  • Thylakoids function like solar panels, absorbing light energy.

Key Processes in Light-Dependent Reactions

1. Photosystem II:

   • Absorbs light, exciting electrons in chlorophyll.
   • Excited electrons leave chlorophyll and transfer into an electron transport chain.
   • To replace lost electrons, water is split, producing:
     • Electrons
     • Hydrogen ions
     • Oxygen (this is the source of oxygen released by plants).

2. Electron Transport Chain:

   • Transports excited electrons and uses their energy to pump hydrogen ions across the thylakoid membrane, creating a concentration gradient.
   • A high concentration of hydrogen ions builds up inside the thylakoid.

3. ATP Production:

   • Hydrogen ions diffuse back across the membrane through ATP synthase, causing it to spin and produce ATP (similar to a hydroelectric dam).
   • ATP is a key product of the light-dependent reactions.

4. Photosystem I:

   • The electrons from the first chain are excited again by light.
   • Electrons then travel down a second electron transport chain and are accepted by NADP+, forming NADPH.
   • NADPH acts as an electron carrier and is crucial for the next stage.

Importance of ATP and NADPH

• Both ATP and NADPH are essential for the Calvin Cycle, where sugars are synthesized from the products of the light-dependent reactions.

Conclusion

• The light-dependent reactions are vital for providing the energy (ATP and NADPH) needed for the Calvin Cycle.
• For interactive learning, the Photosynthesis Interactive at BioMan Biology is recommended.
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