Photosynthesis: AP Biology Crash Course

Key Concept

• Transformation of light energy into chemical energy, stored in carbohydrates.

Light: The Driving Force of Photosynthesis

• Photon: Particle-like behavior of light.
  • Reflection & transmission: Perception of color.
  • Absorption: Allows photosynthesis.
• Chloroplasts: Site of energy conversion.
  • Light energy -> Chemical energy.
• Electron: Energy carrier in chloroplasts.
• Light-dependent reactions: Require light energy to proceed.
• Light-independent reactions: Use energy from light-dependent reactions.

The Light-Dependent Phase

Light Absorption

• Chlorophyll: Absorbs photon energy, transferring it to chloroplast's photosystems.
• Thylakoids: Membrane-bound sacs in chloroplasts.
  • Location of light-dependent reactions.
• Excited Electrons: Transfer energy within thylakoid membranes.
• Photosystem II: Initiates photosynthesis.
  • Splits water into H+ ions, electrons, and oxygen (photolysis).
  • P680: Oxidizing agent in Photosystem II.

The Electron Transport Chain

• Transfers electrons from thylakoid membrane proteins to NADP+.
• Produces ATP and water for plant growth.
• NADPH Formation:
  • NADP+ + 2 electrons + 2 H+ -> NADPH + H+

ATP Production

• Non-cyclical photosynthesis:
  • Equation: 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light -> 2 NADPH + 2 H+ + 3 ATP + O2
• Photon Excitation: Releases electrons in Photosystem II.
• Proton Gradient: Formed across chloroplast membrane.
  • Used to produce ATP from ADP.
• Photosystem I: Further excites electrons for ATP production.

Chemiosmosis

• Protons exit thylakoid via ATP synthase.
  • Rotates ATP synthase, phosphorylating ADP to ATP.

Kinetic, Potential, and Chemical Energy

• Proton gradient created by electron transport chain.
  • Converts kinetic energy to potential energy.
• Potential Energy: Used in ATP synthesis.

Light-Independent Reactions (Calvin Cycle)

• Reduction of CO2: Uses ATP and NADPH.
• Reaction: 3 CO2 + 9 ATP + 6 NADPH + 6 H+ -> C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

The Calvin Cycle

• Fixation: RuBisCo enzyme catalyzes CO2 and RuBP reaction.
  • Forms 3-PGA.
• Reduction: Converts 3-PGA to PGAL using ATP and NADPH.
• Regeneration: PGAL exported and used to regenerate RuBP.

Conclusion

• Light energy stored in PGAL.
• Sugars from photosynthesis are broken down for metabolic energy.
• Similarities with cellular respiration noted.

Additional Resources

• Albert offers AP Biology practice questions and full-length tests.