Light Dependent Reactions (Photosynthesis) Notes

The Light Dependent Reactions Overview

• Light-dependent reactions are often referred to as light reactions because they require light energy.
• These processes primarily take place on the thylakoid membranes of chloroplasts.
• The purpose of light reactions is to convert light energy into high-energy molecules, specifically ATP and NADPH.
• The overall outcome includes the generation of ATP and NADPH used in the dark reactions, alongside oxygen (O2) as a by-product.

Key Components of the Light Reactions

• Photosystems:
  • Photosystem I (PSI) (P700): Best at absorbing light at a wavelength of 700 nm.
  • Photosystem II (PSII) (P680): Best at absorbing light at a wavelength of 680 nm.

Steps of the Light Reactions

1. Photon Absorption in Photosystem II:
   • A photon strikes PSII, exciting an electron from chlorophyll.
   • Light energy is transferred through pigments until it reaches the reaction center P680.
   • The excited electron is captured by a primary electron acceptor, resulting in the oxidation of PSII to P680+.
2. Photolysis of Water:
   • To replace the lost electron in PSII, water (H2O) is split, releasing oxygen (O2) and producing hydrogen ions (H+), occurring rapidly (up to 200 times a second).
3. Electron Transport Chain:
   • The excited electron travels through a series of REDOX reactions within the electron transport chain.
   • Key carriers: After being picked up by the primary acceptor, electron transport involves plastoquinone (PQ) and plastocyanin (PC), leading to PSI.
4. Electron Transport and Chemiosmosis:
   • As electrons pass through the chain, energy is released, pumping H+ ions into the thylakoid space, creating a proton gradient.
   • This gradient is essential for ATP synthesis via chemiosmosis (photophosphorylation).
5. Photon Absorption in Photosystem I:
   • PSI captures light energy, where the reaction center P700 excites electrons, replacing lost electrons through the electron flow from PSII.
6. NADP+ Reduction:
   • Electrons move to ferredoxin and then to NADP+ reductase, where NADP+ acts as the final electron acceptor, forming NADPH.

Chemiosmosis and ATP Production

• The proton gradient generated by the electron transport chain is crucial for ATP synthesis.
• Hydrogen ions (H+) flow back through ATP synthase, driven by the ion gradient, leading to the formation of ATP from ADP and inorganic phosphate (P). This process is known as photophosphorylation because it utilizes light energy.

Non-Cyclic vs. Cyclic Reactions

• Non-Cyclic Reactions: Produce both NADPH and ATP, with PSII and PSI participating in the process.
• Cyclic Reactions: Involve only PSI (P700) where electrons are cycled back. ATP is generated without producing NADPH.
  • Cyclic flow compensates for the ATP demands in the dark reactions, which consume more ATP than NADPH.

Additional Concepts

• Photophosphorylation: Refers to ATP production using light energy in the light reactions.
• Proton gradient formation: Integral for driving ATP synthesis during chemiosmosis, mirroring oxidative phosphorylation from cellular respiration.
• Applications: NADPH and ATP produced in light reactions are utilized in the Calvin cycle (light-independent reactions) to synthesize sugars.

Suggested Study Activities

• Watch related videos and study diagrams in textbooks (e.g., Fig. 4.8).
• Trace the flow of electrons through the light reactions and understand the significance of hydrogen proton accumulation in relation to oxidative phosphorylation.
• Answer textbook questions for reinforcement.