Light-Dependent Reactions in Photosynthesis

Light-Dependent Reaction

• The energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy.
• It produces ATP and NADPH, crucial for the next stage of photosynthesis.
• Photosystems are aggregates of pigments and proteins in the thylakoid membrane.
• Light-harvesting complexes capture photons, transferring energy to the reaction center.
• Reaction-center complexes consist of chlorophyll and a primary electron acceptor.
• The primary electron acceptor accepts electrons from chlorophyll, raising the electron energy.
• Chlorophyll absorbs photons, then transfers that energy through a series of proteins.

Photosystems II and I Comparison

Photosynthesis II and Electron Transport Chain (ETC)

• Chlorophyll and other light-absorbing molecules absorb energy from sunlight transferring energy to electrons, starting the ETC.
• Water molecules separate into oxygen, hydrogen ions, and electrons.
• Electrons move from protein to protein in the electron transport chain and the energy from the movement pumps hydrogen ions against a concentration gradient.
• Hydrogen ions build up within the thylakoid, creating a concentration gradient.
• The energized electrons are added to a molecule called NADP+, for NADPH creation.

Photosystem I and Energy-Carrying Molecules

• In Photosystem I, chlorophyll and other light-absorbing molecules inside the thylakoid membrane absorb sunlight, exciting electrons that leave the molecules.
• NADPH is produced; it functions like ATP in photosynthesis.
• Energized electrons are added to NADP+, forming NADPH.

ATP Production

• Hydrogen ions flow through a protein channel in the thylakoid membrane. This flow uses potential energy to drive ATP synthase.
• ATP synthase adds phosphate groups to ADP, forming ATP.

Calvin Cycle

• The Calvin cycle is also known as the light-independent reactions, occurring in the stroma of the chloroplast.
• Takes place in the stroma of the chloroplast.
• Although it does not directly depend on light, it relies on ATP and NADPH from light-dependent reactions.
• CO2 is incorporated, generating sugars from carbon dioxide.
• Three stages: carbon fixation, reduction, regeneration.

Carbon Fixation

• CO2 is attached to a five-carbon sugar (RuBP) via the enzyme RuBisCo.
• RuBisCo creates a six-carbon molecule that splits into two three-carbon molecules (3-PGA).

Reduction

• Phosphate groups from ATP are added to 3-PGA, forming 1,3-biphosphoglycerate.
• NADPH reduces 1,3-biphosphoglycerate to G3P (glyceraldehyde-3-phosphate).
• Five of the six molecules of G3P are recycled to regenerate RuBP, requiring ATP.
• One G3P will create glucose.

Regeneration of RuBP

• Five G3P molecules convert to three RuBP molecules, requiring ATP to complete the cycle.