Photosynthesis: Light-dependent Reactions

Photosynthesis: Light-dependent Reactions

• Definition: Light-dependent reactions are the first stage of photosynthesis that require sunlight to produce energy-rich compounds.

• Location: Occur in the thylakoid membranes of chloroplasts.

• Key Inputs:

  • Water (H₂O)
  • Light energy (usually from the sun)
  • NADP⁺ (Nicotinamide adenine dinucleotide phosphate)
  • ADP (Adenosine diphosphate)

• Key Outputs:

  • Oxygen (O₂) - released as a byproduct
  • ATP (Adenosine triphosphate)
  • NADPH (reduced form of NADP⁺)

• Process Overview:

  1. Photon Absorption: Chlorophyll absorbs light energy, exciting electrons.
  2. Water Splitting: H₂O molecules are split (photolysis) to release O₂, protons (H⁺), and electrons.
  3. Electron Transport Chain: Excited electrons are transferred through a series of proteins (electron transport chain).
  4. ATP Synthesis: Energy from electrons drives the synthesis of ATP from ADP and inorganic phosphate (Pi) via ATP synthase.
  5. NADPH Formation: Electrons reduce NADP⁺ to form NADPH.

• Importance:

  • Provides energy (ATP) and reducing power (NADPH) for the subsequent light-independent reactions (Calvin Cycle).
  • Contributes to the oxygen content of the atmosphere.

• Photosystems:

  • Photosystem II (PSII): Absorbs light at 680 nm; involved in water splitting and initial electron transport.
  • Photosystem I (PSI): Absorbs light at 700 nm; further energizes electrons to form NADPH.

• Z-scheme: Describes the energy changes of electrons as they travel from PSII to PSI, illustrating the flow of energy and the overall process of electron transport.

• Chemiosmosis: The movement of protons back into the stroma through ATP synthase drives ATP production, illustrating the coupling of electron transport and ATP synthesis.

• Factors Affecting Light-dependent Reactions:

  • Light Intensity: Increased light boosts the rate of reactions until saturation.
  • Temperature: Optimal range enhances enzyme activity involved in the process.
  • CO₂ Concentration: Influences the overall rate of photosynthesis indirectly through the Calvin Cycle.

These notes summarize the essential components and processes of light-dependent reactions in photosynthesis.

Light-dependent Reactions of Photosynthesis

• Definition: First stage of photosynthesis that harnesses sunlight to create energy-rich compounds.
• Location: Occur in the thylakoid membranes within chloroplasts.

Key Inputs

• Water (H₂O): Source of protons and electrons, split during the process.
• Light Energy: Primarily from the sun, crucial for driving the reactions.
• NADP⁺: Serves as an electron acceptor, crucial for forming NADPH.
• ADP: Acts as a precursor to ATP production.

Key Outputs

• Oxygen (O₂): Released as a byproduct of water splitting.
• ATP (Adenosine triphosphate): Energy currency of the cell generated during the process.
• NADPH: Reduced form of NADP⁺, provides reducing power for subsequent reactions.

Process Overview

• Photon Absorption: Chlorophyll captures light energy, exciting electrons to higher energy states.
• Water Splitting (Photolysis): Water is divided into O₂, protons, and electrons, contributing to the electron transport chain.
• Electron Transport Chain: Excited electrons move through a series of proteins, releasing energy used for ATP production.
• ATP Synthesis: Energy transfers convert ADP and inorganic phosphate (Pi) into ATP, facilitated by ATP synthase.
• NADPH Formation: Electrons reduce NADP⁺, resulting in the formation of NADPH.

Importance

• Energy Provisioning: ATP and NADPH generated support the Calvin Cycle, the next stage of photosynthesis.
• Oxygen Generation: Contributes significantly to atmospheric oxygen levels.

Photosystems

• Photosystem II (PSII): Absorbs light at 680 nm; initiates water splitting and starts electron transport.
• Photosystem I (PSI): Absorbs light at 700 nm; further energizes electrons for NADPH synthesis.

Z-scheme

• Describes energy transition of electrons from PSII to PSI, depicting how energy levels change throughout electron transport.

Chemiosmosis

• Process where proton movement back to the stroma via ATP synthase drives ATP generation, illustrating the connection between electron transport and ATP synthesis.

Factors Affecting Light-dependent Reactions

• Light Intensity: Enhances reaction rates until saturation is reached.
• Temperature: Optimal temperatures increase enzyme activity and reaction efficiency.
• CO₂ Concentration: Affects overall photosynthesis indirectly, particularly through the Calvin Cycle.