Light-Dependent Reactions and Photophosphorylation

Light-Dependent Reactions

• Occur in thylakoid membranes of chloroplasts
• Light energy is absorbed by pigments (e.g. chlorophyll a) and converted into ATP and NADPH
• Two types of light-dependent reactions:
  • Cyclic photophosphorylation: produces ATP only
  • Non-cyclic photophosphorylation: produces ATP and NADPH

Photophosphorylation

• Light energy is used to generate a proton gradient across the thylakoid membrane
• Proton gradient is used to produce ATP from ADP and Pi
• Two types of photophosphorylation:
  • Cyclic: electrons cycle back to reaction center, producing only ATP
  • Non-cyclic: electrons are used to reduce NADP+, producing NADPH and ATP

Electron Transport Chain

• Series of electron carriers in thylakoid membrane
• Electrons from light-excited pigments are passed through the transport chain
• Energy from electrons is used to pump protons across the membrane, creating a proton gradient
• Proton gradient is used to produce ATP in photophosphorylation

ATP Synthesis

• ATP is produced from ADP and Pi using energy from proton gradient
• Proton gradient is used to drive the phosphorylation of ADP to ATP
• ATP is produced in the stroma of chloroplasts and used in the Calvin cycle

Photoinhibition

• High light intensities can cause damage to photosynthetic apparatus
• Over-reduction of electron carriers can lead to formation of reactive oxygen species (ROS)
• ROS can damage photosynthetic pigments and proteins, leading to decreased photosynthetic activity
• Photoinhibition can be protected against by mechanisms such as non-photochemical quenching (NPQ)

Light-Dependent Reactions

• Light energy is absorbed by pigments such as chlorophyll a in thylakoid membranes of chloroplasts
• Energy is converted into ATP and NADPH through two types of light-dependent reactions
• Cyclic photophosphorylation produces ATP only, while non-cyclic photophosphorylation produces both ATP and NADPH

Photophosphorylation

• Light energy generates a proton gradient across the thylakoid membrane
• The proton gradient is used to produce ATP from ADP and Pi
• Cyclic photophosphorylation produces only ATP, while non-cyclic photophosphorylation produces both ATP and NADPH

Electron Transport Chain

• A series of electron carriers in the thylakoid membrane passes electrons from light-excited pigments
• Energy from electrons is used to pump protons across the membrane, creating a proton gradient
• The proton gradient is used to produce ATP in photophosphorylation

ATP Synthesis

• ATP is produced from ADP and Pi using energy from the proton gradient
• The proton gradient drives the phosphorylation of ADP to ATP
• ATP is produced in the stroma of chloroplasts and used in the Calvin cycle

Photoinhibition

• High light intensities can cause damage to the photosynthetic apparatus
• Over-reduction of electron carriers leads to the formation of reactive oxygen species (ROS)
• ROS can damage photosynthetic pigments and proteins, leading to decreased photosynthetic activity
• Photoinhibition can be protected against by mechanisms such as non-photochemical quenching (NPQ)