The Light-Dependent Reactions of Photosynthesis (College Board AP Biology)

• Photosynthesis occurs in two stages: the light-dependent stage, which takes place in the thylakoids, and the light-independent stage, which takes place in the stroma
• During the light-dependent stage of photosynthesis:
  • Light energy is used to break down water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
  • A proton gradient is formed due to the photolysis of water resulting in a high concentration of hydrogen ions in the thylakoid lumen
  • Electrons travel through an electron transport chain of proteins within the membrane
  • Reduced NADP (NADPH) is produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
  • ATP is produced during a process known as photophosphorylation

The Electron Transport Chain

• The electron transport chain makes up part of the light-dependent reactions of photosynthesis
• Light energy is absorbed by photosystems I and II, which are embedded in the internal thylakoid membrane of the chloroplast
• Light energy is transferred to electrons in the chlorophyll
• Energized electrons are passed along a chain of electron carriers (known as the electron transport chain)
• The electron carriers are alternately reduced (as they gain an electron) and then oxidized (as they lose the electron by passing it to the next carrier)

The Proton Gradient

• The energized electrons gradually release their energy as they pass through the electron transport chain
• The released energy is used to actively transport protons (H⁺ ions) across the thylakoid membrane, from the stroma (the fluid within chloroplasts) to the thylakoid lumen (the space within thylakoids)
  • A ‘proton pump’ transports the protons across the thylakoid membrane, from the stroma to the thylakoid lumen
  • The energy for this active transport comes from the energized electrons moving through the electron transport chain

• This creates a proton gradient, with a high concentration of protons in the thylakoid lumen and a low concentration in the stroma
• Protons then return to the stroma (moving down the proton concentration gradient) by facilitated diffusion through transmembrane ATP synthase enzymes in a process called chemiosmosis
• This process provides the energy needed to synthesize ATP by adding an inorganic phosphate group (P_i) to ADP (ADP + P_i → ATP)

Products of the Light-Dependent Reactions

• At the end of the electron transport chain, electrons combine with hydrogen ions and the carrier molecule NADP to give NADPH:

  2H⁺ + 2e^- + NADP → NADPH

• ATP and NADPH then pass to the light-independent reactions to be used in the synthesis of carbohydrates from carbon dioxide

Light Dependent Reaction Diagram

The electron transport chain creates a proton gradient linked to the synthesis of ATP from ADP