Light Dependent Reaction

• Light energy is absorbed by the photosystems and converted into chemical energy
• Photolysis splits water into H+ ions, electrons and oxygen
• LDR produces ATP via photophosphorylation
• Protons and electrons reduces NADP to Reduced NADP (NADPH)
• LDR occurs in the thylakoid membrane
• Photoionsation

  Light is absorbed by chlorophyll pigments in a photosystem, ‘exciting‘ the electrons of the chlorophyll molecule, causing some to leave the molecule completely, becoming ionised.

  • 2 electrons are taken up by an electron carrier.
• Photosystem complexes

  Cluster of proteins and chlorophyll molecules that absorb light energy
• LDR products

  Reduced NADP, ATP and Oxygen
• LDR is a chemical process where energy is conserved in chemical bonds.
• Chemiosmotic Theory

  Electrons are passed along a series of electron carriers (in redox reactions) within the thylakoid membrane, forming a transfer chain.

  • Each carrier is at a slightly lower energy level, so electrons lose energy at each stage.
  • Energy lost is used to make ATP
• Adaptation of chloroplasts for LDR

  Thylakoid membrane provides a large surface area for attachment of chlorophyll, electron carriers and enzymes.
• Adaptation of chloroplasts for LDR (Protein network)

  Network of proteins in grana hold the chlorophyll in a very precise manner, allowing maximum absorption of light.
• Adaptations of chloroplasts for LDR (Grana)

  Granal membranes have ATP synthase

  • Catalyses production of ATP and are selectively permeable
  • Allows proton gradient to be established
• Adaptations of chloroplasts for LDR (protein manufacture)

  Contains both DNA and ribosomes for the rapid manufacture of the proteins involved in the LDR
• Photolysis

  Water absorbs light energy, breaking its bonds to form protons, electrons and oxygen.
• Creation of a proton gradient

  Protons from photolysis accumulate in the thylakoid space (lumen)

  • A proton pump in the thylakoid membrane pumps protons into the space from the stroma.
  • Protons can only pass through ATPase synthase channel proteins - rest is impermeable