Light Dependent Reaction

• The light-dependent reaction occurs in the thylakoids of the grana in chloroplasts
• Chlorophyll molecules absorb light energy in a process called photoionisation, exciting a pair of electrons to a higher energy level, leaving the chlorophyll molecules ionized
• The electron passes through an electron transfer chain providing energy to pump hydrogen ions into the inner membrane space
• Photolysis of water requires light energy to break the bonds between oxygen and hydrogen atoms: 2H2O → 4H+ + 4e- + O2
• The photoionized chlorophyll electrons are replaced by the electrons from photolysis of water
• Hydrogen ions pumped into the inner membrane space form an electrochemical gradient and move down it through an ATP synthase channel, producing ATP from ADP + Pi - this is known as the chemiosmotic theory
• Hydrogen ions and electrons combine with NADP to produce reducing NADP
• Cyclic photophosphorylation recycles electrons back to chlorophyll, resulting in the production of only a small amount of ATP and no reduced NADP
• Electron Transport Chain: