Photophosphorylation

• Thylakoid membrane is the site of the light-dependent stage of photosynthesis
• During light-dependent stage of photosynthesis:

  • light energy used to break down water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
• photolysis occurs in the thylakoid lumen
• a proton gradient is formed due to photolysis of water

  • high concentration of hydrogen ions in the thylakoid membrane
• electron travels through an electron transport chain within membrane
• Reduced NADP is produced when hydrogen ions in the stroma and electrons from electron transport chain combine with their carrier molecule, NADP
• ATP is produced during a process known as photophosphorylation.

  • ADP + pi = ATP
  • uses protons from gradient between thylakoid lumen and stroma to drive ATP synthase (enzyme)
• Photophosphorylation of ADP to ATP can be cyclic or non-cyclic
• If photophosphorylation is cyclic or non-cyclic is determined by the pattern of electron flow on photosystem 1 and 2, or both.
• in cyclic photophosphorylation, only photosystem 1 is involved
• in non-cyclic phosphorylation, both photosystem 1 and 2 are involved
• photosystems are collections of photosynthesis pigments that absorb light energy and transfer the energy onto electrons, each photosystem contains a primary pigment
• photosystem 2 primary pigment absorbs light at wavelengths of 680nm, and therefore is called P680
• photosystem 1 has a primary pigment that absorbs wavelengths of 700nm and is therefore called P700
• photosystem 2 is the beginning of the electron tansport chain and is where the photolysis of water takes palce
• photosystem 1 is in the middle of the electron transport chain
• Energy carried by ATP is then used during light-independent reactions of photosynthesis
• Diagram of light dependent stage of photosynthesis

  
• Photolysis - splitting water into oxygen and hydrogen ions using absorbed sunlight energy