light dependent reaction

• Leaf adaptations:

  • Long narrow mesophyll cells with lots of chloroplasts 
  • Many air spaces to maximise diffusion rate 
  • Thin to reduce gas exchange distance 
  • Many stomata for gas exchange 
  • Transparent cuticle and epidermis for light penetration 
  • Stomata that open and close in response to light intensity 
  • Large surface area for light absorption 
  • Network of xylem and phloem to transport water and sugar 
  • Arranged into a mosaic to prevent shading from neighbouring leaves
• Grana is an stack of thylakoids.  
• Thylakoids are where the light dependent stage takes place, chlorophyll is contained within them. 
• Stroma is the fluid matrix where the light independent stage takes place, starch grains are located here. 
• Chloroplasts contain a mixture of pigments to absorb light energy from across the spectrum. The different pigments can be separated using chromatography.
• Different pigments absorb light from different areas of the spectrum, green is usually reflected. 
• Pigments are found in the thylakoid membranes attached to proteins, this is photosystem.   
• Photosynthesis is made of the light-independent (Calvin cycle) and the light-dependant stage. 
• Stage 1:

  Light energy is absorbed by PSII

  Light energy excites 2 electrons in chlorophyll is PSII

  The two electrons move to a higher energy level and leave the chlorophyll 

  The chlorophyll has been photoionised (oxidised) 

  The high energy electrons transfer energy to electron carriers as they move down the electron transport chain to PSI 
• Water splits to replace the lost electron, this is photolysis and produces protons, hydrogen ions, oxygen and electrons 
• Stage 2:

  As the 2 excited electrons from chlorophyll leave PSII to move into the electron transport chain by electron carriers, they must be replaced so Light energy splits water into protons, electrons and oxygen.
• Stage 3:

  The exited electrons release energy as they move down the electron transport chain 

  This energy is used to transport protons into the thylakoid space from the stroma, so that the inside has a higher concentration of protons than the stroma. This forms a proton gradient across membranes. 

  Protons move down their concentration gradient into the stroma via ATP synthase. The energy from this movement combines ADP and Pi to form ATP

  Chemiosmosis is the name of the process where the movement of H+ ions across a membrane makes ATP.
• Stage 4:

  Light energy is absorbed by PSI which excites the 2 electrons to an even higher energy level

  Finally the electrons are transferred to NADP+ with a proton from the stroma to make NADPH