Light dependent stage

• The aim of photosynethesis is to absorb light energy and transfer it into stored chemical energy
• to produce glucose, hydrogen has to be added to carbon dioxide. this means that photosynthesis is a series of reduction reactions.

  for the reduction reactions to take place a reducing agent and a source of energy is required
• the aim of the light dependent reaction is the production of NADPH and ATP.

  the light dependent reaction takes place in the thylakoid membrane
• the light dependent reaction includes:

  • photoionisation
  • photolysis
  • chemiosmosis
  • photophosphorylation
• light energy is absorbed by chlorophyll.

  the absorbed energy excites the electrons and boosts them to a higher energy level so that they are lost from the chlorophyll.

  = PHOTOIONISATION
• energy from the electrons from chlorophyll is used to split water = photolysis

  2H2O -> O2 + 4e- + 4H+

  O2 is a waste product (used in respiration or lost through stomata)

  the e- replace lost electrons in chlorophyll

  the H+ ions reduce NADP to NADPH
• energy is needed to reduce NADP to NADPH.

  the electrons from chlorophyll travel along the ETC. between carriers it travels down an energy level and releases energy for the reaction:

  e- + NADP + H+ -> NADPH

  (the H+ ions are from photolysis)
• electrons from chlorophyll release energy as they travel along the ETC. electron carriers have decreasing levels.

  this energy is used to move protons (H+) against the electrochemical gradient.

  the flow of protons (H+) through the ATP synthase (ATPase) down the electrochemical gradient is called chemiosmosis. it provides energy for the synthesis of ATP. = photophosphorylation
• ETC = electron transfer chain
• The electrons from chlorophyll travel down an ETC.

  the energy they release is used to move H+ against the electrochemical gradient for the synthesis of ATP; and to reduce NADP
• NADP is a reducing agent
• chemiosmosis = the flow of protons across a membrane down their electrochemical gradient
• photophosphorylation = using light energy to produce ATP
• the reduction of NADP:

  e- + NADP + H+ -> NADPH
• photolysis :

  2h2O -> 4H+ + 4e- + O2