Photosynthesis

• What are the stages of Photosynthesis?

  1)Light dependent reaction

  • Thylakoid membrane of chloroplast

  2)Light independent reaction

  • Stroma of chloroplast
• Describe the photoionisation in the light dependant reaction:

  • Chlorophyll absorbs light energy which excites its electrons (higher energy levels)
  • So electrons are released from chlorophyll (chlorophyll becomes positively charged)
• Describe what happens after photoionisation in the light dependent reaction:

  1)Electrons move along electron transfer chain releasing energy

  2)This energy is used to actively pump protons from stroma into thylakoid

  3)Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase

  4)Energy used to join ADP and Pi to form ATP - Phosphorylation

  5)NADP is final electron acceptor and is reduced to form NADPH
• Describe photolysis of water

  Water oxidised to produce protons, electrons and oxygen

  • Electrons replace those lost from chlorophyll
• What is Cyclic Photophosphorylation?

  Electrons can be cycled back by PS1 back into electron transport chain
• Describe the light independent reaction of photosynthesis (Calvin cycle):

  1)CO2 reacts with ribulose biphosphate (RuBP)

  • Catalysed by the enzyme rubisco

  2)Forming two glycerate phosphate (GP) molecules

  3)Glycerate phosphate reduced to triose phosphate

  • Using products from light dependent reaction - NADPH and ATP

  4)Some triose phosphate converted to glucose

  5)Some triose phosphate used to regenerate ribulose biphosphate in the Calvin cycle (using energy from ATP)

  6)Every turn of the cycle results in the loss of one carbon - cycle rounds 6 times to produce hexose (glucose)
• Describe and explain how temperature affects rate of photosynthesis:

  1)As temperature increases, rate increases

  • Enzymes gain kinetic energy e.g. rubisco and ATP synthase
  • So more enzyme-substrate complexes form

  2)Above an optimum temperature, rate decreases

  • Enzymes denature as hydrogen bonds in tertiary structure break
  • Fewer enzyme-substrate complexes form
• Describe and explain how light intensity affects rate of photosynthesis:

  1)As light intensity increases, rate increases

  • Light dependent reaction increases (more photoionization of chlorophyll) so more ATP and reduced NADP produced
  • So light independent reaction increases as more glycerate phosphate is reduced to triose phosphate and more TP regenerates Ribulose Biphosphate

  2)Above a certain light intensity, rate stops increasing and becomes constant

  • Another limiting factor e.g. temperature / CO2 concentration
• Describe and explain how CO2 concentration affects rate of photosynthesis:

  1)As CO2 concentration increases, rate increases

  • Light independent reaction increases
  • As more CO2 combines with ribulose biphosphate to form glycerate phosphate
  • So more GP reduced to triose phosphate
  • So more TP converted to organic substances and more RuBP regenerated

  2)Above a certain CO2 conc. rate stops increasing and becomes constant

  • Another limiting factor e.g. temperature / light intensity