HL 1 Biology

8.2a Photosynthesis Light Reactions

 

Photosynthesis consists of light-dependent and light-independent reactions

Light-Dependent Reactions

  • photoactivation of photosystem II:
      • at 680 nm photons absorbed by pigments
      • chlorophyll a reduced (since gains energy)
      • chlorophyll a oxidized (excited electrons are moving to electron transport systems)
  • photolysis of water:
      • oxidized chlorophyll a lacks electrons
      • replaced as H20, split
      • H+
          • remains in thylakoid interior (lowers pH)
          • contributes to chemiosmotic gradient
          • used in phosphorylation of ATP
      • 2 3l3ctrons
      • 1/2 O2: lost in environment
  • Electron transport system: proteins in thylakoid membrane transfer energy along a pathway in a series of redox reactions:
      • from PsI to NADP+
      • some used to pump H+s from stroma –> thylakoid interior (lower pH)
      • contributes to chemiosmotic gradient (in phosphorlyation of ATP
  • Photoactivation of photosystem I:
      • at 700nm, photons absorbed by pigments
      • chlorophyll a reduced (gains energy)
      • chlorophyll a oxidized
  • Reduction of NADP+ —> NADPH + H+
      • gain of 2 electrons from ETS
      • reduces NADP+ –> NADPH + H+
  • Non-cyclic Photophosphorylation:
      • one way flow of 2 electrons
      • 2 main products: NADPH + H+ and ATP
  • Cyclic Photophosphorylation
      • cyclic flow of electron PsI –> ETS —> PsI
      • Main product: ATP

=======DBQ=======================

Evidence for Chemiosmosis (p. 110)

1 a) The rate of ATP production is faster as the pH of the ADP solution is higher. At the same time, the rate of ATP production is higher at a lower pH as well.

b)  This is due to the fact the magnitude of the concentration gradient (between the inside, outside) in increasing.

2)  At a lower incubation pH, the concentration gradient increases as well as the yield of ATP.

3) The concentration difference is decreased once any change is made and thus results in a decrease in ATP production. The ATP production is generated with the help of the H movement, and goes down the concentration gradient, too.

4) The concentration gradient is affected because of the occurrence of photolysis with the help of light, which produces H.

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