Transcript Week 5
Week 5 Chloroplasts • See Figure 13.27 • Outer membrane • Inner Membrane – contains stroma - contain dark reaction enzymes • Thylakoid membrane – stacked into “granum” – contain light harvesting pigments, electron transport chain • thylakoid space Endosymbiont Theory • Theorizes about the origin of the chloroplast – Cyanobacteria? • Evidence: – – – – DNA RNA antibiotic sensitivity 16s RNA Overall reaction • 6CO2 + 12 H2O + energy---> C6H12O6 + 6O2 + 6H2O • Energy source : sunlight - radiant light energy • Photosynthesis – Light reactions – Dark reactions (syn. Calvin cycle, light independent reactions) Light reactions • Energy capturing reactions – convert light energy into chemical energy • ATP and NADPH • Occur on the thylakoid membrane • Chlorophyll, carotenoids capture light energy – see Figures 13-30 and 13-31 Light reactions • Electrons moving down the electron transport chain establishes proton motive force across the thylakoid membrane – see Figures 13-33 and 13-34 • Water is the ultimate source of electrons • Products of light reaction are: • ATP and NADPH Dark reactions • Carbon fixation by ribulose bisphosphate carboxylase - probably the most abundant enzyme in the world • See Figure 13-36 for pathway Energetics Laws of thermodynamics • 1st law: – energy cannot be created or destroyed, but can be transferred or transformed from one form to another 2nd law of thermodynamics • In the universe, or any isolated system, the degree of disorder can only increase. – Total entropy of the universe is always increasing – Free energy of a system is always decreasing Entropy • Measure of disorder in a system DS – measure of change in S with time DSsystem + DSsurrounding > 0 – spontaneous reaction • Difficult to quantify S Free energy • -DG – spontaneous reaction • exergonic or exothermic reaction • +DG – non spontaneous reaction • endogonic or endothermic Free energy • Williard Gibbs • Measure of energy to do work • G = H - TS DG=DH-TDS – H = enthalpy – T = absolute temperature (K) Free energy DG=DH-TDS D H = D E + D PV D PV is small for biochemical reactions DG=DE-TDS Free energy D G is proportional to equilibrium constant of reaction • See Panel 3-1 D G = D Go + RT ln K – R = 1.987 cal/mole-Ko – T = absolute temperature (Ko) D Go = standard free energy at 1M products and reactants. This is constant - never changes in value. Calculating D Go • At equilibrium D G = 0 • therefore D Go = -RT lnKeq • Example: – glucose 6 P <---> Fructose 6 P – Keq = 0.5 D Go = - (1.987) (298Ko) ln0.5 D Go = + 410 cal/mole Calculating D G • Calculate D G for the following reaction: – dihydroxyacetone phosphate <----> glyceraldehyde 3 P – Concentrations of dhap = 2 x 10-4 M – concentration of G3P = 3 x 10-6 M D Go = - (1.987) (298) ln0.0475 D Go = +1.8kcal/mole Calculation continued D G = D Go + RT ln K D Go = +1.8kcal/mole – K = 3 x 10-6/2 x 10-4 D G = +1.8kcal/mole + (1.987)(298) ln 3 x 10-6/2 x 10-4 D G = - 0.7 kcal/mole • Why is this a negative number when D Go is a positive number?