week 5. chloroplasts see figure 13.27 outer membrane inner membrane contains stroma - contain dark...
TRANSCRIPT
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 systemS
– measure of change in S with timeSsystem + Ssurrounding > 0
– spontaneous reaction• Difficult to quantify S
Free energy
• - G – spontaneous reaction
• exergonic or exothermic reaction
• + G– non spontaneous reaction
• endogonic or endothermic
Free energy
• Williard Gibbs• Measure of energy to do work• G = H - TS G = H - T S
– H = enthalpy– T = absolute temperature (K)
Free energy
G = H - T S H = E + PV PV is small for biochemical reactions G = E - T S
Free energy
G is proportional to equilibrium constant of reaction
• See Panel 3-1 G = Go + RT ln K
– R = 1.987 cal/mole-Ko
– T = absolute temperature (Ko) Go = standard free energy at 1M products and
reactants. This is constant - never changes in value.
Calculating Go
• At equilibrium G = 0• therefore Go = -RT lnKeq
• Example:– glucose 6 P <---> Fructose 6 P– Keq = 0.5
Go = - (1.987) (298Ko) ln0.5 Go = + 410 cal/mole
Calculating G
• Calculate 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
Go = - (1.987) (298) ln0.0475 Go = +1.8kcal/mole
Calculation continued
G = Go + RT ln K Go = +1.8kcal/mole– K = 3 x 10-6/2 x 10-4
G = +1.8kcal/mole + (1.987)(298) ln 3 x 10-6/2 x 10-4
G = - 0.7 kcal/mole• Why is this a negative number when Go is a
positive number?