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?