How do we measure energy changes in cells.......
                      FREE ENERGY        DG      =     DH        -   T DS
                                            free energy     enthalpy       entropy
  DG is a numerical measure of how far a reaction is from equilibrium
  DG is measure amount energy in system able to do work (to stay away from equilibrium)...
                          Disorder increases (thus entropy increases) when useful energy,
                                   that which could be used to do work, is dissipated as heat........
                    most Cells are ISOTHERMAL  -  (4o   to @  45o )  - thus DH = 0
                            (function with a very narrow temp range)

DG can PREDICT..... the Direction of Cellular Reactions......
                                      TOWARD EQUILIBRIUM  and  Maximum ENTROPY



A <---> B     Which Way?
                                                                                 J. Willard Gibbs (1839-1903)
                   DG  = DG0  +  R T ln [ p]/[ r]

    change in free energy content of a reaction...depends upon:
        1.  energy is stored in molecule's covalent bonds
        2. remember, temperature is negligible... cells are isothermal, i.e.,                      
D       =    actual free energy
DGo'     =    standard free energy [change under std conditions]
                    R          =    gas constant ( 2 x 10-3 Kc/mol)
                    T          =    absolute temp (-273oK)
                    ln          =    natural log (conversion log10 = 2.303)

at equilibrium   DG = 0     and    [p]/[r] = Keq
              if we solve above equation for
DG0'  we can see relationship (table 3.1 pg 98)*
                                            of Keq to
DG0                mallery's version of relationship






A <----> B 
   Which way & Why?

    EXERGONIC REACTION - is one which releases free energy
        Product  [B  <<< energy     REACTANT   [A]    [stored in covalent bonds]
            ex:    burning wood (cellulose)
                               glucose monomers  =  potential energy
                               breaks bonds, release heat & light ---> CO2 & H2O

                     cell respiration -  (heterotrophy) -  cellular burning of glucose
                               slower,   multi-step process to capture & release
                                            energy.... as ATP

    ENDERGONIC REACTION -   requires input of energy for   A --> B
        PRODUCT   [B]   >>>energy     Reactant   [A]
            ex:      photosynthesis -  (autotrophy)
                               glucose made from CO2 + H2O  --light--->   C6H12O6
 back                                                energy poor       vs.              energy rich