• amount of energy available for useful work
• negative : exergonic: rxn gives off E
• positive: endergonic : rxn requires E

• free E does not tell us how fast a spontaneous rxn proceeds
• activation E slows biological rxns
• Activation E : amt of E required to bring all molecules in a chemical rxn to the reactive state
• Enzyme : protein that has the ability to catalyze a specific chemical rxn
• Enzymes reduce activation E

• Catalysis : series of oxidation-reduction rxns that liberate E
• Many substrates can serve as e^- donors or acceptors to intermediate e^- cariers
• During metabolism, e^- transferred form donor to acceptorvia e^- carriers
• In catabolism, NAD is most often used (coenzyme)

• Half rxn for NAD reduction: 

1/2 NAD + 1/2 H + e^- -->1/2 NADH

Degree to which substrates serve as e^- donors or acceptors

• Lower E' -->Reducer-->Donates e^-
• Higher E' --> Oxidizer --> Gains e^-
• e^- from top of tower "fall". The further they fall, the more E released
• To release E, donor half-rxn must lie above acceptor half-rxn on the e^- tower
• Glucose: among most favorable e^- donors
• Mid range can be donors or acceptors
• Oxygen among most favorable e^- acceptors

• ATP characterized by presence of high E anhydride bonds
• Other examples include phosphoenolpyruvate , ADP
• High E bonds designated by ~P