Energy Utilization through Coupled Systems

Abstract

Living systems are characterized by a continuous of substance and energy. Both chemical and physical occur. Matter may move from one point to another by diffusion or active transport. Alternatively, it may flow through a sequence of chemically distinct compounds by enzymatically catalyzed reactions. Physically-distant compounds may react with each other through the of electrons along biochemical conductors. Changes in energy are associated with all such movements or conversions. Consider any biochemical system in which a net physical or chemical change occurs without inflow or outflow of matter or energy. A net change can occur only when there is a displacement from equilibrium; therefore, such change must always be accompanied by some negative free-energy change. This negative free-energy change is the difference in free energies of formation of the products as compared with the reactants. It is expressed as AF = AH - TAS, where AH is the heat of the reaction and TAS is the absolute temperature times the changes in entropy. Entropy changes result from rearrangement of chemical bonds, changes in concentration, movements through gradients, etc. AH changes are primarily the result of making and breaking chemical bonds, ionization, electronic excitation, formation of resonance hybrids, etc. Whereas the entropy increase may occasionally be so great that AH is positive (heat absorbed from the surroundings), the more common situation is for AH to be negative. This results in a of heat away from the system (which is commonly isothermal). This energy, together with that associated with the entropy change, is irrevocably lost. In the interests of economy of energy, how might the free-energy loss be diminished? For a net conversion of material to occur in a chemical reaction, the rate of the forward reaction, f, obviously must exceed the rate of the back reaction, b. The ratio of these rates is given by AF = - RT In f/b. If the net rate is to be large and at the same time the energy loss to be small, the reaction must be highly reversible; that is, b must be large. Examples of such reactions are to be found in eperimization of sugar phosphates and in transketolase reactions. We have estimated the free-energy changes of these reactions in the carbon-reduction cycle in