408
9 Bioenergetics
9.22 Metabolic Reactions
Advanced life systems have evolved to optimize the rate and increase the efficiency
of advantageous chemical reactions. Reaction rate is increased through catalytic
proteins (enzymes) and efficiency through the use of a sequence of reactions, each
of which is as close to reversible as possible, rather than through single reaction.
Cellular metabolic efficiencies average about 20%. The production by the Kreb’s
cycle of 36 ATP molecules (ATP storing 0.326 eV/molecule of usable energy)
for each glucose molecule (glucose which can make 29.8 eV/molecule of usable
energy) means this process is about 40% efficient.
In order for an energy-storage molecule to be useful in the chemical environment
in which it exists, it must be metastable, i.e. exist at least over a time period to
be useful. A system will be metastable if its spontaneous disintegration lifetime
is large compared to the times of other needed activities. Metastable molecules
holding energy in the tension of binding may be forced to release this energy by
first adding ‘activation energy’ to open the molecule. (See Fig. 10.7.) Enzymes act
by reducing the required activation energy, using their shape to distort and hold one
or more reactants during a reaction. Enzymes can increase reaction rates by factors
of billions, some catalyzing up to 500,000 molecules per second in a cell.
ATP is produced and continuously used in cellular reactions. The entire supply
of ATP is recycled once per minute in a healthy cell.
Those energy-producing (“catabolic”) reactions which produce ATP allow
energy-consuming (“anabolic”) reactions and generate activity involving work,
such as the coherent motion of molecules.
9.22.1 Process Efficiencies
All processes occurring in a closed system with a large number of entities cannot
cause a decrease in entropy in the system. If there are any sources of dissipation
in the system, i.e. where energy initially in one state (‘coherent’) is spread out into
many states (‘incoherent’), then the entropy in that closed system must increase.
Frictional forces doing work is an example in which coherent motion is partly
converted into incoherent motion (heat, in this case). In contrast, a reversible process
incurs no dissipation. There is no cyclic process that can recover all the energy lost
to dissipation. This is equivalent to the statement that there is no 100% efficient heat
engine, and equivalent to the statement that incoherent motion cannot spontaneously
convert to coherent motion. The ‘free energies’ introduced earlier give a measure,
under given conditions, of the energy in a system that can do useful work.
If a system is open to the transfer of heat to its environment, it can easily lower
in entropy (at the expense of increasing the entropy of its environment). We will
see that if an open system contains a sufficient variety of interacting entities, it
will tend to spontaneously order itself, and expel heat (increasing the entropy of its
9 Bioenergetics
9.22 Metabolic Reactions
Advanced life systems have evolved to optimize the rate and increase the efficiency
of advantageous chemical reactions. Reaction rate is increased through catalytic
proteins (enzymes) and efficiency through the use of a sequence of reactions, each
of which is as close to reversible as possible, rather than through single reaction.
Cellular metabolic efficiencies average about 20%. The production by the Kreb’s
cycle of 36 ATP molecules (ATP storing 0.326 eV/molecule of usable energy)
for each glucose molecule (glucose which can make 29.8 eV/molecule of usable
energy) means this process is about 40% efficient.
In order for an energy-storage molecule to be useful in the chemical environment
in which it exists, it must be metastable, i.e. exist at least over a time period to
be useful. A system will be metastable if its spontaneous disintegration lifetime
is large compared to the times of other needed activities. Metastable molecules
holding energy in the tension of binding may be forced to release this energy by
first adding ‘activation energy’ to open the molecule. (See Fig. 10.7.) Enzymes act
by reducing the required activation energy, using their shape to distort and hold one
or more reactants during a reaction. Enzymes can increase reaction rates by factors
of billions, some catalyzing up to 500,000 molecules per second in a cell.
ATP is produced and continuously used in cellular reactions. The entire supply
of ATP is recycled once per minute in a healthy cell.
Those energy-producing (“catabolic”) reactions which produce ATP allow
energy-consuming (“anabolic”) reactions and generate activity involving work,
such as the coherent motion of molecules.
9.22.1 Process Efficiencies
All processes occurring in a closed system with a large number of entities cannot
cause a decrease in entropy in the system. If there are any sources of dissipation
in the system, i.e. where energy initially in one state (‘coherent’) is spread out into
many states (‘incoherent’), then the entropy in that closed system must increase.
Frictional forces doing work is an example in which coherent motion is partly
converted into incoherent motion (heat, in this case). In contrast, a reversible process
incurs no dissipation. There is no cyclic process that can recover all the energy lost
to dissipation. This is equivalent to the statement that there is no 100% efficient heat
engine, and equivalent to the statement that incoherent motion cannot spontaneously
convert to coherent motion. The ‘free energies’ introduced earlier give a measure,
under given conditions, of the energy in a system that can do useful work.
If a system is open to the transfer of heat to its environment, it can easily lower
in entropy (at the expense of increasing the entropy of its environment). We will
see that if an open system contains a sufficient variety of interacting entities, it
will tend to spontaneously order itself, and expel heat (increasing the entropy of its