CHAPTER
Thermodynamics, Chemical Kinetics,
and Energy Metabolism
Nearly all chemical changes involving breakage and for-
mation of covalent bonds in a living organism (absorption,
digestion, metabolism, locomotion, putrefaction, etc.) are
catalyzed by enzymes. Without these catalysts, the reac-
tions would proceed too slowly for biological systems to
function at any significant rate.
5.1
Methods of Altering the Rate of Reactions
All reaction rates can be enhanced by increasing the con-
centration of reactants; however, this is neither practical
nor desirable for many intracellular chemical substances
whose concentration cannot easily be increased, would be
toxic if increased, or would require physiologically un-
acceptably large volumes of cellular material to maintain
suitable osmotic conditions.
Reaction rates can be accelerated by raising the tem-
perature because this increases thermal motion and en-
ergy. However, the human body functions at a constant
temperature, and so temperature can play no significant
role in altering the rates of chemical processes within the
body. Conceivably, subtle local differences in body tem-
perature may affect chemical reaction rates (e.g., hormone-
receptor interactions or conformational changes of macro-
molecules) that involve weak binding forces. Organisms
such as reptiles have a minimal capacity to regulate their
body temperature; as a result, their body temperature fluc-
tuates depending on environmental temperature. In such
organisms, chemical reactions, which are also enzyme-
catalyzed, largely depend on body temperature. In the
human body, under abnormal conditions leading to ei-
ther increased temperature (fever) or decreased tempera-
ture (hypothermia), metabolism is increased or decreased
frequently with deleterious consequences. In certain clin-
ical conditions, however,
hypothermia
can be useful.
Organs maintained at low temperatures (0-4°C) by perfu-
sion with a cold perfusate (usually a balanced electrolyte
solution) remain viable
1 0
times longer because of reduc-
tion in the rate of chemical reactions and in metabolic
requirements. Hypothermic storage of cadaver kidneys is
extensively used prior to transplantation. Skin, cornea,
and blood are preserved at low temperatures and hy-
pothermia is used in open heart surgery to prevent organ
damage that occurs during interruption of the circula-
tion. A third method of increasing the rate of a reaction
is to add a
catalyst
(an enzyme), which speeds up the
reaction by reducing the free energy of activation (see
below) by providing alternative reaction pathways. Cat-
alysts accelerate reactions without being consumed and
therefore need not appear in a balanced chemical equa-
tion. In an enzyme-catalyzed reaction,
A + B ^ C + D,
the enzyme influences the reaction velocity of both for-
ward and backward reactions to the same extent as long as
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