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Striated
muscle. Energy transformations
When
a chemical reaction occurs, energy is absorbed or released.
In a contracting muscle, chemical reactions release energy
that appears either as mechanical work or as heat. The first
law of thermodynamics, or the law of conservation of energy,
states that the heat and work produced must equal the energy
released by the chemical reactions. The muscles that shortenand
do external work liberate more energy as heat and work than
do those that contract under isometric conditions and do
not shorten or do external work. In light of the law of
conservation ofenergy (first law of thermodynamics), this
finding means that the amount of chemical reaction that
takes place during contraction depends on the type of contraction
performed by the muscle. In other words, the flow of energy
is subject to regulation.
The
efficiency of the process of muscle contraction depends
on the fate of the free energy released in chemical reactions;
i.e., whether it is converted primarily into work or degraded
into heat. The second law of thermodynamics sets limits
to the amount of energy that can be transformed into mechanical
work. Although the production of heat can detract from the
efficiency of working muscle, energy that appears as heat
is not always wasted. In warm-blooded animals, for example,
the heat released by muscles maintains a constant body temperature
regardless of the environmental temperature. When an animal
shivers in the cold alarge amount of heat is generated in
the muscles. The muscles alternately contract and relax,
releasing energy chiefly as heat.
Source
of energy for muscle work
Muscles use the free energy released by chemical reactions
by coupling the chemical reactionto physical changes in
the contractile proteins. The exact molecular details of
this fundamental coupling process are not yet completely
known. Of the reactions that have been identified, the splitting
of ATP is the energy-yielding reaction nearest to the contractile
event. Water participates in this reaction in which ATP
is broken down to ADP and phosphate (Pi ); the reactionthat
occurs in the muscle, during which chemical free energy
is converted into work.
