The first law of thermodynamics, developed from physics and chemistry but fully
valid for biological systems as well, describes the energy conservation
principle:
In any physical or chemical change, the total amount of energy in the
universe remains constant, although the form of the energy may change.
Not until the nineteenth century did physicists discover that energy can be
transduced (converted from one form to another), yet living
cells have been using that principle for eons. Cells are consummate transducers
of energy, capable of interconverting chemical, electromagnetic, mechanical, and
osmotic energy with great efficiency (Fig. 1-7). Biological energy transducers
differ from many familiar machines that depend on temperature or pressure
differences. The steam engine, for example, converts the chemical energy of fuel
into heat, raising the temperature of water to its boiling point to produce
steam pressure that drives a mechanical device. The internal combustion engine,
similarly, depends upon changes in temperature and pressure. By contrast, all
parts of a living organism must operate at about the same temperature and
pressure, and heat flow is therefore not a useful source of energy. Cells are
isothermal, or constant-temperature, systems.
Living cells are chemical engines that function at constant temperature.
Virtually all of the energy transductions in cells can be traced
to a flow of electrons from one molecule to another, in the oxidation of fuel or
in the trapping of light energy during photosynthesis. This electron flow is
"downhill," from higher to lower electrochemical potential; as such, it is
formally analogous to the flow of electrons in an electric circuit driven by an
electrical battery. Nearly all living organisms derive their energy, directly or
indirectly, from the radiant energy of sunlight, which arises from the
thermonuclear fusion reactions that form helium in the sun (Fig. 1-8).
Photosynthetic cells absorb the sun's radiant energy and use it to drive
electrons from water to carbon dioxide, forming energy-rich products such as
starch and sucrose. In doing so, most photosynthetic organisms release molecular
oxygen into the atmosphere. Ultimately, nonphotosynthetic organisms obtain
energy for their needs by oxidizing the energy-rich products of photosynthesis,
passing electrons to atmospheric oxygen to form water, carbon dioxide, and other
end products, which are recycled in the environment. All of these reactions
involving electron flow are oxidation-reduction reactions.
Thus, other principles of the living state emerge:
The energy needs of virtually all organisms are provided, directly or
indirectly, by solar energy.
The flow of electrons in oxidation-reduction reactions underlies energy
transduction and energy conservation in living cells.
All living organisms are dependent on each other through exchanges of energy
and matter via the environment.
The fact that a reaction is exergonic does not mean that it will
necessarily proceed rapidly. The reaction coordinate diagram in Figure 1-6
(bottom) is actually an oversimplification. The path from reactant to product
almost invariably involves an energy barrier, called the activation barrier
(Fig. 1-9), that must be surmounted for any reaction to occur. The breaking and
joining of bonds generally requires the prior bending or stretching of existing
bonds, creating a transition state of higher free energy than
either reactant or product. The highest point in the reaction coordinate diagram
represents the transition state.
Activation barriers are crucial to the stability of biomolecules in living
systems. Although, when isolated from other cellular components, most
biomolecules are stable for days or even years, inside cells they often undergo
chemical transformations within milliseconds. Without activation barriers,
biomolecules within cells would rapidly break down to simple, low-energy forms.
The lifetime of complex molecules would be very short, and the extraordinary
continuity and organization of life would be impossible.
Virtually every cellular chemical reaction occurs because of
enzymes-catalysts that are capable of greatly enhancing the
rate of specific chemical reactions without being consumed in the process (Fig.
1-10). Enzymes, as catalysts, act by lowering this energy barrier between
reactant and product. The activation energy
(ΔG#; Fig. 1-9) required to overcome this
energy barrier could in principle be supplied by heating the reaction mixture,
but this option is not available in living cells. Instead, during a reaction,
enzymes bind reactant molecules in the transition state, thereby lowering the
activation energy and enormously accelerating the rate of the reaction. The
relationship between the activation energy and reaction rate is exponential; a
small decrease in ΔG# results in a very large increase
in reaction rate. Enzyme-catalyzed reactions commonly proceed at rates up to
1010- to 1014-fold greater
than the uncatalyzed rates.
Enzymes are, with a few exceptions we will consider later, proteins. Each
enzyme protein is specific for the catalysis of a specific reaction, and each
reaction in a cell is catalyzed by a different enzyme. Thousands of different
types of enzymes are therefore required by each cell. The multiplicity of
enzymes, their high specificity for reactants, and their susceptibility to
regulation give cells the capacity to lower activation barriers selectively.
This selectivity is crucial in the effective regulation of cellular
processes.
The thousands of enzyme-catalyzed chemical reactions in cells are
functionally organized into many different sequences of consecutive reactions
called pathways, in which the product of one reaction becomes
the reactant in the next (Fig. 1-11). Some of these sequences of
enzyme-catalyzed reactions degrade organic nutrients into simple end products,
in order to extract chemical energy and convert it into a form useful to the
cell. Together these degradative, free-energy-yielding reactions are designated
catabolism. Other enzyme-catalyzed pathways start from small
precursor molecules and convert them to progressively larger and more complex
molecules, including proteins and nucleic acids; such synthetic pathways
invariably require the input of energy, and taken together represent
anabolism. The network of enzyme-catalyzed pathways constitutes
cellular metabolism.
The fact that a reaction is exergonic does not mean that it will
necessarily proceed rapidly. The reaction coordinate diagram in Figure 1-6
(bottom) is actually an oversimplification. The path from reactant to product
almost invariably involves an energy barrier, called the activation barrier
(Fig. 1-9), that must be surmounted for any reaction to occur. The breaking and
joining of bonds generally requires the prior bending or stretching of existing
bonds, creating a transition state of higher free energy than
either reactant or product. The highest point in the reaction coordinate diagram
represents the transition state.
Activation barriers are crucial to the stability of biomolecules in living
systems. Although, when isolated from other cellular components, most
biomolecules are stable for days or even years, inside cells they often undergo
chemical transformations within milliseconds. Without activation barriers,
biomolecules within cells would rapidly break down to simple, low-energy forms.
The lifetime of complex molecules would be very short, and the extraordinary
continuity and organization of life would be impossible.
Virtually every cellular chemical reaction occurs because of
enzymes-catalysts that are capable of greatly enhancing the
rate of specific chemical reactions without being consumed in the process (Fig.
1-10). Enzymes, as catalysts, act by lowering this energy barrier between
reactant and product. The activation energy
(ΔG#; Fig. 1-9) required to overcome this
energy barrier could in principle be supplied by heating the reaction mixture,
but this option is not available in living cells. Instead, during a reaction,
enzymes bind reactant molecules in the transition state, thereby lowering the
activation energy and enormously accelerating the rate of the reaction. The
relationship between the activation energy and reaction rate is exponential; a
small decrease in ΔG# results in a very large increase
in reaction rate. Enzyme-catalyzed reactions commonly proceed at rates up to
1010- to 1014-fold greater
than the uncatalyzed rates.
Enzymes are, with a few exceptions we will consider later, proteins. Each
enzyme protein is specific for the catalysis of a specific reaction, and each
reaction in a cell is catalyzed by a different enzyme. Thousands of different
types of enzymes are therefore required by each cell. The multiplicity of
enzymes, their high specificity for reactants, and their susceptibility to
regulation give cells the capacity to lower activation barriers selectively.
This selectivity is crucial in the effective regulation of cellular
processes.
The thousands of enzyme-catalyzed chemical reactions in cells are
functionally organized into many different sequences of consecutive reactions
called pathways, in which the product of one reaction becomes
the reactant in the next (Fig. 1-11). Some of these sequences of
enzyme-catalyzed reactions degrade organic nutrients into simple end products,
in order to extract chemical energy and convert it into a form useful to the
cell. Together these degradative, free-energy-yielding reactions are designated
catabolism. Other enzyme-catalyzed pathways start from small
precursor molecules and convert them to progressively larger and more complex
molecules, including proteins and nucleic acids; such synthetic pathways
invariably require the input of energy, and taken together represent
anabolism. The network of enzyme-catalyzed pathways constitutes
cellular metabolism. |
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