Metabolism
from Campbell's Biology, Benjamin/Cummings
Publishing Co., 1990
Metabolism is the
sum of all the chemical reactions occurring in the cells of an organism.
Metabolism is complex, efficient, well integrated and responsive to subtle
changes in conditions.
Metabolic Map
- The chemical reactions of
metabolism manage the material and energy resources of the cell.
Aided by enzymes, metabolism
proceeds by steps along interrelated pathways.
- A specific metabolic pathway
is either catabolic or anabolic. Catabolic pathways, such as those
of cellular respiration, break complex molecules into simpler compounds,
releasing energy in the process. Anabolic pathways build up complex
molecules from simpler compounds, requiring energy input usually provided
by catabolism.
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Energy
- Energy is the capacity to do
work by moving matter against an opposing force.
- Kinetic energy, the energy of
motion, does its work by transferring motion from one body of matter to
another.
- Potential energy is stored
energy that results for the specific location or arrangement of
matter. Chemical energy is a form of potential energy stored in the
molecular structure.
- Energy can be changed from
one form to another, governed by the laws of thermodynamics.
- The first law of
thermodynamics, conservation of energy, states that energy cannot be
created or destroyed. Thus, the total amount of energy in the universe is
constant.
- The second law of
thermodynamics states that every time energy changes form, there is an
increase in entropy (S), a measure of disorder, or randomness. Some energy
is dissipated as heat, random molecular motion. Whenever matter
becomes more ordered it does so at the expense of contributing to the
disorder of the surroundings.
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Chemical Energy
- The amount of energy required
to break a particular chemical bond is the same as the amount released
when the bond forms. This quantity of energy is called bond energy.
- Enthalpy (H) is the total
potential energy of a molecule. The heat of reaction (DH) is the net change in stored heat
content when reactants are converted to products.
- Reactions with a net release
of heat (- DH) are
exothermic. Endothermic reactions absorb heat form the surroundings
(+DH), reflecting the upgrading of
chemical energy in the products at the expense of the energy from the
surroundings.
- Exergonic (spontaneous)
reactions are those that occur without net input of energy from the
surroundings. They proceed only if there is a net decrease in free
energy (-DG) of the system, which
depends on the magnitudes of the enthalpy and entropy changes and the
temperature (T). These relationships are expressed in the formula: DG = DH
- TDS
- Endergonic (nonspontaneous)
reactions cannot occur without a supply of energy from the surroundings (+DG).
- In metabolism, exergonic
reactions are used to power endergonic reactions.
- A reaction approaches
equilibrium spontaneously (DG is
negative). To move a reaction away from its equilibrium, a cell must
add free energy.
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ATP and Cellular Work
- ATP (adenosine triphosphate)
serves as the main every shuttle in cells. Hydrolysis of one of its
weak phosphate bonds produces ADP (adenosine diphosphate) and inorganic
phosphate, in an exergonic reaction that releases energy.
- ATP drives endergonic
reaction in the cell by the enzymatic transfer of the phosphate group to
specific reactants. The phosphorylated intermediates formed are more
reactive than the original molecules. In this way, cells can carry
out work such as movement, active transport, and anabolism.
- The regeneration of ATP from
ADP and phosphate is an endergonic reaction, driven primarily by cellular
respiration and light-driven reactions in photosynthesis.
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Enzymes
- Enzymes are proteins that function
as biological catalysts, agents that change the rate of a reaction without
being consumed in the reaction.
- Before a reaction can occur,
the reactants must absorb enough energy to break existing bonds.
This free energy of activation is usually provided in the form of heat
absorbed from the surroundings, which causes the reactants to reach an
unstable transition state required for the reaction to proceed.
Biological macromolecules, with their orderly structure (low entropy),
would decompose spontaneously if not for high activation energies.
- Enzymes allow molecules to
react in metabolism by lowering activation energies. This allows
bonds to break at the fairly low body temperatures characteristic of most
organisms.
- Each type of enzyme has a
uniquely shaped active site, which gives it specificity in combining with
its particular substrate molecules.
- The active site of an enzyme
can lower activation energy in a number of ways: by providing a template
for substrates to come together in proper orientation; by binding to the
substrate in such a way that critical bonds of the substrate are strained;
and by providing suitable microenvironments. Some enzymes even
participate covalently in the reaction, after which they are chemically
restored to their original state.
- As proteins, enzymes are very
sensitive to environmental conditions that influence the weak chemical
bonds responsible for their three-dimensional structure. Each enzyme
has optimal conditions of temperature, pH, and salt concentration which determine
the most active conformation. Outside these narrow limits,
conformation changes and activity decreases.
- Cofactors are nonprotein ions
or molecules required for the function of some enzymes. If the
cofactor is organic, it is known as a coenzyme.
- Enzyme inhibitors are
chemicals that selectively inhibit enzyme function, either reversibly
through the formation of weak bonds or irreversibly through covalent
bonds.
- A competitive inhibitor is
structurally similar to the substrate and can bind to the active site in
its place. A noncompetitive inhibitor binds to a place on the enzyme
other than the active site, disrupting the shape and function of the
active site.
- Some enzymes change shape
when regulatory molecules, either activators or inhibitors, bind to
specific allosteric receptor sites. Allosteric sites are usually
located between the subunits of complex enzymes. Induced fir by the
binding of substrate activates other attached subunits in a phenomenon
called cooperativity.
- Biomolecule
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Control of Metabolism
- One of the most common
methods of regulating metabolism is by feedback inhibition, in which the
end product of a metabolic pathway inhibits the first enzyme in that
pathway. In this way, a cell can conserve resources by producing
certain molecules only when they are in low concentration.
- Some enzymes occur in
multienzyme complexes, organized in assembly-line fashion for efficient
catalysis of key reaction sequences. Enzymes may also be built into
membranes or dissolved in relatively high concentration within specialized
cell compartments.