Introduction to
Metabolism
Metabolic Pathways
Laws of
Thermodynamics
Free Energy
Energy Coupling &
ATP
Enzymes
Metabolic Pathways
All the chemical reactions in an
organism make up its metabolism.
 There are two types of metabolic
pathways: Anabolic and Catabolic

The Two Types of Pathways
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Anabolic:
Ex. Dehydration
synthesis
Photosynthesis
Protein synthesis
Uses energy
Endergonic
reactions
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Catabolic:
Ex. Hydrolysis
Cellular
respiration
Releases energy
Exergonic
reactions
Energy and Organisms
Remember that energy is the
capacity to perform work.
 Chemical energy is just the potential
energy stored in molecules.
 Catabolic reactions transform
potential energy into kinetic energy.
 Anabolic reactions transform kinetic
energy to potential energy.

The Laws of
Thermodynamics
Living systems are subject to the
laws of thermodynamics just like
cars and furnaces.
 1st Law of Thermodynamics
(Principle of Conservation of Energy)
Energy cannot be created or
destroyed.

Laws of Thermodynamics
(Continued)
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2nd Law of Thermodynamics:
Every energy transfer or transformation
increases the entropy (disorder) of the
universe.
In living systems, much of the energy
involved in transformations is lost as
heat. (heat is energy in its most random
state)
The quantity of energy in the universe is
constant but the quality is not.
Changes in Living Systems
Spontaneous:
Occur without outside help (energy)
Can be harnessed to perform work
Only occur when free energy is
available
Increase the entropy (disorder) of
the universe and increase the
stability of the system.

Free Energy
Portion of a system’s energy that
can perform work when the
temperature is uniform throughout
the system.
 We can quantify free energy by
using the symbols on the next slide.

Free Energy Equation
G= free energy of a system (a
measure of its instability)
 T=absolute temperature in Kelvins
 H=total energy of the system
 S= entropy
 Relationship is expressed:
G=H-TS

Free Energy Changes
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Expressed as delta G (triangle symbol)
Systems that spontaneously change to a
more stable state must give up energy,
give up order, or both.
So:
Delta G= delta H-T delta S
Delta G will have a negative value in
these types of changes.
Equilibrium and Free Energy
Equilibrium is a state of maximum
stability.
 As a reaction proceeds toward
equilibrium its free energy
decreases.
 For a reaction at equilibrium delta G
is 0.(A reaction at equilibrium
performs no work)

Exergonic Reactions
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There is a net release of free energy in an
exergonic reaction.
Delta G is negative.
Magnitude of delta G is the maximum
amount of work the reaction can perform.
Products store less energy than the
reactants.
Example Pg.92
Cellular Respiration
Endergonic Reactions
Stores free energy in molecules
 Delta G is positive
 Magnitude of delta G is the quantity
of energy required to drive the
reaction.
 Ex. Photosynthesis
 Products store more energy than the
reactants.

Metabolic Disequilibrium
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If cells reached equilibrium they would
have no free energy(In other words, they
would be dead!)
So one of the defining features of living
organisms is that they maintain metabolic
disequilibrium.
They can do this because they are open
systems and there is a constant flow of
materials in and out of the cell.
Energy Coupling and ATP
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Energy coupling is using exergonic
reactions to drive endergonic reactions.
ATP is the “energy coupler” for cellular
work, like:
Mechanical Work-muscle contraction
Transport Work- Na-K Pump
Chemical Work- protein synthesis (or
pushing endergonic reactions)
Adenosine Triphosphate
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Structure:
Nitrogenous base adenine bonded to
ribose with three phosphate groups
attached
Reaction:
ATP + Water yields ADP + inorganic
phosphate
Under cellular conditions(pg 94) this
reaction yields about –13kcal/mol
Characteristics of ATP
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Bonds between phosphates are unstable
because phophates are negatively
charged.
Release of energy is due to instability-in
losing a phosphate it becomes more
stable.
The terminal phosphate is added
(phosphorylation) to another molecule
which energizes it.
Most cellular work is done by ATP.
Enzymes
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It is important to understand that
enzymes are essential to chemical
reactions in our bodies.
Normally, chemical reactions occur very
slowly and would not allow life as we
know it to exist.
Enzymes make everything possible by
speeding up reactions as they lower
activation energy.
Enzymes (continued)
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Enzymes are catalytic proteins-they
change the rate of reaction without being
used up by the reaction.
Activation energy is the energy needed to
get the reaction to proceed. Your text
refers to it as the amount of energy
needed to push the reactants over an
energy barrier so that the downhill part of
the reaction can proceed.
Enzymes (continued)
Proteins have specific shapes and
enzymes are proteins. Remember
that how one molecule recognizes
another is related to its shape.
 Therefore, sucrase only recognizes
and reacts with sucrose. It will not
react with maltose.
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Enzymes (continued)
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Enzymes have an active site, a pocket or
groove on the surface of the enzyme
where the substrate, or substance the
enzyme acts on or changes.
Once the substrate enters the active site,
it makes the enzyme slightly change its
shape. The aligns molecules so that the
ability of the enzyme to catalyze reactions
is enhanced. This is called induced fit.
Enzyme Regulation
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Enzymes are regulated naturally by
temperature and pH. They function most
effectively in humans within the range of
human body temperature.
Enzymes in humans generally function
best between pH 6-8.
An exception would be pepsin which
functions best in the stomach at a pH of
2.
Enzyme Helpers
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Cofactors and coenzymes are substances
that aid enzymes in their catalytic
activity.
Some cofactors are minerals like Zinc and
Copper. (The minerals in vitamins and
mineral supplements)
Coenzymes are organic molecules that
help enzymes. (some vitamins are
cofactors)
Enzyme Inhibitors
Some substances can prevent the
action of enzymes, either totally or
temporarily.
 These are called inhibitors. If they
bond covalently to the enzyme, the
effect is permanent. If the bonds are
weak then the action can be
reversed.

Competitive and
Noncompetitive Inhibitors.
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Competitive
inhibitors mimic the
substrate and
compete with the
substrate for a
place in the active
site.
They can be
overcome with
more substrate in
the environment.

Noncompetitive
inhibitors do not
mimic the substrate
but change the
shape of the
enzyme by binding
to it somewhere
else making the
active site
essentially inactive.
Metabolic Control
The body needs not only speed in
chemical reactions but control so
tasks are done in order and
efficiently.
 It can regulate the activity of
enzymes once they are made or
turn genes on and off to make
enzymes as needed.

Methods of Metabolic
Control
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The allosteric site provides a place for
activator and inhibitor molecules to bind
and (you guessed it)activate or inhibit the
enzyme.
Allosterically regulated enzymes often
have binding sites where polypeptide
chains join.
The enzyme becomes stable in its active
state if an activator binds to it and stable
in its inactive state if an inhibitor binds to
it.
Control of ATP by enzymes
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Some catabolic enzymes have allosteric
sites that bind inhibitors and activators.
If there was an excess of ATP, it would
bind to these and act as an inhibitor,
slowing catabolism.(catabolism
regenerates ATP)
If AMP (product of broken down ATP)
accumulates it binds to these sites, acting
as an activator and increasing the rate of
catabolism so that more ATP is
regenerated.
Feedback Inhibition
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Feedback inhibition is how ATP is
controlled in the example on the last
slide.
The end product, in that case ATP, slows
down its own production so the cell is not
wasting resources making substances
that it already has enough of……
The next slide shows another example of
this process.
Cooperativity
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This type of metabolic control relies on
the substrate molecule.
It is similar to allosteric regulation and
induced fit.
When one substrate molecule binds to an
enzyme’s active site, it “primes” the
enzyme to accept more substrate
molecules by making all the subunits fit
the substrate.
Enzyme Complexes
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Groups of enzymes involved in a
particular metabolic pathway can form
complexes that keep them in a particular
place.
This makes metabolism more efficient
because it acts like an assembly line, with
the product of one enzyme becoming the
substrate for the next one.
Some enzyme complexes are bound to
particular organelles like those in
membranes of the mitochondria that
perform cellular respiration.
Prokaryotic vs. Eukaryotic
Cells
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While prokaryotes are very
successful organisms, the
complexity of eukaryotes may be
partially explained by the
compartmentalization of complex
reactions on membranes like those
present in chloroplasts and
mitochondria
Prokaryotes do not have organelles
or an endomembrane system like
eukaryotes
 The presence of organelles is
presently explained by the
Endosymbiosis Theory
 This states that when smaller cells
became part of larger cells by
predation, some remained
functioning in the host cell (pg.5167)
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