Download Notes 24

4/4/11
Energy
• Energy can take a huge number of forms
A Bit About Enzymes
by
Genghis Khan
Example: A nuclear power plant uses radiation
energy to heat water and turn it into steam, which
moves a turbine, which generates electricity.
– motion
– radiant energy (light, UV, radio waves, etc.)
– heat
– chemical energy
– radioactivity
• The First Law of Theormodynamics states that:
Energy cannot be created or destroyed; it can only
be changed from one form to another.
Example: An athlete eats Wheaties (which contain
chemical energy stored in the matter that makes them
up) and converts their energy into heat and motion—
and also into chemical energy (e.g. for making the
proteins used in building muscle tissue).
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Energy
Another example: A falling basketball hits the ground
and converts kinetic energy to elastic potential energy.
This is immediately converted back into kinetic energy,
which forces the ball back upwards.
• For our purposes here, all energy can essentially
be classified into two types: potential energy and
kinetic energy
– Potential energy is "stored" in a system
– Kinetic energy is the energy that is acting on a system
– Example: If you compress a bedspring, you exert
kinetic energy on it as you push. When the spring is
fully compressed, it stores potential energy. Release the
spring, and, with a loud "boing", its potential energy is
converted into kinetic energy of motion.
The reason the ball doesn't bounce eternally is that energy is
constantly "leaking" out of the system—in the form of
sound waves, friction with the air and the floor, etc.
Chemical Energy
• Potential energy is also stored in chemical bonds
—in the bonds that hold atoms together in a
molecule.
• Kinetic energy is released when chemical bonds
are broken.
• However, kinetic energy is needed to form
chemical bonds in the first place.
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So a plant captures kinetic energy (light) from the
Sun, and is able to use that energy, in a beautiful and
complex way, to stick atoms together, form chemical
bonds, and build up more plant tissue, as it grows.
If that plant burns, then the potential energy that was
stored in all those chemical bonds is released, as
kinetic energy (light, heat, motion).
Reactions
Reactions
• So a chemical reaction both consumes energy and
releases energy.
• If it produces more than it consumes, the reaction
is self-propagating, and is said to be exothermic.
– Simplest example: Fire.
• If it consumes more than it produces, the reaction
must have a source of energy to proceed, and is
said to be endothermic.
• Exothermic reactions, once started, will continue
until completion. – When gasoline is burned, for example, the chemical
bonds that make it up break apart and reform with
oxygen molecules. We write it as 2 C8H18 + 25 O2 ->
16 CO2 + 18 H2O.
• However, even an exothermic reaction requires an
input of energy to get it started—such as the spark
that sets off a fire, or that starts the burning of
gasoline in your car's engine
• This is the activation energy.
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Catalyst
• A catalyst is a substance that lowers the
activation energy of a chemical reaction.
• A catalyst is not affected by the chemical
reaction—in other words, it's not "burned
up" or otherwise consumed by the reaction.
Enzymes
• Enzymes are proteins that act as catalysts in living
systems.
– Small organic molecules that aren't proteins but that
play various "helping roles" in driving chemical
reactions are known as coenzymes.
– A number of vitamins are coenzymes, such as B1
(thiamine), B2 (riboflavin), B9 (folic acid), and others.
• One enzyme generally catalyzes only one
chemical reaction.
• The molecule(s) that an enzyme acts on is/are
called the substrate.
We can graph the amount of potential energy in a
chemical system over time. Notice that the activation
energy appears as a "hump" that has to be "gotten
over" for the reaction to "roll downhill".
If an enzyme is present, it lowers the activation
energy—making it far more likely that the reaction
will proceed to completion.
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Enzyme Shapes
Diagram of how the "lock and key" model works. . .
• "Lock and key" model
– An enzyme molecule has a single region called
the active site, which the substrate fits into like
a key in a lock
– Once the substrate molecule is bound to the
active site, the enzyme molecule may put stress
on it, or may bring it together with another
molecule. . .
– . . . and thus the substrate may be either broken
up, or joined together, as the case may be. The enzyme beta-amylase. . .
The enzyme beta-amylase with a substrate
molecule in the active site. . .
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Another real
example:
phenylalanine
hydroxylase,
shown here,
with a
substrate
molecule in
the active site.
Another real
example: betalactamase, shown
here with a substrate
molecule (penicillin)
in the active site.
Bacteria that can
make beta-lactamase
are resistant to
penicillin, because
they can break it
down!
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