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2–4 Chemical Reactions and
Enzymes
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THINK ABOUT IT
Living things are made up of chemical
compounds, but chemistry isn’t just what life is
made of—chemistry is also what life does.
Everything that happens in an organism—its
growth, its interaction with the environment, its
reproduction, and even its movement—is based
on chemical reactions.
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2–4 Chemical Reactions and Enzymes
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Chemical Reactions
Chemical Reactions
A chemical reaction is a process that changes
one set of chemicals into another set of chemicals
by changing the chemical bonds that join atoms in
compounds.
This can occur slowly, like the formation of rust, or
quickly, like an explosion.
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2–4 Chemical Reactions and Enzymes
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Chemical Reactions
The elements or compounds that enter into a
chemical reaction are known as reactants. A and B
(below) are reactants.
The elements or compounds produced by a chemical
reaction are known as products. C and D are
products.
A + B = C + D
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2–4 Chemical Reactions and Enzymes
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Chemical Reactions
What happens to chemical bonds during
chemical reactions?
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2–4 Chemical Reactions and Enzymes
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Chemical Reactions
Chemical reactions always involve changes
in the chemical bonds that join atoms in
compounds.
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2–4 Chemical Reactions and Enzymes
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Energy in Reactions
Energy in Reactions
Energy is released or absorbed whenever
chemical bonds form or are broken.
Because chemical reactions involve breaking and
forming bonds, they involve changes in energy.
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2–4 Chemical Reactions and Enzymes
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Energy in Reactions
How do energy changes affect whether a
chemical reaction will occur?
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Energy in Reactions
Energy Changes
Chemical reactions that release energy
often occur spontaneously.
Chemical reactions that absorb energy
will not occur without a source of energy.
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Energy in Reactions
When hydrogen gas reacts with oxygen to produce
water vapor, it is an energy-releasing reaction in
which energy is given off as heat (and maybe sound
and light).
2H2 + O2 2H2O + energy
Reversing this reaction would require the addition of
large amounts of energy, so it will not occur
spontaneously.
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2–4 Chemical Reactions and Enzymes
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Energy in Reactions
In order to stay alive, organisms need to carry out
reactions that require energy.
Every organism must have a source of energy to
carry out chemical reactions.
Plants get their energy from the sun.
Animals get their energy from eating plants or other
animals. The energy needed to grow, breathe, think,
and even dream is released through the chemical
reactions that occur when we metabolize, or break
down, digested food.
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2–4 Chemical Reactions and Enzymes
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Energy in Reactions
Activation Energy
All reactions, ones that release energy AND ones
that require energy, need an input of energy to get
started.
Chemists call this energy the activation energy.
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Energy in Reactions
Chemical reactions that release energy often occur
spontaneously. Chemical reactions that absorb energy will
occur only with a source of energy. The peak of each graph
represents the energy needed for the reaction to go forward. The
difference between this required energy and the energy of the
reactants is the activation energy.
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2–4 Chemical Reactions and Enzymes
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Enzymes
Enzymes
Some chemical reactions that make life possible are too slow or have activation
energies that are too high to make them practical for living tissue.
These chemical reactions are made possible by catalysts.
A catalyst is a substance that speeds up the rate of a
chemical reaction.
Catalysts work by lowering a reaction's activation energy.
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2–4 Chemical Reactions and Enzymes
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Enzymes
Why are enzymes important to living
things?
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2–4 Chemical Reactions and Enzymes
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Enzymes
Enzymes are proteins that act as biological
catalysts.
Enzymes speed up chemical reactions
that take place in cells.
Enzymes act by lowering the activation
energy.
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2–4 Chemical Reactions and Enzymes
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Enzymes
Lowering the activation energy has a dramatic effect
on how quickly the reaction is completed.Enzymes speed
up chemical reactions that take place in cells. Notice how the addition of an
enzyme lowers the activation energy in this reaction. This action speeds up the
reaction.
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Enzymes
Enzymes are very specific, generally catalyzing only
one chemical reaction.
For this reason, part of an enzyme’s name is usually
derived from the reaction it catalyzes.
For example, lipases are enzymes that catalyze the
breakdown of lipids.
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Enzyme Action
Enzyme Action
For a chemical reaction to take place, the reactants
must collide with enough energy so that existing
bonds will be broken and new bonds will be
formed.
If reactants do not have enough activation energy,
no reaction will take place.
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2–4 Chemical Reactions and Enzymes
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Enzyme Action
The Enzyme-Substrate Complex
Enzymes provide a site where reactants can be
brought together to react, reducing the energy
needed for reaction.
The reactants of enzyme-catalyzed reactions have
a special name. They are known as substrates.
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Enzyme Action
For example,
A. The enzyme carbonic anhydrase converts the substrates carbon dioxide and water into carbonic acid (H2CO3). See pg. 52, Fig. 2-22.
The fit is so precise that the active site of the enzyme and the substrates are often compared to a lock and key.
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Enzyme Action
B. The enzyme and substrates remain bound together until the reaction is done and the substrates are converted to products.
C. The products of the reaction are released and the enzyme is free to start the process again.
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Enzyme Action
An Enzyme-Catalyzed ReactionThe enzyme hexokinase converts the substrates glucose and ATP into glucose-6-
phosphate and ADP.
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Enzyme Action
Regulation of Enzyme Activity
Temperature, pH, and regulatory molecules are all
factors that can affect the activity of enzymes.
Enzymes work best at certain pH values.
Many enzymes are affected by changes in
temperature.
The activities of most enzymes are regulated by
molecules that carry chemical signals within cells,
switching enzymes “on” or “off” as needed.
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2–4
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2–4
The elements or compounds produced by a
chemical reaction are known as
a. reactants.
b. enzymes.
c. products.
d. waste.
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2–4
Chemical reactions always involve
a. changes in energy.
b. enzymes.
c. catalysts.
d. changes in the atomic number of the
reactants.
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2–4
The factor that prevents many energy-releasing
reactions from occurring at relatively low
temperatures is called
a. catalytic energy.
b. chemical bond energy.
c. enzyme energy.
d. activation energy.
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2–4
Which of the following statements is true?
a. All proteins are enzymes.
b. All catalysts are enzymes.
c. All enzymes are catalysts.
d. All catalysts are proteins.
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2–4
What happens to an enzyme after the reaction it
catalyzes has taken place?
a. The enzyme is destroyed, and the cell must
make another.
b. The enzyme holds on to the product until
another enzyme removes it.
c. The enzyme is unchanged and ready to
accept substrate molecules.
d. The enzyme changes shape so it can accept
a different kind of substrate.
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