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Energy and Chemical Reactions
Ch. 2
Biology
Ms. Haut
Chemical Reactions
• Reactants—the
starting materials
• Chemical bonds
are broken, atoms
are reorganized
and new bonds
formed
• Products—the
resulting
material(s)
– 2H2 + O2  2H2O
http://www.chem.ufl.edu/~itl/2045/matter/FG03_003.GIF
Reactants → Products
Chemical Reactions
• Occur when bonds
between the
outermost parts of
atoms are formed
or broken
• Involve changes in
matter, the making
of new materials
with new
properties, and
energy changes
http://upload.wikimedia.org/wikipedia/c
ommons/8/8c/Wohler_synthesis.gif
http://www.personal.kent.edu/~cearley/ChemWrld/bala
nce/H2_O2.gif
Chemical Equations
• Show the kind of reactants and
products and their relative amounts
in the reaction
4 Al + 3 O2 → 2 Al2O3
Chemical Equations
• Because of the
principle of the
conservation of matter,
an equation must be
balanced.
• It must have the same
number of atoms of the
same kind on both
sides
http://www.chem.ufl.edu/~itl/2045/matter/FG03_003.GIF
Energy in Reactions
• Energy is released when chemical
bonds are broken
• Energy is absorbed when chemical
bonds are formed.
• Because chemical reactions involve
breaking and forming bonds, they
involve changes in energy.
Energy
• Energy—the capacity
to do work
– Kinetic energy—
energy of motion
(heat)
– Potential energy—
stored capacity to
perform work
(chemical energy
stored in chemical
bonds)
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Chemical Reactions
• Cells carry out thousands of chemical
reactions
– The sum of these reactions constitutes
cellular metabolism
• Chemical reactions either store or release
energy
– Endergonic reactions—absorb and store
energy in a reaction
– Exergonic reactions—energy is released
from reaction
• Metabolism = endergonic rxns + exergonic rxns
Energy in Reactions
• In order to stay alive, organisms need to carry out
reactions that require energy.
• Because matter and energy are conserved in
chemical reactions, 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.
Energy in Reactions
• Chemical reactions that release energy
do not always occur spontaneously.
• Chemists call the energy that is needed
to get a reaction started the activation
energy.
Energy in Reactions
• Activation energy is a factor in
whether the overall chemical reaction
releases energy or absorbs energy.
Energy in Cells
• ATP is used to shuttle chemical energy within
the cell
• ATP—energy-rich covalent bonds b/w
outer 2 phosphate bonds
• When ATP gives up its energy, it forms ADP
and an energy shuttle, the phosphate group.
• Energy coupling—using energy released from
exergonic reactions to drive endergonic
reactions
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Enzyme
• 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
• Enzyme—catalytic protein that speeds up the
chemical reactions by lowering the activation
energy (end with –ase)
EA
barrier
Reactants
1
Products
2
Figure 5.5A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Energy Profile
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Enzymes are substrate-specific—has
unique 3-D shape which determines
what the enzyme works on (substrate)
• The 3-D shape of the enzyme creates
a “pocket” called the Active Site in
which the substrate binds
• Active site has
particular amino
acid side-chains
that match up
with side-chains
of substrate
http://www.pickens.k12.sc.us/phsteachers/edmunds/Enzyme%20Active%
20Site.gif
”Induced Fit Model”
• How an enzyme works
• The enzyme
is unchanged and can repeat the
process
Affecting Enzyme Function
Biology
Ms. Haut
•Enzymes speed up the rate of chemical
reactions because they lower the energy of
activation.
•Enzymes lower the energy of activation by
forming an enzyme-substrate complex.
Enzyme Activity
• Enzyme activity is influenced by
– temperature
– salt concentration
– pH
– Presence of other molecules
Enzyme Activity
• A cell’s physical and chemical
environment affects enzyme activity
• Each enzyme has optimal
environmental conditions that favor the
most active enzyme shape
Effects of Temperature
• Optimal temp. allows greatest number
of molecular collisions without
denaturing the enzyme
Reaction rate  when temperature 
Kinetic energy increases and collisions
increase
Beyond optimal temperature, reaction
rate slows
 Too high, agitation disrupts weak bonds of the
tertiary structure of enzyme (enzyme unfolds)
 Too low, collisions b/w substrate and active site
don’t occur fast enough
Effects of pH
• Optimal pH range for most enzymes
is pH 6 - 8
Beyond optimal pH, reaction rate slows
Too low (acidic) H+ ions interact with
amino acid side-chains and disrupt weak
bonds of the tertiary structure of enzyme
Too high (basic) base interacts with H+
ions on amino acid side-chains and disrupt
weak bonds of the tertiary structure of
enzyme
Effect of Amount of
Substrate or Enzyme
• Increase Concentration of Substrate
– More substrate available to make contact with
enzyme
– Speeds up reaction rate up to a point
– Reaction rate decreases as substrate is
converted into product
• Increase Concentration of Enzyme
– More enzyme available to make contact with
more substrate molecules
– Speeds up reaction rate up to a point
– Reaction rate decreases as substrate is
converted into product
Chemical Reactions and
Enzymes
Chemical Reactions and
Enzymes
The elements or compounds produced
by a chemical reaction are known a
a)reactants.
b)enzymes.
c)products.
d)waste.
Chemical Reactions and
Enzymes
Chemical reactions always involve
a)
b)
c)
d)
changes in energy.
enzymes.
catalysts.
changes in the atomic number of the
reactants.
Chemical Reactions and
Enzymes
The factor that prevents many energyreleasing reactions from occurring at
relatively low temperatures is called
a)
b)
c)
d)
catalytic energy.
chemical bond energy.
enzyme energy.
activation energy.
Chemical Reactions and
Enzymes
Which of the following statements is true?
a)
b)
c)
d)
All proteins are enzymes.
All catalysts are enzymes.
All enzymes are catalysts.
All catalysts are proteins.
Chemical Reactions and
Enzymes
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.
Acknowledgements
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Unless otherwise noted, illustrations are credited to Prentice Hall
and have been borrowed from Biology by Miller and Levine, ©
2007. These images have been produced from the originals by
permission of the publisher. These illustrations may not be
reproduced in any format for any purpose without express written
permission from the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by
Campbell, Reece, Mitchell, and Taylor, ©2003. These images
have been produced from the originals by permission of the
publisher. These illustrations may not be reproduced in any
format for any purpose without express written permission from
the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by
Campbell, Reece, Mitchell, and Taylor, ©2001. These images
have been produced from the originals by permission of the
publisher. These illustrations may not be reproduced in any
format for any purpose without express written permission from
the publisher.