Biology
Chapter 2-4
Chemical Reactions and Enzymes
A. Chemical Reaction—is a process that changes one set
of chemicals into another set of chemicals. Chemical
reactions always involve breaking chemical bonds and the
formation of new chemical bonds.
1. Reactants—the substances that exist before the reactions
occurs.
2. Products—the substances that are produced by the
reaction.
reaction arrow
2H2 + O2 ----> 2H2O
reactants
products
Chemical reactions like the one
above contain two parts: The
reactants and the products.
The reactants are on the left of the
arrow, while the products are to
the right.
3.
Examples:
CO2 + H2O
Reactants
H2CO3
Reactants
 H2CO3
Product
 CO2 + H2O
Product
B. Energy in Reactions—energy is released or absorbed
whenever chemical bonds form or are broken.
1.
Energy changes:
a.
Exothermic—the term used to describe a chemical
reaction that gives off energy during a chemical reaction.
Exothermic reactions often occur spontaneously.
HEAT
b. Endothermic—this refers to chemical reactions that
absorb energy, these will not occur without a source of
energy.
c.
Activation energy—the energy that is needed to get a
reaction started. For example, you must apply force when
striking a match to get the fire to start.
Reaction pathway
without enzyme
Activation energy
without enzyme
Activation
energy
with enzyme
Reactants
Reaction pathway
with enzyme
Products
C. Enzymes—are biological catalysts that speed up the rate
of a chemical reaction.
1. In living organisms, chemical reactions need a catalyst
to speed up or lower the activation energies that are required
to carry out the reactions. A catalyst speeds up or lowers the
activation energy required in a reaction.
2. Living organisms produce their own catalyst. These are
known as enzymes. Enzymes are proteins especially made
for a specific reaction.
D. Enzyme Action—for a chemical reaction to occur, the
reactants must collide with enough energy so that existing
bonds will be broken and new bonds will be formed. If there
is not enough energy, then the reaction will not occur.
1. Enzyme-substrate Complex—enzymes provide a
place where reactants can brought together to react. This
reduces the energy needed for the reaction. The reactants of
the enzyme-catalyzed reactions are known as substrates.
Glucose + ATP
hexokinase
ADP + glucose-6-phosphate
Glucose + ATP
hexokinase
ADP + glucose-6-phosphate
2. Regulation of enzyme activity-enzymes are affected by
any variable that influences chemical reactions. These can
include:
a.
PH values
b.
Temperature
c.
Cells can turn the enzymes “on or off” at certain times.