Most of the chemical reactions that occur in your cells do not occur
spontaneously.  Instead, cells rely on proteins, called enzymes,
to kick start chemical reactions and speed them up, enabling
cells to get the most out of the energy sources available to them.
In fact, if it weren't for enzymes, chemical reactions would proceed
too slowly to support life.
This graph illustrates how enzymes speed up chemical reactions.
Remember that in a chemical reaction, the reactants interact to form
a product.
The chemical reactants for this example are shown on the left, and the 
and the products on the right.  The wall that separates them represents
the activation energy.  You can think of this wall as an energy 
"speed bump."  The larger the bump, the slower the reaction.
The yellow speed bump represents a chemical reaction without an 
enzyme, and the orange speed bump represents the same reaction
with an enzyme.  As you can see, the orange speed bump is a lot
lower.  This is because the enzyme acts to physically bring the
reactants together.  By doing so, it increases the efficiency
of the reaction, and lowers the amount of energy needed for the reaction to
occur.  Since less energy is required, the
reaction occurs at a faster rate.  Notice that the enzyme
does not influence the energy level of the reactants or the products,
but only the amount of energy that is required during the process of the chemical
reaction.  Without the use of enzymes, many of our body's
processes, such as digestion, and the processing of nerve impulses,
would simply occur too slowly.
