¨ Everything that happens in an organism – its growth, its interaction
with the environment, its reproduction, and even its movement – is based on chemical reactions.
Chemical Reactions
¨ A chemical reaction is a process that changes one set of chemicals into another set of chemicals.
¨ Reactants are elements or compounds that enter into a chemical reaction.
¨ Products are elements or compounds produced by a chemical reaction.
¨ Chemical reactions always involve the breaking of bonds in reactants
and the formation of new bonds in products.
v An important chemical reaction in your body involves carbon dioxide.
v Carbon dioxide in your blood reacts with water to produce a highly
soluble compound called carbonic acid, H2CO3.
CO2 + H2O ----> H2CO3
v This reaction enables the bloodstream to carry carbon dioxide to
the lungs.
v In the lungs, the reaction is reversed.
H2CO3 -----> CO2 + H2O
v This reverse reaction produces carbon dioxide gas, which is released
as you exhale.
Energy in Reactions
¨ Energy is released or absorbed whenever chemical bonds form or are
broken.
¨ Chemical reactions involve changes in energy.
Energy Changes
¨ Chemical reactions that release energy often occur spontaneously.
Ex) hydrogen
gas burning, or reacting, with oxygen to produce water vapor
Ø energy is released in the form of heat
Ø hydrogen gas explodes – light and sound
¨ Chemical reactions that absorb energy will not occur without a source
of energy.
Ex) water
changed into hydrogen and oxygen gas
Ø Must pass an electrical current through water to decompose water
into hydrogen and oxygen gas
¨ Plants get their energy from trapping and storing sunlight and animals
when they consume plants or other animals.
¨ Humans release energy when they metabolize, or break down, digested
food.
Activation Energy
¨ Chemical reactions that release energy do not always occur spontaneously.
Ex)
The cellulose in paper burns in the presence of oxygen releasing heat and light but will burn only if you light it with a
match.
¨ Activation energy is energy needed to get a reaction started.
¨ Activation energy is a factor in whether the overall chemical reaction
releases energy or absorbs energy.
Enzymes
¨ Some chemical reactions are too slow or have activation energies
that are too high to make them practical for living tissue.
¨ A catalyst
is a substance that speeds up the rate of a chemical reaction.
¨ Catalysts work by lowering a reaction’s activation energy.
¨ Enzymes are proteins that act as biological catalysts.
¨ Enzymes speed up chemical reactions that take place in cells.
¨ Enzymes are very specific, generally catalyzing only one chemical
reaction.
¨ Part of an enzyme’s name is usually derived from the reaction
it catalyzes.
Ex) Carbonic
anhydrase catalyzes the reaction that removes water from carbonic acid.
The Enzyme-Substrate Complex
¨ Enzymes provide a site where reactants can be brought together to react.
¨ This site reduces the energy needed for reaction.
¨ The reactants of enzyme-catalyzed reactions are known as substrates.
¨ The substrates bind to a site on the enzymes called the active site.
¨ The substrate and active site have complementary shapes and are
often compared to a “lock and key”.
¨ Once the reaction is over, the products of the reaction are released
and the enzyme is free to start the process again.
