All matter is made up of molecules. Many molecules can peacefully co-exist almost indefinitely. Some molecules, however, cause some kind of reaction when they come into contact with certain molecules. For this reaction to occur, the molecules must be brought extremely close together and in a particular orientation. Activation energy is also involved in many reactions, because typically, reactions also involve the breaking of pre-existing bonds.
A substantial amount of energy is often required for a chemical reaction to take place, due to the strength of the bonds that need to be broken. The amount of activation energy required to start a reaction is often called the energy barrier. This energy is rarely provided by the molecules colliding, so other factors are necessary to help the molecules clear the energy barrier and facilitate the chemical reaction. Heat, a physical factor, and adding an appropriate enzyme, a chemical factor, are two examples of factors that activate molecules.
Once a chemical reaction has started, it often releases enough energy, usually as heat, to activate the next reaction and so on in a chain reaction. This is precisely what happens with a fire. Wood can lie in a woodpile for years without bursting into flames spontaneously. Once set on fire, activated by a spark, it literally consumes itself as the heat that is released supplies the activation energy to keep the rest of the wood burning. Heating a mixture will increase the rate of the reaction.
For most biological reactions, heating is impractical as body temperature is limited to a very small range. Heat can only be used as a way to overcome the energy barrier to a very limited extent before cells are damaged. For the reactions for life to take place, cells must use enzymes to selectively lower the activation energy of reactions.
Enzymes are protein molecules that act as biological catalysts. A catalyst is a molecule that speeds up a chemical reaction, but remains unchanged at the end of the reaction. Almost every metabolic reaction that takes place within a living organism is catalyzed by an enzyme. Enzymes have precise three-dimensional shapes and possess an active site, which is where a molecule can attach itself to the enzyme. The shape of the active site allows certain molecules to bind to it perfectly, so each type of enzyme will usually act on just one type of molecule, called the substrate molecule. Reactions that are catalyzed by enzymes will take place rapidly at much lower temperatures than without them.
For example, during respiration, glucose molecules react with oxygen molecules and are broken down to form carbon dioxide and water and release energy. Because glucose and oxygen are not naturally reactive, a small amount of activation energy must be added to start the respiration process. When one of the substrate molecules binds to the required enzyme, the shape of the molecule will be slightly changed. This in turn makes it easier for that molecule to bind to other molecules or change into the product of the reaction. As such, the enzyme has reduced the activation energy of the reaction, or made it easier for the reaction to take place.
If the energy barrier did not exist, the complex high-energy molecules on which life depends would be unstable and break down much more easily. The activation energy barrier therefore prevents most reactions from taking place. This ensures a stable environment for all living things.