Molecules can coexist peacefully or cause reactions when brought close together with activation energy. Heat and enzymes can activate molecules, with enzymes being biological catalysts that lower activation energy. Enzymes have active sites that bind to substrate molecules, making it easier for them to react and release energy. The activation energy barrier prevents unstable reactions and ensures a stable environment for living things.
All matter is made up of molecules. Many molecules can coexist peacefully almost indefinitely. Some molecules, however, cause some kind of reaction when they come into contact with certain molecules. For this reaction to take place, the molecules must be brought very close together and in a particular orientation. Activation energy is also involved in many reactions, because reactions typically also involve the breaking of pre-existing bonds.
Often a significant amount of energy is required for a chemical reaction to occur, due to the strength of the bonds that need to be broken. The amount of activation energy required to initiate a reaction is often called the energy barrier. This energy is rarely provided by the colliding molecules, so other factors are needed to help the molecules release the energy barrier and facilitate the chemical reaction. Heat, a physical factor, and the addition of an appropriate enzyme, a chemical factor, are two examples of factors that activate molecules.
Once a chemical reaction has begun, it often releases enough energy, usually in the form of heat, to start the next reaction and so on in a chain reaction. This is exactly what happens with a fire. Wood can sit in a woodpile for years without spontaneously catching fire. Once set on fire, activated by a spark, it literally burns out as the heat that is released provides 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, rewarming is impractical as body temperature is limited to a very small range. Heat can only be used to overcome the energy barrier to a very limited extent before cells are damaged. For the reactions of life to take place, cells must use enzymes to selectively lower the activation energy of the 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 occurs 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 to the enzyme. The shape of the active site allows certain molecules to bind tightly to it, so each type of enzyme will usually act on only one type of molecule, called a substrate molecule. Reactions catalyzed by enzymes will occur 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. Since glucose and oxygen are not naturally reactive, a small amount of activation energy must be added to initiate 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 turn into the product of the reaction. As such, the enzyme reduced the activation energy of the reaction or made it easier for the reaction to proceed.
If the energy barrier did not exist, the complex high-energy molecules on which life depends would be unstable and decompose much more easily. The activation energy barrier therefore prevents most reactions. This ensures a stable environment for all living things.
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