The amount of energy needed to start or continue a reaction is called activation energy. Reactions use energy in the form of ATP to get started. Sometimes energy is released in a reaction in the form of heat. Reactions that release heat are called exothermic reactions. The end result of this type of reaction is generally the breakdown of a larger molecule into several smaller molecules. A type of reaction that requires energy and stores that energy in the bonds of the chemical is called an endothermic reaction. Both types of reactions requires activation energy to get them started. The problem lies in the cell. Sometimes the energy required to get a reaction started is much more then the cell can handle. That is where the enzyme come in. Enzymes are proteins that have a distinct 3 dimensional structure. Enzymes are used by the cell to reduce the activation energy needed to start a chemical reaction. Enzymes also reduce the amount of time a reaction needs to take place. It is for these two reasons that an enzyme is called a catalyst. A catalyst lessons the amount of energy needed for a reaction to occur, lessens reaction time and is not changed in a reaction. All enzymes are catalysts but not all catalysts are enzymes.
All biological reactions use enzymes, because the energy needed to cause a reaction to take place would be so great that the cell would not be able to survive. First it more than likely would not be able to store that much energy, and second it is likely that the heat involved in the release of great amounts of ATP would harm the cell irreparably. Enzymes allow the cell to survive these chemical reactions needed to survive.
Another aspect of enzymes that are important to know is that they are substrate specific. A substrate is a substance that the enzymes act upon. There is only one type of substrate for each type of enzyme. For example, your saliva is filled with salivary amylase, an enzymes that digest certain carbohydrates. Amylase will not digest proteins, lipids or nucleic acids just certain types of carbohydrates. The lock and Key Hypothesis demonstrates that there is only one enzyme for each substrate, just as there is only one lock for each type of key. In this analogy the key is the substrate and the lock is the enzyme. An enzyme will not function without its' substrate just as a lock will not function without its' key.