At any given moment, all of the work that is done inside a cell is being done by enzymes. If you understand enzymes, you understand cells. A bacterium like E. coli has about 1,000 different types of enzymes floating around in the cytoplasm at any given time.
Enzymes have extremely interesting properties that make them little chemical-reaction machines. The purpose of an enzyme in a cell is to allow the cell to carry out chemical reactions very quickly. These reactions allow the cell to build things or take things apart as needed. This is how a cell grows and reproduces. At the most basic level, a cell is really a little bag full of chemical reactions that are made possible by enzymes!
Enzymes are made from amino acids and are proteins. When an enzyme is formed, it is made by stringing together between 100 and 1,000 amino acids in a very specific and unique order. The chain of amino acids then folds into a unique shape. That shape allows the enzyme to carry out specific chemical reactions. An enzyme acts as a very efficient catalyst for a specific chemical reaction by speeding up the reaction tremendously.
Inside a bacterium, there are about 1,000 types of enzymes (lactase being one of them). All of the enzymes float freely in the cytoplasm waiting for the chemical they recognize to float by. There are hundreds or millions of copies of each different type of enzyme, depending on how important a reaction may be to a cell and how often the reaction is needed.
For example, the sugar maltose is made from two glucose molecules bonded together. The enzyme maltase is shaped in such a way that it can break the bond and free the two glucose pieces. The only thing maltase can do is break maltose molecules, but it can do that very rapidly and efficiently. Other types of enzymes can put atoms and molecules together. Enzymes’ job is to break molecules apart and put them together, and there is a specific enzyme for each chemical reaction needed to make the cell work properly.
An example of how crucial enzymes are to proper cell function can be found in those who are lactose intolerant. The problem arises because lactose, the sugar in milk, does not get broken into its glucose components, which keeps it from being digested. The intestinal cells of lactose-intolerant people do not produce lactase, the enzyme needed to break down lactose. This problem shows how the lack of just one enzyme in the human body can lead to problems. Lactose intolerance is easily fixed by swallowing a drop of lactase prior to consuming dairy, but many other enzyme deficiencies are not as easily fixed.
Enzymes do all of the work inside cells from breaking glucose down for energy to build cell walls to constructing new enzymes and allowing the cell to reproduce. Even the slightest inconsistency in an enzyme can cause problems with in the cell.