Enzymes are biological molecules that catalyze chemical reactions. In enzymatic reactions the substrates are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell. A popular example of an enzyme is lipase. Milk contains the enzyme lipase, which assists with the breakdown of fats. In the image below there is a carton of 2% milk that contains lipase.