Enzymes are specified proteins that catalyze chemical reactions in biological systems without being permanently changed or used up. This unit was developed to help students gain a more functional, intuitive understanding of how enzymes work, what factors affect enzyme activity, and how enzyme activity can be regulated by organisms. Without enzymes, cells and organisms could not break down or construct molecules quickly, and the metabolic reaction—the chemical reactions in cells and organisms—would proceed slowly. The rate of any reaction in a cell can be turned up or down by changing the activity of the enzyme responsible for that reaction. In this unit, the exercises let students discover firsthand how cells might regulate enzymes.