In ► Chap. 3 on diffusion, we already explored how fast chemical reactions can occur for a two-state model (► Sect. 3.4.4) or for two molecules that need to meet in space (► Sect. 3.4.5), and how long it takes for chemical equilibrium between reactants and products to establish. Many biological processes, such as the formation of amino acid chains, would take far too long on their own and are catalyzed by enzymes. Additionally, nature uses chemical energy, for example from the hydrolysis of ATP, to speed up biological processes or to induce conformational changes in certain enzymes, allowing them to move directionally. These molecular motors enable the cell to transport substances over long distances, where diffusion alone would be too slow. Another process that allows the cell to move and dynamically regulate its membrane boundary is the polymerization of various cytoskeletal components, especially actin. This process also relies on the conversion of chemical energy into mechanical movement, resembling that of a conveyor belt or caterpillar track. In this chapter, we have grouped all these processes under the name “active” processes, and they can be well explained from the perspective of enzyme kinetics. After some examples at the beginning of the chapter, we will describe them using the Michaelis-Menten equation and then go into more detail about the dynamics of molecular motors and the dynamics of fiber proteins in a cell.