ABSTRACT
Molecules diffuse, transporting matter and heat. For example, soft drinks lose their fizz when CO2 diffuses out of their plastic containers. Drugs diffuse out of clever encapsulation devices into the body according to tailored time schedules. Metabolites flow in and out of cells through biological membranes. The electrical currents that power household appliances, modern electronics, and neurons result from the flows of ions and electrons that are driven by gradients of electrical potentials. These processes are described by laws of kinetics; they are outside the scope of equilibrium thermodynamics. What are the rates at which molecules flow from one place to another? What forces drive them? In this chapter, we discuss the phenomenology; in the following chapter, we take a more microscopic view.
