This document discusses the movement of substances across the plasma membrane in three chapters. It defines the plasma membrane and explains that substances can pass through it through passive transport mechanisms like diffusion and facilitated diffusion, which move substances down their concentration gradient without energy use, or active transport, which moves substances against their gradient by using cellular energy. Specifically, it describes the structure of the plasma membrane as a phospholipid bilayer with embedded proteins, and the mechanisms of simple diffusion, osmosis, and facilitated diffusion for passive transport and active transport using carrier proteins.