The document discusses the structure and function of the plasma membrane. It notes that the plasma membrane is a phospholipid bilayer that forms a fluid mosaic with embedded proteins. This structure allows the membrane to regulate what passes in and out of the cell while maintaining the cell's shape. The document also outlines the different types of passive transport including diffusion, osmosis, and facilitated transport. It describes active transport processes like pumps and endocytosis that require cellular energy to move molecules against gradients.