The plasma membrane functions as a selective barrier controlling what passes in and out of cells. It is composed of lipids and proteins arranged in the fluid mosaic model, where lipids form a fluid bilayer and proteins float freely within it. Materials cross the membrane through passive transport mechanisms like diffusion, osmosis, and facilitated diffusion which do not require energy, or active transport processes like carrier-mediated transport, endocytosis, and exocytosis which use cellular energy.