The cell membrane has a bilayer structure with phospholipids arranged tail-to-tail and proteins free to move within. It defines the cell boundaries and contains specialized proteins for functions like receptor and transport roles. Diffusion and facilitated diffusion allow passive movement of solutes down concentration gradients through membrane pores or protein channels. Osmosis allows water to diffuse through aquaporin pores down its concentration gradient. Active transport uses proteins like the sodium-potassium pump and endocytosis and exocytosis involve vesicle transport of substances into and out of the cell using membrane invagination and fusion.
