Cells take in nutrients and expel waste through their membranes using two main processes: passive transport and active transport. Passive transport moves substances across membranes without cell energy expenditure, including diffusion, osmosis, and facilitated diffusion using carrier proteins or ion channels. Active transport requires cell energy in the form of ATP to pump substances against their concentration gradient, as seen with the sodium-potassium pump. Endocytosis and exocytosis use vesicles to transport larger molecules into and out of cells. Membrane receptor proteins also allow cells to communicate.