There are several modes of secretion that cells use to transport molecules. The main modes are exocytosis, where vesicles fuse with the cell membrane to release their contents to the outside; and active transport, where proteins in the cell membrane use energy to pump molecules across the membrane against a concentration gradient. Examples of cells that use these modes include neurons releasing neurotransmitters via exocytosis and kidney tubule cells pumping ions like sodium and potassium via active transport.