Exocytosis is a biological process where cells transport molecules out of their interior. This occurs when vesicles, which are small membrane-bound sacs, fuse with the cell membrane. Once fused, the contents of the vesicle are released into the external environment. This process is essential for various cellular functions, including the secretion of hormones and neurotransmitters. During exocytosis, the cell uses energy in the form of ATP to facilitate the movement of vesicles. This mechanism is crucial for maintaining cellular communication and regulating the composition of the extracellular environment. It plays a significant role in processes such as digestion and immune responses.