The function of the plasma membrane in regulating cellular activity is foundational for maintaining homeostasis within cells. This selectively permeable barrier controls the movement of substances in and out of the cell, allowing essential nutrients to enter and waste products to exit. It plays a critical role in communication and signaling between cells by containing receptors that respond to external stimuli. Additionally, the plasma membrane is involved in various cellular processes, such as endocytosis and exocytosis. By maintaining a stable internal environment, the plasma membrane is crucial for overall cellular health and function.