The plasma membrane plays a crucial role in maintaining homeostasis by regulating the movement of substances into and out of the cell. It acts as a selective barrier that allows essential nutrients and ions to enter while keeping harmful substances out. Moreover, it facilitates communication with other cells and responds to external signals. The fluid mosaic model of the plasma membrane highlights its dynamic structure, crucial for processes like endocytosis, exocytosis, and signal transduction, all vital for cellular functions.