A membrane is a selective barrier that surrounds cells, controlling the movement of substances in and out. It is primarily composed of a phospholipid bilayer, which provides structural integrity while allowing for fluidity and flexibility. Membranes play a crucial role in maintaining homeostasis within cells, facilitating communication and transport. In addition to cell membranes, membranes can also refer to structures in various biological systems, such as neuron membranes that help transmit signals. Membranes are essential for processes like osmosis and diffusion, ensuring that cells function effectively and respond to their environment.