A membrane is a thin, flexible layer that acts as a barrier or filter. It can be found in various biological and artificial contexts. In biology, membranes surround cells and organelles, controlling the movement of substances in and out. They are primarily composed of lipids and proteins, forming a structure known as the phospholipid bilayer. In addition to biological membranes, there are synthetic membranes used in technology, such as in water filtration systems and fuel cells. These membranes can selectively allow certain molecules to pass while blocking others, making them essential for processes like osmosis and diffusion.