Membranes are thin layers of material that separate different environments or compartments. They can be found in various forms, such as biological membranes that surround cells, or synthetic membranes used in filtration processes. These structures play a crucial role in controlling the movement of substances in and out of spaces, ensuring that essential materials are retained while waste products are removed. In biological systems, membranes are primarily composed of lipids and proteins, forming a phospholipid bilayer. This arrangement allows for selective permeability, meaning that only certain molecules can pass through. Membranes are vital for processes like cell communication, nutrient transport, and maintaining homeostasis within organisms.