Membrane

A membrane is a selective barrier that separates two systems. A biological membrane, also called a plasma membrane, is a thin layer of biochemically active material that surrounds every cell in the body. The cell membrane regulates what goes in and out of the cell, and also controls chemical reactions within the cell. Membranes are made up of lipids (fats) and proteins.

The lipid part of the membrane is made up of a double layer of phospholipids. Phospholipids are molecules that have a hydrophilic (water-loving) head and a hydrophobic (fat-loving) tail. When these molecules are placed in water, they line up so that their hydrophobic tails point inward, away from the water. This creates a barrier between the inside and outside of the cell. Proteins are embedded in this lipid bilayer or float freely in it. Some proteins function as channels that selectively allow certain substances to cross the membrane; others serve as receptors for specific molecules on the outside of the cell.

The structure of membranes allows them to be selectively permeable—they can separate two areas while still allowing some communication between them. For example, cells need to take in nutrients from their environment but they also need to keep harmful substances out. The right balance must be maintained for the cell to function properly. Membranes play an important role in many cellular processes, including energy production, signal transduction, and movement across membranes (transport).