The cell membrane is the boundary between a living cell and its outside environment, and it is responsible for regulating what molecules pass into and out of the cell. Cell membranes can be thought of as fluid mosaics of phospholipids and proteins.
Phospholipids are the main component of cell membranes. Each phospholipid molecule has a hydrophilic (attracted to water) head region and a hydrophobic (repelled from water) tail region. To form a membrane, phospholipids are arranged in a bilayer with the hydrophobic tails pointing inside the membrane and the hydrophilic heads facing out.
Phospholipids are constantly moving laterally within the membrane. Cholesterol helps keep membrane fluidity constant by slowing down the movement of phospholipids at high temperatures and by preventing them from packing close together at lower temperatures.
Cells recognize each other by the carbohydrates in their cell membranes. Cell-cell recognition (how one cell recognizes other cells) is important in sorting different types of cells into organs during embryonic development, and it is how the immune system is able to identify and attack foreign cells.
Cell membranes are selectively permeable. Hydrophobic molecules dissolve in the phospholipid bilayer and easily pass through the membrane. Other molecules can be brought into the cell through transport proteins that span the membrane.
Cell membranes are about 8 nanometers thick.