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A cell is enclosed by a membrane layer which is primarily composed of phospholipids, proteins, and carbohydrates.

Phospholipids are polar molecules that spontaneously arrange, often as a continuous bilayer about seven nanometers thick.

The hydrophilic, or water-loving, polar heads face the outside and inside of the cell. The hydrophobic, or water-fearing, non-polar tails line up in the middle to avoid exposure to water.

While phospholipids provide the basic structure, the other components, the proteins and carbohydrates, associate with the phospholipids and contribute to the membrane's function.

Depending on the cell's needs, some proteins function as receptors that detect extracellular signals and transmit them inside the cell.

Others act as transporters to provide a path across the membrane for different types of molecules. Additional proteins are structural links that connect the cytoskeleton to the extracellular matrix or adjacent cells.

Carbohydrates can bind to either the phospholipids forming glycolipids or to proteins on the membranes, creating glycoproteins. Both can bind hormones or neurotransmitters, as well as allow cells to recognize each other.