The cell membrane separates a cell from its watery environment. It does so because it is composed of water-insoluble phospholipids and proteins. The phospholipids form a bilayer sheet surrounding the cellular contents; much like a bubble surrounds air. Proteins are embedded in the phospholipid bilayer and they can move laterally throughout the lipid layer. The proteins serve to limit what goes in and out of the cells by forming channels or by other active processes. Other proteins act as receptors for hormones and other regulatory factors.
The phospholipids consist of a three-carbon glycerol molecule with two long-chain fatty acids chemically attached to two of the carbons and a phosphate and small water-soluble organic group attached to the third carbon. Fatty acids are typically 12-24 carbons long (making them water insoluble) with a chemical group on the end that can attach to the glycerol molecule. Thus one side of the phospholipid is water insoluble and goes to the inside of the bilayer, whereas the other side is water soluble and is located on the watery outside or inside (cytoplasmic) surface of the membrane. The water insoluble parts attract each other so that millions of phospholipids form the bilayer structure described above.
The proteins can be completely inserted into the phospholipid bilayer or they can be peripheral proteins. Proteins consist of long chains of twenty different amino acids. Inserted proteins usually are arranged so that their chains go back and forth through the membrane seven times. The amino acids of the proteins are water-insoluble for the parts that go through the membrane and water-soluble for the parts that stick out into the outside of the cell or the cytoplasm. Often times these proteins form channels for specific chemicals to enter or leave the cell. Other membrane proteins act as enzymes, and their chemical reactions provide the energy for influx or efflux of chemicals.
Carbohydrates (sugars) represent another molecular family present on the outside surface of the cell membrane. Sugar molecules are attached to certain membrane lipids (called glycolipids) or membrane proteins, allowing these structures to be more water soluble. The sugar molecules also serve as receptors for other cells or proteins, such as antibodies. For example, whether your blood type is O, A, B, or AB is determined by specific sugar structures on your blood cell surface glycolipids or glycoproteins.
The membrane proteins that are receptors are usually peripheral to the membrane. Upon binding to a specific regulator (like a hormone), the receptor protein can change its shape, which, in turn, changes the shape of other embedded membrane proteins. This shape-change signal is further transmitted by inducing other chemical reactions or shape changes by proteins inside the cell. Thus, signals from the outside of a cell can be transmitted to the inside of the cell, changing a cell's behavior.
In addition to surrounding the outside of cells, membranes also surround internal structures of the cell, called organelles. Organelles surrounded by a membrane include the nucleus, mitochondria, endoplasmic reticulum, Golgi bodies, vacuoles, and lysosomes. Chloroplasts in plants are also surrounded by a membrane. Plant cells have an additional cell wall, composed of cellulose, surrounding their cell membrane.