Cell membranes are very important structures to cells because they function as a barrier between the components of the cell and the outside environment. The cell membrane is not only responsible for creating a wall between inside and outside the cell, it must also act as a threshold through which select molecules can enter and exit the cell when necessary. The cell membrane is what defines the cell and keeps its components separate from outside cells or organisms.
The cell membrane is composed of a double layer of fat cells called a lipid bilayer in which membrane proteins are embedded. The structure of the lipid bilayer prevents the free passage of most molecules into and out of the cell. We will begin our discussion of the structure of the cell membrane by discussing the structure and properties of the lipid bilayer. We will then go on to discuss the role of membrane-bound proteins, and finally, will discuss membrane transport structures.

Active transport - The transport of molecules across a membrane and against their natural flow; mediated by carrier proteins and requiring outside energy.
Carrier protein - A protein responsible for mediating the active transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by a conformational change that occurs within the protein that forms an opening for specific molecules to pass through.
Channel protein - A protein responsible for mediating the passive transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by its membrane-spanning hydrophilic structure which, when open, allows molecules to pass through.
Diffusion - The transport process in which molecules naturally travel from an area of higher concentration to an area of lower concentration.
Glycocalyx - A layer of carbohydrates that coats the exterior of higher-ordered cells. Functions in protecting the cell from damage.
Hydrophilic - A polar molecule that selectively associates itself with water through hydrogen bonds.
Hydrophobic - A nonpolar molecule that does not readily associate with water through hydrogen bonds.
Integral protein - A membrane protein that cannot be easily removed from the lipid bilayer.
Ionophore - A class of membrane transport proteins. Small, hydrophobic molecules that increase membrane permeability to certain ions.
Lipid bilayer - A thin double layer of phospholipid molecules. Provides the structure of a cell membrane. Structure is a result of hydrophobic and hydrophilic forces.
Lipid-bound protein - Membrane proteins that are located entirely within the lipid bilayer, having no part touching either the inside or outside of the cell.
Multi-pass protein - Transmembrane proteins that cross the lipid bilayer more than one time.
Osmosis - The process by which water naturally travels from an area of high concentration to one of lower concentration.
Passive transport - Transport mediated by channel proteins. The movement of molecules across a membrane according to the natural flow.
Peripheral protein - A membrane protein that can be easily removed from the lipid bilayer.
Single-pass protein - A transmembrane protein that only crosses the lipid bilayer one time.
Transmembrane protein - A membrane protein that spans the lipid bilayer having portions in contact with both the inside and outside of the cell. Area within the lipid bilayer forms an alpha-helix.
Lipid Bilayer Structure

The lipid bilayer is a universal component of all cell membranes. Its role is critical because its structural components provide the barrier that marks the boundaries of a cell. The structure is called a "lipid bilayer" because it is composed of two layers of fat cells organized in two sheets. The lipid bilayer is typically about five nanometers thick and surrounds all cells providing the cell membrane structure.

Lipids and Phospholipids

The structure of the lipid bilayer explains its function