Consists of biological molecules, lipids, and proteins involved in many functions, including cellular communication and the transport of materials into and out of the cell. made up of a double layer known as a bilayer of phospholipid molecules and also contains protein molecules. Phospholipid molecules sit on the inner and outer sides of the cell membrane with their tails touching. They have hydrophilic (water-loving) heads on the outside and then hydrophobic tails that touch each other. This seals the membrane, making it impervious to water-soluble molecules. To survive the cell needs materials to enter and exit. Some gases and small ions can pass through the bilayer, but larger molecules require the use of proteins. There are different types of them, including intrinsic proteins that directly cross the membrane and then there are others that are only found on the inner or outer surface of the membrane and are called extrinsic proteins. The membrane is formed to protect the cell from the external environment and therefore contains almost everything it needs to function. These proteins allow substances to pass through the surface membrane in several ways. These processes can be passive or active, examples of these processes