It is made up of biological molecules, lipids and proteins involved in many functions, including cellular communication and the transport of materials into and out of the cell . consisting of a double layer known as a bilayer of phospholipid molecules and also contains protein molecules. Phospholipid molecules lie 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 touching each other. This seals the membrane, making it impermeable 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 for larger molecules the use of proteins is necessary. There are different types, including intrinsic proteins that cross the membrane, and then others that are only found on the inner or outer surface of the membrane, called extrinsic proteins. The membrane is formed in such a way as to protect the cell from the external environment and therefore contains almost everything it needs to function. These proteins allow substances to cross the surface membrane in different ways. These processes can be passive or active, examples of these processes