The cell membrane is made up of a phospholipid bi-layer. One end of each phospholipid is hydrophobic (avoids water) and the other end is hydrophilic (attracted to water). There are also a number of protein and carbohydrate structures which perform many functions including giving structural support to the membrane, acting as channels for materials entering and exiting the cell and receptors which are used for various purposes including cell to cell communication.
