The cell membrane is the outer layer around cells that is flexible. The cell membrane limits the passage of large molecules into the cell and separates the cell from its external environment.
The cell membrane is described as semi-permeable because only certain molecules are allowed to pass through the membrane.
The cell membrane is made up of two layers of phospholipids. Phospholipids have a phosphate head (shown in orange) and two fatty acid tails (shown in red). The phospholipid molecules arrange themselves into a bilayer because the phosphate heads of the molecules are attracted to water molecules and the fatty acid tails repel water. When a molecule is attracted to water it is known as hydrophilic and when it is repelled by water it is known as hydrophobic.