A major type of lipid found in cell membranes is the phospholipid. The phospholipid is made up of a polar head and two non-polar tails.
The composition of a water-loving negatively charged phosphate head and two water-fearing tails causes billions of phosopholipids to organize naturally into a three-dimensional spherical bilayer when suspended in water. Two phosopholipids join together using their tails until billions have clustered side by side and the sheet curves into a closed sphere. This three-dimensional spherical bilayer becomes the cell membrane. The tails are fatty acid chains, and their length and properties determine the fluidity of the cell membrane. When animals hibernate in the winter at a reduced temperature, the type and amount of phosopholipids adjust to maintain the fluidity of the membrane.