Biological membranes are bilipid layers . In a real cell the membrane
phospholipids create a spherical three dimensional lipid bilayer shell
around the cell. However, they are often represented two-dimensionally
as:
Each 
represents a phospholipid. The circle, or head,
is the
negatively charged phosphate group and the two tails are the two highly
hydrophobic hydrocarbon chains of the phospholipid. The tails of the
phospholipids orient towards each other creating a hydrophobic environment
within the membrane. This leaves the charged phosphate groups facing out
into the hydrophilic environment. The membrane is approximately 5 nm thick.
This bilipid layer is semipermeable, meaning that some molecul