1. All cells are surrounded by a structure that divides the inside of the cell from the outside. This structure, termed a membrane, is composed mainly of phospholipid molecules. They form into a flat sheet with their hydrophobic tails on the inside of the structure and their hydrophilic head groups on the outside in contact with the surrounding water molecules. Hydrophilic molecules in solution inside or outside of the molecule can not easily cross this hydrophobic barrier.
2. Cell membranes also contain non-lipid molecules, for example membrane proteins. The structure of membranes is described by the "fluid mosaic model". According to this model, membranes are composed of a mosaic of lipid and protein molecules, with the proteins freely moving within the confines of the membrane.
3. Some membrane proteins exist to allow passage of molecules through the membrane. These proteins are specific for particular molecules or classes of molecules.

Pages in this unit:

1. Membranes are not simply made of lipid molecules (the fluid mosaic model)
2. Functions of membrane proteins
3. Transport: protein mediated, endocytosis and exocytosis