What are cell membranes primarily composed of?

Answer 1

Phospholipids

Answer 2

The plasma membrane is composed primarily of two types of molecules—lipids, which are fatty or oily molecules, and proteins. The basic structural framework of the plasma membrane is formed by two sheets of lipids, each sheet a single molecule thick. Within this double layer, or bilayer, of lipids, the protein molecules are embedded. Proteins are responsible for a host of functions, including transporting substances across the membrane, aiding communication between cells, and carrying out chemical reactions. In most cells, the plasma membrane is about 40 percent lipid and 60 percent protein, but these proportions vary greatly, from as little as 20 percent to as much as 75 percent protein depending on the type of cell.

Answer 3

The basic structure of the cell membrane is a bilayer of phospholipids with proteins embedded in it. Cholesterol and carbohydrate chains also play a part.

See plenty of diagrams here:

http://www.altavista.com/image/results?itag=ody&q=plasma+membrane&kgs=1&kls=0

Answer 4

Phospholipid bi-layers.