The ____ portion of the cell membrane functions as a barrier while the ____ portion determines specific functions, including pumps, receptors, adhesion, etc.

A. carbohydrate, nucleic acid

B. lipid, protein

Yes, lipids of membranes form a barrier to hydrophilic molecules, including sugars, proteins, nucleic acids, charged ions and other polar molecules. While lipids form the barrier, we will later show that lipids can participate in membrane signaling.

The majority of the functions of membranes, including transport, receptors, and adhesion, are determined by membrane proteins. The fluid mosaic model of membrane structure says that peripheral proteins (associated with membrane lipids and proteins) and integral proteins (embedded in or passing through the lipids) float in the lipid bilayer.

The inside of the membrane has a viscosity about equal to olive oil (very fluid). Cholesterol is also part of cell membranes and alters membrane fluidity.

C. lipid, carbohydrate

D. nucleic acid, lipid