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Cell Membranes Problem Set

Problem 1: Membrane components

Tutorial to help answer the question

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

C. lipid, carbohydrate

D. nucleic acid, lipid


Graphic of Fluid Mosaic Model for Cell Membrane


Phospholipid molecule

Areas of the molecule enriched for "C" and "H" are hydrophobic.

elements of the phospholipid molecule: Hydrogen, Carbon, Nitrogen, Oxygen, Phosphate

Fatty acids tails are composed of carbon and hydrogen and are hydrophobic.


Phospholipids are amphipathic, and have hydrophilic and hydrophobic character in the same molecule. Interactions with water cause the phospholipids to form a bilayer, with hydrophobic fatty acid tails inside the bilayer and hydrophilic groups exposed to water.

This basic structure makes the membrane a barrier to hydrophilic molecules. The interior of the membrane is fluid, with a viscosity about equal to olive oil. Cholesterol in membranes alters fluidity.


Proteins of membranes are divided into two general classes:

Integral and peripheral proteins of  membranes
  • Integral membrane proteins penetrate or are embedded in the phospholipid bilayer. Some membrane proteins have one or more alpha-helical regions that can span the bilayer multiple times.

  • Peripheral membrane proteins lack hydrophobic regions, and they frequently associate with integral membrane proteins or hydrophilic surfaces of the bilayer.

Transport of polar molecules through the cell membrane usually requires channels made up of proteins that span the lipid bilayer. Thus, we associate the majority of functional properties of cell membranes with the membrane proteins. The lipid is predominately a barrier. Later we shall see that both lipids and proteins of membranes can function in cell signaling.

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