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Cell Membranes


The Lipid Bilayer

Membrane Proteins

Problem : Identify the lipid bilayer in the following diagram of a cell.


The lipid bilayer is the outer-most layer surrounding the cell.

Problem : What is the main function of the lipid bilayer?

The lipid bilayer acts as a barrier between the inside and outside of the cell. It is highly impermeable and does not allow most molecules to freely pass through it into or out of the cell.

Problem : Why is the structure called a lipid bilayer?

It is called a lipid bilayer because it is composed of two layers of fat, or lipid, molecules.

Problem : Fill in the blanks.
A phospholipid molecule contains two distinct regions. The __________ region is attracted to water and the ___________ region is repelled from water. As a result of its both polar and nonpolar regions, it is classified as a(n) __________ molecule.

Hydrophilic. Hydrophobic. Amphipathic.

Problem : Are the two layers of the lipid bilayer identical in composition?

No. As a result of the bilayer's fluidity, structures such as lipids and proteins can freely move around within the lipid bilayer.