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The cell membrane is made up of two different types of molecules, lipids and proteins (you may wish to review the basic structures of these molecules from Chapter 2). We will focus on the lipids in this section and discuss proteins later.
The cell membrane is composed mostly of phospholipids. Recall that phospholipids contain two regions; a polar head (comprised of a phosphate group) and a nonpolar tail (made up of two fatty acid chains). The head region is hydrophilic (water loving) and the tail is hydrophobic (water hating). This poses a problem for the molecule in the aqueous environment of the cell. The lipids solve this problem by forming a lipid bilayer: the hydrophobic tails associate to exclude water, while the hydrophilic heads are left exposed to the watery external and internal environments.
The plasma membrane also contains glycolipids and sterols (such as cholesterol). Proteins that span the membrane (transmembrane proteins) are responsible for transport of molecules and ions.

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