A cell membrane consists of a lipid bilayer, which is a double layer of phospholipids. Each phospholipid has a hydrophilic phosphate head and a
hydrophobic lipid tail. These phospholipids are not chemically bound to
one another, but rather maintain their structure due to the fact that the
lipid tails do not like water. They're hydrophobic. If they all buddy up, then
they can all stay dry. (See the figure below.) What is the function of the
phospholipid bilayer ?

The answer is;  hydrophilic heads; hydrophobic tails

Explanation:

The hydrophilic heads of a phospholipid are composed of a phosphate and glycerol molecule. Since the phosphate has a charge, the head are able to interact with water molecules through hydrogen bonds (Remember water is a polar molecule).  The hydrophobic tails are composed of hydrocarbon chains that are uncharged hence do not interact with water. When phospholipids are immersed in water, they form a bilipid layer - with the hydrophobic  tails clumped up together in the center and the hydrophilic heads on the outside.

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