Phospholipid

A phospholipid is a lipid molecule composed of a glycerol backbone attached to two fatty acid tails and a phosphate group head. Phospholipids are amphipathic (having a hydrophobic tail region and hydrophilic head region) and are the primary component of cell membranes, forming lipid bilayers.

• In water, phospholipids spontaneously arrange into a bilayer: the hydrophobic tails cluster away from water, and the hydrophilic phosphate heads face the water. This self-assembly is what creates cell membranes.
• Each phospholipid has two fatty acid "tails" (nonpolar) and one polar "head" (the phosphate group often with an additional polar group like choline or serine). In contrast, a triglyceride has three fatty acid tails and no polar head (so triglycerides don't form bilayers).
• Because of their amphipathic nature, phospholipids allow membranes to be fluid yet selectively permeable: small nonpolar molecules can pass through the hydrophobic core, while charged or large molecules generally cannot without transport proteins.

• If an exam question describes a molecule with "hydrophilic head and two hydrophobic tails" forming membranes, it's describing a phospholipid.
• Be ready to distinguish a phospholipid from other lipids. For example, a common trap is confusing phospholipids with fats: a triglyceride has 3 fatty acids and is entirely hydrophobic, whereas a phospholipid has 2 fatty acids + a phosphate (making it amphipathic).
• Membrane-related questions often hinge on phospholipid behavior. For instance, "what structural feature of phospholipids enables cell membranes to self-assemble in water-" Answer: their amphipathic nature (dual hydrophobic/hydrophilic properties).