What is the structural difference between a triglyceride and a phospholipid, and how does this difference relate to their functions in the body?

A triglyceride consists of a glycerol backbone with three fatty acid chains attached, making the whole molecule nonpolar. A phospholipid is a modified triglyceride with only two fatty acid chains and a phosphorus-containing polar head group in place of the third fatty acid. Because triglycerides are nonpolar and energy-dense, they are stored as body fat and insulate/protect organs; because phospholipids have both a polar head and nonpolar tail, they are the chief building material for cell membranes.

Structurally, triglycerides are formed when three fatty acid chains are bound to a glycerol molecule by dehydration synthesis, producing a large E-shaped molecule whose hydrocarbon chains are nonpolar. Phospholipids are diglycerides: they have only two fatty acid chains attached to glycerol, plus a phosphorus-containing group. This phosphorus-containing group is polar, while the two fatty acid chains form a nonpolar tail, giving phospholipids both a polar head and a nonpolar tail. Triglycerides, lacking any polar head, are nonpolar and thus function mainly as compact stored energy, insulation, and mechanical protection. The dual polarity of phospholipids lets them serve as the chief structural material of cellular membranes, because their polar heads can interact with water while their nonpolar tails interact with other nonpolar molecules.