What is a major structural difference between triglycerides and phospholipids?

The key difference between triglycerides and phospholipids lies in the composition of their structures, particularly concerning the presence of phosphate groups. Triglycerides are composed of one glycerol molecule bonded to three fatty acids. In contrast, phospholipids consist of a glycerol backbone, but they have two fatty acids and one phosphate group attached to it.

In the case of phospholipids, one of the fatty acids in the triglyceride structure is replaced by a phosphate group. This substitution is crucial because it gives phospholipids their unique properties, allowing them to form bilayers in biological membranes, which is essential for the structure and function of cells. This difference is also what contributes to the amphipathic nature of phospholipids; they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts, whereas triglycerides are entirely hydrophobic.

Thus, the correct identification of phospholipids having one fatty acid replaced by a phosphate group highlights the fundamental structural distinction between these two types of lipids.