Which type of lipids is classified as amphipathic?

Phospholipids are classified as amphipathic lipids because they possess both hydrophilic (water-attracting) and hydrophobic (water-repelling) properties. The structure of a phospholipid includes a hydrophilic "head," which is polar and interacts well with water, and two hydrophobic "tails," which are nonpolar and avoid water. This unique configuration allows phospholipids to form bilayers that are fundamental to cell membrane structure, enabling the separation of the internal environment of the cell from the external environment while allowing selective permeability.

In contrast, triglycerides consist of a glycerol molecule bonded to three fatty acids, making them hydrophobic and not amphipathic. Steroids, which are characterized by a four-ring structure, also do not have distinct hydrophilic and hydrophobic regions. Fatty acids, while they can have a hydrophilic carboxyl group, typically behave as hydrophobic molecules due to their long hydrocarbon chains. Thus, phospholipids stand out as the primary type of amphipathic lipid critical for biological functions.