What term is used for molecules that have both hydrophobic and hydrophilic ends?

The term for molecules that possess both hydrophobic (water-repelling) and hydrophilic (water-attracting) ends is amphipathic. This characteristic is crucial in biological systems, particularly for the formation of cellular membranes.

Amphipathic molecules can arrange themselves in such a way that their hydrophilic parts face outward towards the aqueous environment, while their hydrophobic regions are oriented towards each other, away from water. This property is fundamental in the structure of phospholipids, which are key components of cell membranes, allowing for the formation of lipid bilayers that create a barrier separating the internal environment of the cell from the external environment.

Hydrophobic molecules do not have this dual affinity; they are entirely water-repelling. Polypeptides are chains of amino acids and do not inherently possess both hydrophobic and hydrophilic characteristics unless specified by their amino acid composition. Steroids, while they may have some polar functional groups, are primarily composed of hydrophobic carbon rings and do not fit the definition of amphipathic as clearly as phospholipids do. Thus, amphipathic accurately describes the unique structural property of such molecules.