Lipids Practice Test

A student builds artificial membranes using phospholipids. Each phospholipid has a glycerol backbone, two fatty acid tails, and a phosphate-containing head group. In water, the phosphate head is polar and interacts with water, while the hydrocarbon tails are nonpolar and avoid water. When phospholipids are shaken in water, they spontaneously form bilayers with heads facing outward and tails facing inward, creating a hydrophobic interior. Which statement best explains why a small nonpolar molecule crosses this bilayer faster than a charged ion?

A phospholipid has a glycerol backbone with two fatty acid tails and a phosphate head group. In one variant, the head group includes an additional charged group, increasing its overall polarity; the tails are unchanged. Increased head-group polarity strengthens interactions with water molecules at the membrane surface, while the hydrophobic tails still cluster away from water. When bilayers made from the two variants are compared in water, both remain intact. Which statement best predicts a molecular-level difference caused by the more polar head group?