Elements of Life Practice Test

A phospholipid contains a glycerol backbone (C, H, O), two fatty acid tails (mostly C and H), and a phosphate-containing head group (P and O). In an aqueous solution, phospholipids spontaneously arrange so that the phosphate heads contact water while the fatty acid tails cluster together. The phosphate group’s negative charge and polar bonds create strong interactions with water, while the long hydrocarbon tails interact mainly through weak dispersion forces and avoid water. Which statement best describes the molecular basis for phospholipid self-assembly in water?

A membrane protein has a transmembrane region enriched in amino acids with nonpolar side chains composed mostly of C and H, and an external region enriched in polar side chains containing O and N. The phospholipid bilayer interior is largely hydrocarbon, while the surrounding cytosol is water-based. Nonpolar side chains interact favorably with the bilayer’s hydrocarbon tails via dispersion forces, whereas polar side chains can form hydrogen bonds with water. Which statement best explains why the C- and H-rich segment spans the membrane?