A teacher assigns the following experiment to a class. Draw an outline of a bird's feather, and color one half of the outline with a thick coat from a wax crayon. Leave the other half uncolored. Next, sprinkle a few drops of water on both halves of the feather. You will observe that the water soaks into the plain paper. The waxy coat, however, allows the paper to remain dry. Water forms tiny beads on top of the wax instead of soaking through it. The results of the experiment are MOST USEFUL for explaining which of these functions of lipids in organisms?