The sac-like shape of lipocytes allowing them to expand to store more fat is an example of the theme "structure relates to function" in biology. This theme states that the form and structure of a biological organism or cell is closely related to its function, and that the specific shape and arrangement of a organism's parts enables it to perform its specific roles and tasks. In the case of lipocytes, their sac-like shape allows them to expand and store more fat, which is essential for their function as fat storage cells.