Cell Size and Compartments

Why cells stay small

As a cell grows, volume increases with the cube of its radius while surface area increases only with the square. The surface-area-to-volume ratio therefore falls.

Since all exchange with the environment happens across the surface while all demand comes from the volume, a cell that grows too large cannot supply itself. This single constraint explains why cells divide, why cells that need high exchange are flattened or folded (microvilli, root hairs, alveoli), and why large organisms are multicellular rather than made of giant cells.

Organelles as compartments

Compartments let incompatible reactions run at once. Lysosomes keep digestive enzymes at pH $5$ away from the cytoplasm; mitochondria maintain a proton gradient across their inner membrane; the nucleus separates transcription from translation.