Final answer:
Cell size is largely limited by the rate of diffusion, which affects how substances move across the cell membrane. Prokaryotic cells rely on simple diffusion, but eukaryotic cells have adaptations to facilitate transport and support a larger size.
Step-by-step explanation:
The upper limit to the size of a cell is largely determined by the diffusion rate of substances across the cell membrane. Because volume increases much more quickly than cell surface area, a larger cell faces challenges in efficiently moving materials in and out. This diffusion challenge limits cell size, as essential materials and wastes must be transported across the cell's surface.
Prokaryotic cells and eukaryotic cells differ not just in size, but also in complexity. While prokaryotic cells rely on simple diffusion due to their small size, eukaryotic cells have evolved structural adaptations to facilitate transport, such as a network of membranes and organelles, to overcome the limitations of diffusion. The increased surface area provided by these structures supports the larger size of eukaryotic cells.