Final answer:
Cells are small primarily because a high surface area-to-volume ratio is necessary for efficient diffusion of substances in and out of the cell. Prokaryotic cells are generally smaller than eukaryotic cells and rely on their simplicity for rapid diffusion, while eukaryotic cells have complex structures to aid transport.
Step-by-step explanation:
The fundamental reason cells are small is due to the surface area-to-volume ratio. As a cell grows, its internal volume increases faster than its surface area, leading to a decreased ratio. This makes it harder for the cell to move nutrients and waste products in and out efficiently. The diffusion distance within the cell becomes greater, and the cell cannot sustain itself adequately. Thus, most cells remain small to optimize this ratio and ensure efficient diffusion of substances.
While most cells are small, there are variations in form and function. Prokaryotic cells are typically much smaller than eukaryotic cells, and their simple structure allows for quick diffusion of substances. Eukaryotic cells, being larger, have developed complex organelles and structures to facilitate cellular transport despite their size.