4.2: Cell Size
Cell sizes vary widely among and within organisms. Bacterial cells range between 1-10 micrometers (μm)and are considerably smaller than most eukaryotic cells. The smallest bacteria are 0.1 μm in diameter—about a thousand times smaller than eukaryotic cells, which typically range from 10-100 μm.
Cells can take in nutrients and water via diffusion through the plasma membrane itself or through specific channels in the membrane. The area of the membrane surrounding the cells limits the exchange rate of these materials. Smaller cells tend to have a higher surface area-to-volume ratio than larger cells. When a sphere increases in size, the volume grows proportionally to the cube of its radius, while its surface area grows proportional to the square of its radius. Smaller cells have relatively more surface area compared to their volume than larger cells of the same shape. A larger surface area means more plasma membrane where materials can pass into and out of the cell. Substances also need to travel within cells. As a result, the diffusion rate may limit processes in large cells.
Prokaryotes are often small and divide before they face limitations due to cell size. Larger eukaryotic cells have organelles that facilitate intracellular transport and structural changes that help overcome limitations. Some cells that must exchange large amounts of substances with the environment develop long, thin protrusions that maximize the surface area to volume ratio. An example of such a structure is the root hair of plant cells that facilitate water intake and nutrients.