Comparing Cell Size

Cell Size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm (see image below). The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

This figure shows relative sizes of microbes on a logarithmic scale. Recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured. Image Attribution: OpenStax Biology (CC BY 4.0)

Small size vs big size

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from geometry that the formula for the surface area of a sphere is 4πr², while the formula for its volume is 4πr³/3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube (see image below).

Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. The cell on the left has a volume of 1 mm³ and a surface area of 6 mm², with a surface area-to-volume ratio of 6 to 1, whereas the cell on the right has a volume of 8 mm³ and a surface area of 24 mm², with a surface area-to-volume ratio of 3 to 1. Image Attribution: OpenStax Biology

If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Similarly, another way is to develop organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells called eukaryotic cells.