Cell Size & Surface Area

Biology: IB Diploma Higher Level, topic 2.2.2

Cell Size

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic.

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.
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.

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic.
Let’s examine why that is so.
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 (ie much more rapidly).

So, 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.
- 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; another way is to develop organelles that perform specific tasks.
These adaptations lead to the development of more sophisticated cells called eukaryotic cells.

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.

As mentioned before, most cells are approximately spherical in shape.
A sphere is the shape with the largest surface area-to-volume ratio.
As nutrients diffuse into the cell, a sphere is the shape where nutrients would have to travel the least distance to reach the center.
This is important because nutrients and wastes are always exchanged at the periphery of the cell.
The shorter the distance these nutrients and wastes have to travel, the faster the exchange of these molecules are.