The only way the cell can get nutrients is through its sides, right? After all, it has to absorb nutrients through the plasma membrane. So, the cell's SURFACE AREA is important in determining how quickly it can uptake nutrients. However, the ENTIRE cell needs the nutrients, so they have to be distributed over the cell's VOLUME. Therefore, the surface area to volume ratio gives an indication of how well a cell will survive.
Now, let's imagine that our cubic cell is 1 micrometer on a side. Since a cube has six sides, its total surface area is 6 times the area of one side. Area = length x width, so one side is 1 micrometer x 1 micrometer = 1 micrometer². Now multiply that by six sides and you get a total surface area of 6 micrometers².
The cell's VOLUME is its length x width x height, or 1 micrometer³. So its surface area to volume ratio is 6 micrometers²:1 micrometer³.
Now let's say the cell grows so that it is 2 micrometers on a side. Now the area of a side is 2 micrometers x 2 micrometers = 4 micrometers². Its total surface area is 24 micrometers².
However, the cell's volume is 2 micrometers x 2 micrometers x 2 micrometers = 8 micrometers³. The surface area to volume ratio is now 24 micrometers²:8 micrometers³, which reduces to 3:1. The surface area to volume ratio decreases as the cell grows.
If the cell gets too large, it won't have sufficient surface area to meets its nutritional needs. So, cells have to divide every so often to keep their overall volume small relative to the surface area.
