Cell Surface Area vs Volume
Every living organism is made up of tiny cells. The average human cell is about 50 micrometers or 0.05 mm in diameter. Why are organisms made up of so many tiny cells instead of fewer large cells? The answer to that question is partly due to the relationship between the surface area of a cell and it’s volume.
Everything that the cell needs in order to survive must diffuse through the cell membrane and travel to the location at which it is needed inside the cell. At the same time, wastes that accumulate in the cell must pass out of the cell through the cell membrane. The mechanism for this movement of materials and wastes is diffusion, which is random movement. Nutrient and waste particles bounce around as they collide with other particles. The process of diffusion is very efficient over extremely tiny distances. However, it is a very poor mechanism for transport over large distances. If cells were much larger than they are, it would take so long for nutrients to diffuse from the surface to the centre that the cell would not function properly. To function best, a cell needs a large surface area and a small volume.
The applet below allows you to easily see the relationship between surface area and volume of as cell as it increases in size. You can enter either a volume or a surface area in the labelled boxes and then click the Add Point button. Points for both surface area and volume will be plotted on the graph. When you have plotted enough points to get the idea, click the Graph button and the applet will join the points that you plotted. Click on the Surface Area to Volume ratio button to see what happens to this ratio as a cell becomes larger.
What did you notice about the ratio of surface area to volume for large cells compared to small cells? What would limit a cell from growing very large without dividing? What benefits would a cell have if it divided to create more small cells rather than fewer large ones? What disadvantages would there be?