Expert: Walter Hintz - 5/9/2006

Question
Why are animals made up of more cells rather than bigger cells?

Answer
Good question David;  I have a whole lecture on this topic but I will try to summerize it for you.  The better question is "Why are cells small?"
 It has to do with a mathematic principle called the scale effect.  In geometry there is a ratio between volume and surface area of and solid body of uniform size and this ratio applies to cells. It tells us that as the size of a cell increases the ratio decreases.  Look at this

 cell size    Surface area    Volume     SA/Vol ratio
    1 mm      6 square mm      1 cubic mm   6-1
    2 mm      24 sq mm         8 cu mm      3-1
    3 mm      54 sq mm         27 cu mm     2-1
 Now consider this
  The gas and nutrient exchanges that a cell must make are proportional to the volume and the exchanges must take place through the surface area.  When we double the volume we decrease the SA proportionally;  the exchanges increase but the surface are needed to do this is less.

  What happens? The cell divides restoring the ratio.