Cell Size and Surface Area   a.k.a. “Pink Gel-O”


Purpose:  What is the relationship between diffusion rates, cell size and cell volume? When cells grow to a certain size, their rate of growth slows down until they stop growing entirely.  They have reached their size limit.  At this point, they might under go mitosis or not. When one of these larger cells divides into two smaller cells, the rate of growth again increases.  In this investigation you will explore one of the factors that limits cell size and growth rate.   Materials needed for cell activity and growth must in some way gain entrance into the cell.  At the same time wastes must be eliminated from the cell. 



3 “ice cubes” of  2% gel-o (agar) + 0.04% bromthymol blue 

millimeter ruler

100 ml white vinegar

plastic spoon, plastic table knife, paper towel

small bowl

white piece of paper (to serve as a background to observe the reactions)



  1. Measure the sides of one of the cubes of gel-o.  Record.  Put it aside.
  2. Cut one other cube so that you have one cube ~3x3x3 cm  and one cube ~2x2x2 cm
  3. Cut the third cube into a 1x1x1 cm cube
  4. Place the cubes in the bowl and pour the white vinegar on until the blocks are submerged.  Record the time.  Use the spoon to gently turn the blocks frequently for the next 10 minutes.
  5. Prepare a table and do the *calculations necessary to fill it in while waiting and stirring.

Cube shape

Cube Dimension

Surface Area


Simplest Ratio






















Surface area of a cube = length x width x number of surfaces

Volume of a cube = length x width x height

Ratio of surface area to volume = surface area



The ratio may also be written “surface area: volume”  The ratio should be expressed in its simplest form.  To do this, divide both terms by the second term.  Example:  The surface area of a cube is determined to be 24cm.  The volume is calculated to be 8cm  The ratio of surface area to volume is 24/8 or 3/1 or 3:1.


  1. AFTER 10 MINUTES  remove the gel-o blocks from the vinegar and gently blot them dry.  AVOID OVER HANDLING THE BLOCKS UNTIL THEY ARE BLOTTED DRY.
  2. Measure the depth of diffusion of the H+ ions from the vinegar (the area that has turned clear) into the cubes.
  3. In the space below, draw each cube before and after

Smallest ______cm








Medium ______cm

Largest  ______cm



  1. List the gel-o cubes in the order of the ratios of surface area to volume, and also from the largest to smallest ratio.   How do these two lists compare?
  2. What evidence is there that H+ ions from the vinegar diffuses into the gel-o cube?  What can you conclude about the rate of diffusion of H+ ions?
  3. If the gel-o cubes were living cells and the vinegar H+ ions were a vital substance, which block would have the most efficient ratio of surface area to volume?  Explain.
  4. What happens to the surface area to volume ratio of a cell as it grows?  How would this effect it’s ability to move substances evenly throughout it?
  5. When one cube shaped cell divides by mitosis into two equal parts, how does the volume of each smaller cell compare with the one large cell?  Does the surface area change in the same proportion?  Explain using math..
  6. Propose a hypothesis to answer the following questions.  Why have larger organisms developed from more cells rather than larger cells?  How does cell division affect each new cell’s ability to absorb materials for growth?
  7. Why are most cells very, very thin (height) compared to length and width?