Rana's Cell Size and Diffusion Argumentation

Rana Srouji

AP Biology

Mr. Hammer

December 1, 2014

Cell Size and Diffusion Argument

Diffusion is the movement of molecules from a high concentration to a low concentration.  Almost all living organisms have cells that are dependent on diffusion to obtain the essential nutrients needed to survive.  Once the cells take in these nutrients, they break them down to create more components for the cell.  The cell size will then increase, but cells always remain small.  An explanation for this is, cells that have a larger surface area to volume ratio are more efficient at diffusing essential nutrients.

My lab partners and I proved this explanation by creating and testing models of cells using agar.  Originally, the agar was blue in color.  We created four model cells of agar, two of which represented small cells while the other two represented larger cells.  The cells were cut into rectangular prisms for easy calculations.  The small cells were cut with length of 2 cm, width of 1.5 cm, and height of 1.5 cm.  Their volumes were 4.5 cm cubed, and their surface areas were 16.5 cm squared.  The larger cells were cut with length of 4 cm, width of 1.5 cm, and height of 2.5 cm.  The volume of the large cells were 15 cm cubed and their surface area were 39.5 cm squared.  The surface area to volume ratio for the small cells is 3.67 while the ratio for the large cells is 2.63.  Afterwards, all of the cells were then placed into a plastic container filled with vinegar.   All of the cells were placed in the vinegar for 27 minutes.  Within seconds the acid began to diffuse into the agar.  Once the cells were taken out of the vinegar, we took the measurements of the blue colored agar remaining, which is the amount that had not been diffused.  The remaining blue agar in the small cells had a length of 1 cm, width of 0.5 cm, a height of 0.5 cm, and a volume of 0.25 cm cubed.  The amount remaining in the large cells had a length of 3 cm, width of 1 cm, height of 0.5 cm, and volume of 1.5 cm cubed.  In addition, using the volumes, we found that of the small cells only 5.6% of each cell was not diffused within the 27 minutes, and 94.4% of each cell was diffused.  Of the larger cells, 10% of each cell was not diffused, while 90% was diffused.  

Additionally, we used the agar in our experiment because it is a gel-like substance that chemicals can diffuse through, and that can be cut into different shapes.  We created the model cells in two different sizes to show the difference in the amount diffused within the same amount of time, and the difference in the surface area to volume ratios.  It was to show that the diffusion was directly related to the ratio.  The larger cells have a smaller ratio, while the smaller cells have a larger ratio.  Also, we created two cells per size for both the small and the large cells in case any error were to occur.  Phenolphthalein was previously added to the agar before we used it.  The phenolphthalein is a chemical indicator that changes color when coming into contact with acid, so that the amount of acid diffused through the agar would be easily seen.  Once the cells were placed in vinegar, their color would change from blue to yellow because vinegar is an acid which will diffuse through the cell and change the color once it comes in contact with the phenolphthalein.  In conclusion, cells are small because they have a large surface area to volume ratio, which allows them to diffuse most efficiently.  The larger cells had a less amount diffused, only 90% of each large cell diffused, and a smaller surface area to volume ratio, (a ratio of 2.63).  Unlike the small cells which had a larger surface area to volume ratio, of 3.67, and diffused 4.4% more than the large cells, within the same amount of time.  Thus, the cells that have a larger surface area to volume ratio are more efficient at diffusing essential nutrients.  

However, the reasoning for why cells are so small cannot be because the rate of diffusion is related to cell size.  Nutrients do not diffuse at a faster rate through small cells than they do through large cells.  This is because, as my group tested the agar cells, we collected data that showed the distance of diffusion through the cells.  We gathered this data in order to find the rate of diffusion, since the rate of diffusion is the smallest distance of diffusion per cell divided by the amount of time diffused.  Both the small and large cells had the same distance of diffusion, which was 0.5 cm.  The rate of diffusion then is 0.5 divided by the 27 minutes, which then equals 0.019.  Thus, the rate of diffusion cannot be related to the cell size since both the small and large cells have the same rate of diffusion.  

This is an image of the "cells"/agar as soon as they were placed into the container of vinegar.  The acid began to diffuse into the cells as seen by the light yellow coloring on the outer edges of the agar.  

This is an image of the cells/agar after the 27 minutes, once they have been removed from the acid.  The blue inside is the amount that has not been diffused, the yellow coloration is the acid that diffused though the agar and has come into contact with the phenolphthalein.