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The Importance of Surface Area

Is it possible that surface area also determines the maximum size of a cell?

When cells divide, the new cells usually grow to a certain size and then stop growing. They seem to have reached their size limit. If one of these new cells divides, the process is repeated, with the newest cells growing only as large as the parent cells. So there seems to be a limitation to cell size. If it were not for this limitation, perhaps our bodies would be just one huge highly specialized cell instead of several trillion cells. Nutrients needed for growth and activity must enter the cell through its membrane by diffusion. The surface area of the cell membrane determines the quantity of nutrients that will pass into the cell. As the cell grows, its surface area increases, allowing it to absorb more nutrients. But as the cell grows, its volume also increases, creating a demand for more nutrients. In this exercise, you will consider the relationship between the cell's surface area and its volume.

Because living cells are very small and hard to work with, we will create larger-than-life models of cells using agar, which is a gelatine-like substance. The agar contains a pH indicator called bromcresol green. At a pH greater than 5.6, the indicator looks blue, but at a pH less than 4, the indicator turns yellow. Right now, the agar is at a neutral pH of about 7, so the agar looks blue. If we set the agar in vinegar, an acid of about pH 3, the agar should turn yellow. However, the agar won't turn yellow immediately—it will take time for the vinegar to travel into the agar by diffusion.

In this experiment, our cells are represented by the agar cubes, and the vinegar represents a nutrient that enters the cell by diffusion. We will use agar cells of different sizes, then see how much of the cell gets fed with vinegar in a set amount of time.

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