Agar Cube Surface Area

Описание: Why are cells so small? Cells need to allow molecules in and out of their membranes, and if the cell gets too big then the centre cannot be serviced efficiently. This is because, as the size of an object increases, the volume increases at a greater rate than surface area. For a cell, this means that efficiency of the exchange of materials across a membrane is reduced and so the cell may not take in enough nutrients, or toxins may be retained for too long. Using agar cubes with indicator, some vinegar, and some maths, we can see how much effect a small increase in surface area has on volume.

In this practical, students are given agar cubes of three different sizes, impregnated with indicator to display an alkaline pH. Initial measurements make it clear that the surface-area-to-volume ratio is large for small cubes, with the ratio reducing as cube size increases. The cubes are placed in vinegar, and as the acid diffuses into the cell to reduce the pH and change the colour of the indicator, students can see that a small difference in size can make a large difference to the time it takes to completely penetrate to the centre. Students can calculate the results themselves and draw up graphs, or a spreadsheet can be used instead with the equations programmed in, according to your preferences.

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