Cells require a constant supply of glucose and oxygen to produce energy.

Cellular respiration occurs in the mitochondria of a cell. As a result of this process, large quantities of waste carbon dioxide must be expelled from the cell. All materials involved in this reaction are either imported or exported through the cell membrane by diffusion. Diffusion occurs quickly over short distances. For example, a molecule of oxygen can diffuse from the centre of a small cell to the outside, through the cytoplasm and cell membrane, in less than one second. However, the same molecule would take longer to diffuse from the centre of a larger cell to outside of the cell. Since the rapid import and export of chemicals is critical to the efficient operation of cells, the mechanism of diffusion effectively places a limit on cell diameter.

In this demonstration, students soak agar “cells” containing phenolphthalein indicator in a solution of sodium hydroxide, NaOH(aq). Students observe the effect of cell volume and surface area on the extent of diffusion of the sodium hydroxide solution through the “cell”.

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