Domains of Life; Prokaryotic vs. Eukaryotic Cells

ID #1001

Can you explain why cells are so small? I am confused on the surface area-to-volume ratio.

If a cell gets any larger than the fairly narrow range of sizes we encounter, it has increased volume for performing more reactions, theoretically allowing for a more complex cell. But there are energy needs for all these reactions, and that energy production requires raw materials, like glucose for example. Also, waste products need to be removed, or they will simply accumulate. But as we've mentioned and will see in more detail very soon, the cell membrane is selective about what it lets across in either direction. Since the amount of surface area doesn't increase proportionally to the increase in volume as the cell gets larger (see the "cubes" in that lecture slide), you can't keep up with the required intake and export, and all those "extra" reactions can't be sustained. So you can't just keep getting bigger.

Conversely, if usable surface area is so important (and it is), you might think that getting smaller would be better. But as you decrease in size, you lose a lot of volume, and now you don't have the necessary space for carrying out even the basic set of reactions required to keep the cell alive, much less anything extra or fancy. So you can't just keep getting smaller, either.

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