Relative sizes of prokaryotic and eukaryotic cells
In lecture on Friday, we introduced the two basic classes of cellular organization: eukaryotes, which surround their genetic material (DNA) with a membrane to form a structure called the nucleus, and prokaryotes, which lack a membrane-bound nucleus. Although the two types of cells differ greatly in a number of ways, one way that they differ is in size. Prokaryotic cells are generally smaller than eukaryotic cells, and then we launched into a brief discussion of why this might be so. In this image from our textbook, we can see a human cheek cell (a eukaryote) that is covered with many bacteria (prokaryotes,) and it should be immediately apparent that the human cell greatly dwarfs the bacteria. This relationship generally holds true when comparing eukaryotic cells with prokaryotic cells. In our explanation of why this is the case, we used the concepts of surface area and volume of a cell to try and understand what happens when a cell gets bigger.
In order to explain this in a different way, consider two balls constructed of a porous material such as a sponge. The balls are in every way identical in terms of composition, however one of them is ten times wider than the other, so that the first has a radius of 1 centimeter, and the second has a radius of 10 centimeters. To appreciate how much bigger the second one is, we could weigh them on a scale. Since they are composed of the same material, they have the same density, which I will say is equal to 1 gram per cubic centimeter. If we were to weigh them both, we would find that the smaller one weighs just about 4 grams, and the larger one weighs 1000 times more, or just over 4 kilograms. Now consider if we put each of these sponges into a pool of water; how quickly would each of them get completely soaked? The smaller one would become saturated much more quickly, as water can penetrate to the center more easily. The larger one takes longer to get soaked to the center. If we now think of these as cells, and the sponge material actually represents enzymes and cell parts that require nutrients for them to work, you can see that it will be harder to get sufficient nutrients all through the larger cell, as everything has to come through the outside surface in order to get to the center.
In class, we said that eukaryotic cells solve this dilemma by having internal membrane compartments (organelles) that in effect make the surface area of the cell bigger relative to a small cell. In this version of the two cells, I have drawn the larger one so that the surface penetrates deep inside the cell. Now nutrients can get to the center much more quickly of the larger cell by diffusing into the invagination and entering the cell all along the length of the channel. The larger cell is able to take up nutrients (and get rid of metabolic wastes) much more quickly with these structures than if it lacked them. The complicated internal structure of eukaryotic cells has allowed them to attain quite remarkable sizes relative to prokaryotic cells, resulting in the very largest cells, the oocyte.
There are however, examples of prokaryotic cells that achieve extremely large sizes. They are able to accomplish this feat of giganticism by the same mechanism that eukaryotic cells do: they have internal membranes to effectively increase their relative surface area, allowing them to carry out a useful rate of growth.