Discuss why prokaryotes tend to be small relative to eukaryotic cells

Prokaryotic cells are "simple" cells. What I mean by this is that they are "simpler" than eukaryotic cells. Now, don't be misled. The use of "simple" here is only used in a comparative sense. They are still vastly complex organisms, way beyond the knowledge of the entire scientific community in the world today. Think about that. So, "simple" doesn't exactly mean simple.

One of the main reasons why we might postulate that prokaryotic cells are smaller than eukaryotic cells has to do with internal functions. Eukaryotic cells have a significant system of internal membranes in order to divide the cytoplasm into compartments for specialized functions. A prokaryotic cells has no regular endomembrane system; therefore, it must remain small. Think about it. From a cell's point of view, the interior of a cell is a vast three dimensional ocean. It's huge -- from a molecular point of view, right? Therefore, without internal membranes to traffic molecules around the cell, the internal space would be too large for reactions to occur in a reasonable amount of time. Remember, reactions take place when molecules collide. If molecules can't find each other, then no reactions can take place. The endomembrane system in eukaryotic cells helps facilitate the interaction of molecules. As a result, the cell's internal space can increase accordingly. With prokaryoties, however, the absence of internal membranes imposes a limitation on the internal size of the cell.

The last point to consider pertains to why cells are small at all. I mean, why can't they just be huge? Most scientists point to the surface area/volume ratio problem. The smaller a sphere (or cube), the higher the surface area/volume ratio it has. As a sphere or a box increases in size, of course, the surface area must increase too (or else the sphere or box would explode); however, the surface area doesn't increase to the extent that the volume increases. Therefore, large objects have a low surface area/volume ratio. Small objects have a high ratio. What's the point? Cells need to exchange molecules with their environments. Whether these molecules are waste products or nutrients, they must pass through the plasma membrane. If there isn't enough membrane to keep up with the cell's needs, then we'd have a problem. The cell would die due to build up of toxic materials or lack of sufficient nutrients being brought in. This limitation severely constrains even eukaryotic cells to remain small.

Examples can be provided from anywhere. Just open up your textbook and you'll see examples of small eukaryotic cells. Pick a few and then explain why you think they're small based on the preceding paragraph. Nevertheless, here's two examples. Muscle cells are small because they must bring in lots of sugars in order to metabolize them to ATP to provide energy for contraction. If they get too big, they won't get enough sugars in, nor enough O2 in, nor enough energy production. Epithelium cells need to be small in order to allow for the transport of molecules from one surface to the other. Think of the simple epithelial cells that line the small intestine as an example.