Prokaryotic vs. Eukaryotic Cells

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By aaron

One of the most fundamental concepts of biology is the fact that all life is made up of cells. And based on the structure of the cells that comprise an organism, we can classify every organism into one of two categories known as Prokaryotes and Eukaryotes. Let's look at how these two groups differ from one another.

Nucleus vs. no nucleus

When asked to define the difference between prokaryotes and eukaryotes, the most common definition people give is that eukaryotic cells have a cell nucleus where the genetic material is stored, while prokaryotes do not have a nucleus. This is true, but that is only the most prominent difference between the two groups. There are quite a few other distinctions worth knowing about.

More broadly speaking, eukaryotic cells have membrane-bound components called organelles. You can think of a cell like a machine, with all sorts of little components that make it survive and reproduce. In a eukaryotic cell, many of those components are surrounded by a membrane that is composed primarily of lipids. The membrane also has various proteins that allow specific molecules to move through the membrane. If a cell component is surrounded by such a membrane, that component is known as an “organelle”. Eukaryotic cells, by definition, have organelles. In a prokaryotic cell, there are no organelles. Prokaryotes do have a membrane around the cell itself, just like eukaryotes, but the subcomponents of a prokaryotic cell mostly just float around inside the cell without any separation between them. Some components do have a protein shell around them, but because they are made mostly from proteins rather than lipids, those aren't considered to be membranes and those components are not considered to be organelles.

The nucleus is an organelle

Now that we've covered the concept of organelles, let's go back and talk about the nucleus again. I said that a eukaryotic cell has a nucleus and a prokaryotic cell does not. This fact, while true, is a bit lacking. The nucleus consists primarily of genetic material surrounded by a membrane. Because it is a cell component surrounded by a lipid membrane, the cell nucleus counts as an organelle. Thus, we can start to see the bigger picture and realize that organelles are the true distinction between eukaryotes and prokaryotes. In other words, while it is true to say that eukaryotes have a nucleus and prokaryotes don't, a more comprehensive definition would be to say that eukaryotes have membrane-bound organelles, while prokaryotes do not. The nucleus is just one of many such organelles.

Unicellular vs. multicellular

In addition to organelles, there are some other differences between eukaryotes and prokaryotes. One important difference is that eukaryotes can be unicellular or multicellular. That is, they can be comprised of only one cell, like protists, or of many cells, like any large plant or animal. In contrast, prokaryotes are all made of only one cell.

Specialized tissue types

This leads us to the next point of distinction; specialized tissue types. Because eukaryotes can be multicellular, they can have various tissue types that perform specific functions. For example, in humans, this would be skin cells, or nerve cells, or blood cells. Plants also have various tissue types for their needs. But because prokaryotes are unicellular, they can't have specialized tissues. The cell is the organism and must perform all of the functions required to keep itself alive.

Reproduction methods

Other than their structural differences, eukaryotes and prokaryotes also differ in their methods of reproduction. Some eukaryotes like animals reproduce sexually, with two individuals... shall we say... exchanging genetic information. Other eukaryotes, such as protists and some fungi, reproduce asexually, which means they don't have to mate in order to produce offspring. In contrast, prokaryotes are exclusively asexual.

Size differences

One final difference that is worth noting is the size difference. Compared to prokaryotes, eukaryotic cells are huge. It is hard to give precise numbers, because even within each group there is a wide range of different sizes, but the average sized eukaryotic cell is roughly 5-10 times as big as the typical prokaryotic cell. Now, it is worth noting where eukaryotes and prokaryotes fit into the the bigger picture of organism classification. You might be familiar with the three main domains of life, known as eukarya, bacteria, and archaea. Originally, there were only two domains called Eukarya and Eubacteria, and classification into one of those two domains was based simply on whether or not the organism's cells had a nucleus. But in the early 1990's, it became a little more complicated.

Based on advances in biochemistry, it became clear that some organisms classified as eubacteria are structurally similar to bacteria, but biochemically and genetically more similar to eukarya. With this discovery, eubacteria was split into two domains, leaving us with the three-domain system we have today that includes eukarya, bacteria, and archaea. All organisms in the domain eukarya are eukaryotic, while all bacteria and all archaea are prokaryotic. In short, observing whether or not a cell has a nucleus can be used to classify it as either a eukaryotic or prokaryotic cell, but that is no longer enough to classify it into one of the three domains of life. If it has a nucleus, it is in the domain eukarya, but if it doesn't have a nucleus, it could be in either bacteria or archaea. A true classification into one of those two domains would involve genetic analysis. #science #biology #prokaryotes #eukaryotes #biochemistry #cell

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