What are the Differences Between Prokaryotic and Eukaryotic Cells?

Life on Earth is usually characterized into two main classes (also called domains): Prokaryotes and Eukaryotes. And prokaryotes are divided into two domains — Bacteria and Archaea.

Each of these types contains unique structural and biochemical differences which differentiate them from one another.

So, what are the differences between Prokaryotic and Eukaryotic cells? How are they similar, and how are they different?

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Types of Organisms

Living things are generally divided into three different groups: Bacteria, Archaea, and Eukarya.

Bacteria and Archaea are primarily single-celled organisms classified as Prokaryotes. By a matter of definition, prokaryotic cells tend to be the simplest, tiniest, and most ancient of cells here on earth.

Most other organisms that fall outside of the Bacteria and Archaea categories are placed in the Eukarya group and are made up of eukaryotic cells.

Eukaryotes are not limited by the number of cells there are in the organism. They can be single-celled or have many millions of cells. Eukaryotes include plants, animals, and fungi, as well as organisms such as protists and some algae. 

Prokaryotes developed earliest in the history of life on earth, with Eukaryotes developing around 2.7 billion years ago, roughly 1 to 1.5 billion years after prokaryotic organisms.

One theory is that eukaryotic cells evolved from a symbiotic association of prokaryotes, called endosymbiosis — for example, bacteria living inside a larger host cell. The theory posits that, over time, the symbiotes evolved together until one could not function without the other.

Still, though, what separates these two types of cells exactly?

How do prokaryotes and eukaryotes differ?

To properly break down the differences of these cells, we'll break down the explanation into specific parts of each cell — starting with the nucleus and DNA.

Nucleus/DNA:

Eukaryotic cells are comprised of a nucleus surrounded by an envelope made up of two membranes. The nucleus in eukaryotic cells holds the DNA.

In Prokaryotic cells, on the other hand, there is no nucleus, but rather a nucleoid region that is not surrounded by a separate membrane. This region of the cell holds the DNA, which is generally free-floating.

If we look deeper into the DNA of the respective cells, in both types of cells, the DNA is contained in chromosomes.

Eukaryotic cells have multiple linear chromosomes which undergo meiosis and mitosis as the eukaryotic cells replicate. However, the cells of prokaryotic organisms usually contain just one, circular chromosome. Although, some studies have shown that some prokaryotes may have as many as four chromosomes.

Organelles in Eukaryotic Cells:

Eukaryotic cells contain multiple, membrane-bound organelles that are not present in the simpler prokaryotic cells. These include organelles such as the mitochondria (the powerhouse of the cell), rough and smooth endoplasmic reticulum, the Golgi complex, and, in plant cells, chloroplasts. All of these individual organelles are contained in the cytoplasm of the eukaryotic cell.

Prokaryotic cells also have cytoplasm, but it doesn't house any membrane-bound organelles.

Ribosomes:

Both types of cells have ribosomes, but in eukaryotic cells, the ribosomes are bigger and more complex. They are also bound by a membrane and may be found in the cytoplasm, nuclear membrane, and even the endoplasmic reticulum. In prokaryotic cells, on the other hand, the ribosomes are scattered and floating freely throughout the cytoplasm.

Stepping back for a moment, Ribosomes are complex macromolecules which synthesize proteins. These proteins are essential to cell function and repair.

One notable difference between the ribosomes in both cells are the size of the subunits that make them up. In both types of cells, the ribosomes are made up of two subunits, a small one and a large one.

In Eukaryotes, these subunits are larger, recognized as 60S and 40S (the S stands for 'svedbergs,' a unit used to measure how fast molecules move in a centrifuge). In Prokaryotes, these subunits are smaller, recognized as 50S and 30S.

This all may sound like a lot of jargon, but the difference in subunits is important, as it has allowed scientists to develop antibiotic drugs that attach to certain types of bacteria. 

The difference is also notable in more negative aspects. For example, the poliovirus uses the differences in ribosomes to seek out and attach to ribosomes in eukaryotic cells, affecting their ability to translate messenger RNA into proteins.

Reproduction:

Most Eukaryotic cells reproduce sexually, through meiosis, whereas prokaryotes generally reproduce asexually, through binary fission, which is similar to mitosis. This means that the resulting cells of prokaryotes are exact clones of the parent cells. Some prokaryotic cells also have pili, which are hair-like projections used to exchange genetic material during a process called conjugation, which can be considered a type of sexual reproduction.

In meiosis, the number of chromosomes in the cell is reduced by half, creating four haploid cells, which are each genetically distinct from the parent cell that gave rise to them. 

Cell Walls:

Most Prokaryotic cells have a rigid cell wall around their plasma membrane. In single-celled organisms, this rigid cell wall gives shape to the organism. Some Eukaryotes, such as vertebrates, do not have cell walls, although others, such as plant cells, do. 

Comparing the cell walls of prokaryotes and eukaryotic plant cells, the two also differ chemically. Plant cell walls are primarily composed of cellulose, but in bacteria, for example, the cell walls are composed of peptidoglycans, which is essentially a combination of sugars and amino acids. 

After all of that individual breakdown, you may still be wondering how you can tell the two different cells apart, so here's a straight and simple answer.

The main differences between the Prokaryote and Eukaryote cells are the presence or absence of a nucleus; the size and complexity of the ribosomes; the method of reproduction; and the presence or absence of a cell wall. 

Eukaryotes have a nucleus with a membrane, larger ribosomal subunits, usually reproduce sexually, and may not have a cell wall. Prokaryotes have no membrane around the nucleus, smaller ribosomal subunits, reproduce asexually, and do have cell walls. 

What do prokaryotes and eukaryotes have in common?

These two types of cells do have several things in common; after all, each type is still a cell. Here are the key similarities:

• Both types of cells have DNA, which codes, and determines the characteristics of the cells.
• Both types of cells have an outer membrane that separates them from the environment and acts as a sort of gate for incoming and outgoing substances.
• Eukaryotes and Prokaryotes both have cytoplasm inside their cells.
• Finally, as mentioned before, both cells have ribosomes, which make proteins.