What is the difference between prokaryotic and eukaryotic cells?

The main difference between prokaryotic and eukaryotic cells lies in their structure.

Prokaryotic cells and eukaryotic cells are the two types of cells that exist on Earth. There are several differences between the two, but the biggest distinction between them is that eukaryotic cells have a distinct nucleus containing the cell's genetic material, while prokaryotic cells don't have a nucleus and have free-floating genetic material instead. 

What are prokaryotic and eukaryotic cells?

All living things can be divided into three basic domains: Bacteria, Archaea and Eukarya. The primarily single-celled organisms found in the Bacteria and Archaea domains are known as prokaryotes. These organisms are made of prokaryotic cells — the smallest, simplest and most ancient cells.

Organisms in the Eukarya domain are made of the more complex eukaryotic cells. These organisms, called eukaryotes, can be unicellular or multicellular and include animals, plants, fungi and protists. Many people are unclear on whether yeasts or fungi are prokaryotes or eukaryotes. Both are eukaryotes and share similar cell structure to all other eukaryotes.

There is evidence to suggest that eukaryotes are the descendants of separate prokaryotic cells, according to Berkeley University of California. 

Eukaryotes developed at least 2.7 billion years ago, following 1 to 1.5 billion years of prokaryotic evolution, according to the National Institutes of Health (NIH). Scientists hypothesize that the nucleus and other eukaryotic features may have first formed after a prokaryotic organism swallowed up another, according to the University of Texas. 

It’s also been suggested that tiny organelles in eukaryotic cells – called mitochondria – may also be the descends of prokaryotic living-bacterium which were engulfed by other cells and remained in the cell as a permanent guest, according to Berkeley University. 

What do prokaryotes and eukaryotes have in common?

Although prokaryotic and eukaryotic cells have many differences, they share some common features, including the following:

• DNA: Genetic coding that determines all the characteristics of living things.
• Cell (or plasma) membrane: Outer layer that separates the cell from the surrounding environment and acts as a selective barrier for incoming and outgoing materials.
• Cytoplasm: Jelly-like fluid within a cell that is composed primarily of water, salts and proteins.
• Ribosomes: Organelles that make proteins.

How do prokaryotes and eukaryotes differ?

Eukaryotic cells have a nucleus surrounded by a nuclear envelope that consists of two lipid membranes, according to Nature Education. The nucleus holds the eukaryotic cell's DNA. Prokaryotic cells do not have a nucleus; rather, they have a membraneless nucleoid region (open part of the cell) that holds free-floating DNA, according to Washington University.

The entire DNA in a cell can be found in individual pieces known as chromosomes. Eukaryotic cells have many chromosomes which undergo meiosis and mitosis during cell division, while most prokaryotic cells consist of just one circular chromosome. However, recent studies have shown that some prokaryotes have as many as four linear or circular chromosomes, according to Nature Education. For example, Vibrio cholerae, the bacterium that causes cholera, has two circular chromosomes.

Eukaryotic cells have several other membrane-bound organelles not found in prokaryotic cells. These include the mitochondria (convert food energy into adenosine triphosphate, or ATP, to power biochemical reactions); rough and smooth endoplasmic reticulum (an interconnected network of membrane-enclosed tubules that transport synthesized proteins); golgi complex (sorts and packages proteins for secretion); and in the case of plant cells, chloroplasts (conduct photosynthesis). All of these organelles are located in the eukaryotic cell's cytoplasm.

Although only eukaryotes carry membrane-bound organelles, recent evidence suggests that both eukaryotes and prokaryotes can produce organelle-like structures that lack membranes, according to a 2020 report published in the journal Proceedings of the National Academy of Sciences (PNAS).

For instance, in the bacterium Escherichia coli, molecules and proteins cluster together to form liquid "compartments" within the cytoplasm, according to the PNAS study. These compartments form similarly to how oil forms droplets when mixed with water, according to a statement from the University of Michigan. Such membraneless structures have been reported in many bacterial species, including Mycobacterium tuberculosis, which causes tuberculosis, and cyanobacteria, a type of photosynthetic bacteria that can also cause disease.

In eukaryotic cells, the ribosomes are bigger, more complex and bound by a membrane. They can be found in various places: Sometimes in the cytoplasm; on the endoplasmic reticulum; or attached to the nuclear membrane (covering on the nucleus).

In prokaryotic cells, the ribosomes are scattered and floating freely throughout the cytoplasm. The ribosomes in prokaryotic cells also have smaller subunits. All ribosomes (in both eukaryotic and prokaryotic cells) are made of two subunits — one larger and one smaller. In eukaryotes, these pieces are identified by scientists as the 60-S and 40-S subunits. In prokaryotes, the ribosomes are made of slightly smaller subunits, called 50-S and 30-S.

The difference in types of subunits has allowed scientists to develop antibiotic drugs, such as streptomycin, that attack certain types of infectious bacteria, according to the British Society for Cell Biology. On the downside, some bacterial toxins and the polio virus use the ribosome differences to their advantage; they're able to identify and attack eukaryotic cells' translation mechanism, or the process by which messenger RNA is translated into proteins.

Most eukaryotes also reproduce sexually (although some protists and single-celled fungi may reproduce through mitosis, which is functionally similar to asexual reproduction). Prokaryotes reproduce asexually, resulting in the offspring being an exact clone of the parent. Some prokaryotic cells also have pili, which are adhesive hair-like projections used to exchange genetic material during a type of sexual process called conjugation, according to Concepts of Biology. Conjugation can occur in bacteria, protozoans and some algae and fungi.

Most prokaryotic cells have a rigid cell wall that surrounds the plasma membrane and gives shape to the organism. In eukaryotes, vertebrates don't have a cell wall but plants do. The cell walls of prokaryotes differ chemically from the eukaryotic cell walls of plant cells, which are primarily made of cellulose. In bacteria, for example, the cell walls are composed of peptidoglycans (sugars and amino acids), according to Washington University.

Additional resources

Check out this animated video by the Amoeba Sisters that explains the difference between prokaryotic and eukaryotic cells. To take a more indepth look into all the cells in the world take a look at Looking Inside Cells: Life Science by Kimerberly Fekany Lee. 

Bibliography

Lesli J Favor, “How Eukaryotic and Prokaryotic Cells Differ (Britannica Guide to Cell Biology),” Rosen Publishing, 2014.

Frantisek Baluska et al, “Eukaryotic Cells and their Cell Bodies: Cell Theory Revised”, Annals of Botany, Volume 94, Jukly 2004, https://doi.org/10.1093/aob/mch109 

James Wagstaff & Jan Lowe, “Prokaryotic cytoskeletons: protein filaments organizing small cells”, Nature Reviews Microbiology, Volume 16, January 2018, https://doi.org/10.1038/nrmicro.2017.153 

Avadhesha Surolia & Abhijit Chakrabarti, “Biochemical Roles of Eukaryotic Cell Surface Macromolecules”, Springer International Publishing, 2014.