Prokaryote
n., plural: prokaryotes
[prəʊˈkærɪɒt]
Definition: An organism lacking a distinct membrane-bound nucleus
Table of Contents
Prokaryote refers to any of the group of living organisms primarily characterized by the lack of a true nucleus and other membrane-bound organelles, such as mitochondria and chloroplasts, and by the possession of a single loop of stable chromosomal DNA in the nucleoid region and cytoplasmic structures, such as plasma membrane, vacuoles, primitive cytoskeleton, and ribosomes. Examples of prokaryotes are bacteria and archaea.
Prokaryote Definition
A prokaryote is defined as any organism that is chiefly characterized by a cell devoid of a well-defined (i.e., membrane-bound) nucleus as opposed to a eukaryote that has a nucleus. Instead of a nucleus, the prokaryotes have a nucleoid region where the genetic materials are located.
Etymology: The term prokaryote (plural: prokaryotes) came from the Latin pro, meaning “in favor of” or “on behalf of” and káry(on), meaning “nut”, “kernel”. The term prokaryotic is a derived word and used to refer to a prokaryote.
Variant: procaryote
Compare: eukaryote
Prokaryotic Cell Characteristics
Here are the distinctive characteristics of prokaryotic cells:
Cellularity
Prokaryotes are unicellular organisms that lack a well-defined nucleus. They have instead a nucleoid region in their cytoplasm where their genetic material occurs in most instances as a single, circular molecule of DNA. Although mostly unicellular, some prokaryotes are capable of forming stable aggregate communities.
Reproduction
They generally reproduce asexually, which is by binary fission or by budding. Conjugation, apparently, is the counterpart of sexual reproduction in eukaryotes where two cells exchange genetic materials via a conjugation tube.
Prokaryotic organelles
Although prokaryotes lack a nucleus, mitochondria, and other membranous organelles found only in a eukaryotic cell, they do possess special intracellular membranes referred to as prokaryotic organelles. These prokaryotic organelles may be classified as follows:
Bound by a lipid bilayer:
- Thylakoids. Some bacteria also have special intracellular membranes, like thylakoids. Plants and algae have thylakoids, too, but they are found inside the chloroplast. In photosynthetic bacteria, such as cyanobacteria, the thylakoids are located in the cytoplasm. Thylakoids are necessary for trapping light energy, for photosynthesis. Example: thylakoids in photosynthetic bacteria, such as cyanobacteria.
Thylakoids of prokaryotes are shown as infoldings of the plasma membrane. Image Credit: Bartee, L. et al. and OpenStax, CC BY-SA 4.0. - Magnetosomes. These are intracellular structures for magnetotaxis, especially found in magnetotactic bacteria. They contain magnetic mineral crystals of magnetite (Fe3O4) or greigite (Fe3S4) that help in orienting magnetotactic bacteria to Earth’s magnetic field lines.
- Anammoxosomes, paryphoplasm, and riboplasm. Anammoxosomes are the largest lipid bilayer membrane-bound intracytoplasmic compartments found in anammox bacteria (anaerobic ammonium oxidizing bacteria). The other two are paryphoplasm and riboplasm. Anammoxosomes are particularly for energy metabolism, and thus could be analogous to the eukaryotic mitochondria. (van Teeseling et al., 2013)
- Thylakoids. Some bacteria also have special intracellular membranes, like thylakoids. Plants and algae have thylakoids, too, but they are found inside the chloroplast. In photosynthetic bacteria, such as cyanobacteria, the thylakoids are located in the cytoplasm. Thylakoids are necessary for trapping light energy, for photosynthesis. Example: thylakoids in photosynthetic bacteria, such as cyanobacteria.
Bound by a lipid monolayer:
- Lipid bodies (lipid droplets). They are single-lipid layer membrane-bound structures involved in lipid storage and metabolism.
- Vacuoles. Some bacteria have vacuoles, which are fluid-filled single-membrane-bound ‘organelle‘ containing a wide range of ions, wastes, and other substances. Note: Lipid bodies and vacuoles are found in various eukaryotic cells as well.
Bound by a proteinaceous coat:
- Carboxysomes. These protein-shell internal structures are involved in carbon fixation in certain bacteria.
Phase-defined boundary:
- Nucleolus-like compartments. These are similar to the nucleolus of eukaryotes. Prokaryotes do not have nuclei and nucleoli. However, some bacteria have something quite similar to the eukaryotes’ nucleoli. Fast-growing bacteria, such as E. coli, tend to form nucleolus-like compartments at the surface of the nucleoid region. Nucleolus-like compartments serve as transcription factories for ribosomal RNA synthesis and ribosome biogenesis in prokaryotes. (Mata Martin et al., 2018)
- Chlorosomes. These spherical bodies are attached to the inner side of the cell membrane of sulfur bacteria and other anoxygenic phototrophic prokaryotes. They are light-harvesting complex. They are chiefly made up of aggregates of bacteriochlorophyll c, d, or e.
Other cell structures
- Cell wall. Some prokaryotes have a cell wall that surrounds the cell membrane. Bacterial cell walls are composed chiefly of peptidoglycan. Its thickness can be used to determine whether bacterial cells are Gram-positive (essentially, with a thicker cell wall) or Gram-negative (with a thinner cell wall). As for the archaea, their cell wall is made up of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides rather than peptidoglycan (except for a group of methanogens).
- Flagella. Some prokaryotes have flagella (singular: flagellum). In particular, the bacterial flagellum is a structure used primarily for motility (cell movement) and chemotaxis, a basic cell physiological response. The flagellar filament protein, flagellin, is the cytoskeletal protein that confers structural support to flagellated bacteria. It is one of the most significant cytoskeletal proteins because of its role in chemotaxis.
- Inclusion bodies. These cytoplasmic structures are insoluble protein aggregates in prokaryotes. Inclusion bodies are present in many eukaryotic cells, too.
Eukaryotes vs. Prokaryotes
Based on the cellular organization, there appear two main types of organisms — eukaryotes and prokaryotes. Eukaryotes are organisms in which at the cellular level most of their genetic material is located inside a double-membraned nucleus. Other genetic materials outside the nucleus are found in the mitochondria and the chloroplasts (if present). The chromosomes of eukaryotes are linear strands of DNA.
In prokaryotes, the chromosome is mainly circular. Prokaryotes are smaller in size than eukaryotes. Therefore, prokaryotes have a large surface area to volume ratio. And because of this, they have a high metabolic rate and high growth rate.
Both eukaryotes and prokaryotes store their genetic information in their genes and the main source of metabolic energy is ATP. Also, both of them have ribosomes that serve as the site of protein synthesis. However, the composition of their ribosomes differs:
- The prokaryotic ribosome is the 70S and it is made up of 50S (large subunit) and 30S (small subunit).
- The eukaryotic ribosome is the 80S and it consists of 60S (large subunit) and 40S (small subunit).
[N.B. the S units do not add up since they represent measures of sedimentation rate, not mass.]
Watch this vid to further understand the major differences between prokaryotes and eukaryotes’ cell biology and molecular biology
General Differences Between Prokaryotic Cells and Eukaryotic Cells | |
|---|---|
| Prokaryotic Cells | Eukaryotic Cells |
| Mostly, unicellular; others may form colonies or occur in pairs | Some are unicellular, others are multicellular (i.e., cells function in unison as tissues/organs) |
| Lacking nucleus; nucleoid region only, where the bacterial genome or chromosome is found (cytoplasm) | With a nucleus that contains the nuclear chromosomes and nucleolus (where ribosomal RNAs are produced) |
| Many prokaryotes do not have intracellular compartmentalization that is as complex as that in most eukaryotes. | With complex internal compartmentalization that defines clearly the eukaryotic cytoplasmic organelles, particularly, the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus |
| With 70S ribosomes | With 80S ribosomes |
| Reproduces asexually, by binary fission (bacterial cell division). Nevertheless, some bacteria (especially Gram-negative) have pili or fimbriae, which they use to attach to other bacteria. Pili that are involved in conjugation, attachment, and DNA transfer are referred to as conjugation tube (or conjugation pili) | Some reproduce asexually, others by sexual reproduction, and others still can reproduce by either means |
| Mostly have a cell wall (exception: very few prokaryotic species, particularly Mycoplasma species), which, in bacteria, is characteristically made up of peptidoglycan whereas in archaea, mostly of polysaccharides and glycoconjugates. | Many eukaryotic cells have cell walls. Plant cell walls are made up of cellulose; fungal cell walls are chitin, glucans, and glycoproteins; algal cell walls are made up of glycoproteins and polysaccharides (e.g., carrageenan and agar). Animal cells lack cell walls. |
| Example: Bacterial cell
| Example: Animal cell
|
Data Source: Maria Victoria Gonzaga of Biology Online
Examples
Prokaryotes include the domains, Eubacteria and Archaea. Thus, examples of prokaryotes include bacteria, archaea, and cyanobacteria (blue-green algae).
Bacteria
Bacteria are microscopic, single-celled organisms that belong to Domain Eubacteria (true bacteria). Their cells lack a nucleus and other subcellular compartments such as mitochondria, endoplasmic reticulum, and Golgi bodies. Their DNA is found in the cytoplasm (nucleoid region) rather than inside a nucleus.
Thus, DNA replication occurs in the cell’s cytoplasm. They reproduce by fission or by forming spores. They can inhabit all kinds of environments, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earth’s crust, in the stratosphere, and even in the bodies of other organisms. Bacteria include the bacilli, the cocci, the spirochetes, and the vibrios.
Archaea
Archaea belong to the Domain Archaea. They are unicellular microorganisms that are genetically distinct from bacteria and eukaryotes. Similar to prokaryotes, they lack a nucleus and other organelles. However, the genes of archaebacterial are more similar to eukaryotes.
Both of them produce certain enzymes that are used in transcription, translation, and other metabolic pathways. Many archaebacterial are found thriving in extreme habitats. Archaebacteria include the halophiles (those inhabiting extremely salty environments), the methanogens (archaea species producing methane), and the thermophiles (those that can thrive in extremely hot habitats).
Cyanobacteria
Cyanobacteria, also called blue-green algae, are microorganisms that are formerly considered protists for being single-celled and photosynthetic. However, they now belong to a group or phylum of photosynthetic bacteria that inhabit aquatic habitats and moist soils.
Cyanobacteria are ecologically significant because a huge percentage of gaseous oxygen comes from their photosynthetic activity. They may lack a nucleus but they possess microcompartments (e.g. thylakoids and carboxysomes).
They also have photosynthetic pigments (particularly, phycobiliproteins) that account for the bluish-green color of their cells. Cyanobacteria include Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, and
Stigonematales.
Featuring… “The Ancient Prokaryotes”
Prokaryotes are said to be older than eukaryotes! Scientists believe that the first prokaryotes on Earth existed about 3.5 billion years ago and could be the earliest ancestors of living things.
They could be where the eukaryotes evolved from. One of the popular theories is that the earliest forms of prokaryotes interacted with other ancient cells through endosymbiosis, which led to the occurrence of eukaryotes. This theory is called the Endosymbiotic theory, which attempts to explain the origin of cytoplasmic organelles, particularly mitochondria and chloroplasts that are found only in eukaryotic cells.
According to this theory, these semi-autonomous organelles originated from ancient prokaryotes that have been taken in or engulfed by the larger cells. This theory is supported by the following points:
- Mitochondria and chloroplasts contain a genetic material of their own that is single and circular, similar to that of prokaryotic genetic material. These organelles’ genetic material, DNA, is referred to as “extranuclear” to distinguish it from nuclear DNA, which as the name implies, is found in the nucleus.
- Prokaryotes reproduce by binary fission; mitochondria and plastids multiply by a similar process.
Take the Prokaryote Biology Quiz!
References
- Armstronf, J. E. (2015). How the Earth Turned Green: A Brief 3.8-Billion-Year History of Plants. The University of Chicago Press. Retrieved from https://www.press.uchicago.edu/ucp/books/book/chicago/H/bo16465693.html
- Chemistry. 282 (40): 29323–35. doi:10.1074/jbc.M703896200.
- Long, B. M., Badger, M. R., Whitney, S. M., & Price, G. D. (October 2007). “Analysis of carboxysomes from Synechococcus PCC7942 reveals multiple Rubisco complexes with carboxysomal proteins CcmM and CcaA”. The Journal of Biological
- Mata Martin, C., Sun, Z., Zhou, Y. N., & Jin, D. J. (2018). Extrachromosomal Nucleolus-Like Compartmentalization by a Plasmid-Borne Ribosomal RNA Operon and Its Role in Nucleoid Compaction. Frontiers in Microbiology, 9. https://doi.org/10.3389/fmicb.2018.01115
- Prokaryotic vs. Eukaryotic. (2019). Retrieved from Nku.edu website: https://www.nku.edu/~whitsonma/Bio150LSite/Lab%205%20Cells/Bio150LRCellTypes.htm
- van Teeseling, M. C. F., Neumann, S., & van Niftrik, L. (2013). The Anammoxosome Organelle Is Crucial for the Energy Metabolism of Anaerobic Ammonium Oxidizing Bacteria. Microbial Physiology, 23(1-2), 104–117. https://doi.org/10.1159/000346547
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