Prokaryotes: examples to learn from

by Alex Bolano

November 5, 2018

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Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and cellular organelles. Prokaryotes represent one of the two fundamental divisions of living organisms and are contrasted with eukaryotes, uni- or multicellular organisms that contain cells with a membrane-bound nucleus and distinct organelles.

Os procariotos são divididos em dois tipos principais de organismos: bactérias e archaea. Alguns exemplos de organismos procariotas incluem as bactérias comuns E. coli, a archaea M. Smithii que ajuda os humanos a degradar os polissacarídeos nos intestinos e o Deinococcus radiodurans, uma espécie de bactéria conhecida por sua extrema resistência à radiação.

Pyrococcus furiosus, an extremophile archaeon that lives in ocean thermal vents. Credit: WikiCommons.

Prokaryotes are as old as life itself, as prokaryotic organisms were the first kind of living creatures to exist on earth. The first prokaryotes popped up around 4 billion years ago and dominated the planet for a longer time than any other kind of life since. Because prokaryotes have been around for so long, evolution has had time to mold and adapt them in many ways.

Prokaryotic organisms exhibit a staggeringly diverse range of characteristics: different metabolic pathways, cell wall structures, cellular appendages, locomotive techniques, genetic mechanisms, and reproductive mechanisms. In fact, prokaryotic organisms are probably the single most diverse grouping of living organisms on the planet, despite the fact that most of them are invisible to the naked eye. They owe this extraordinary diversity to their quick reproductive rates, environmental adaptability, and the ability to mutate very quickly to overcome environmental pressures. It is also thought that eukaryotic life originally emerged from prokaryotic cells that engulfed other single-celled organisms, a theory called endosymbiosis.

Prokaryotic Anatomy – Bacteria & Archaea

Prokaryotic organisms are so named because they consist of prokaryotic cells. There are two fundamental kinds of cells, prokaryotic and eukaryotic cells. Eukaryotic cells contain a membrane-bound nucleus of DNA and several well-defined independent cellular organelles. Eukaryotic cells are the basic building blocks of all complex multicellular life, including human beings.

Prokaryotic cells, on the other hand, are much “simpler” than eukaryotic cells. Prokaryotic cells do not have a membrane-bound nucleus, so their DNA, RNA, and associated proteins float freely in the intracellular matrix surrounded by cytoplasm. Prokaryotic cells also lack well-defined specialized cellular organelles, though some cellular regions of prokaryotic cells seem to be loosely specialized to perform some specific function. All prokaryotic cells have thick rigid cell walls, with a handful of exceptions.

The general structure of a prokaryotic cell. The exact structure will differ depending on the specific prokaryotic organism. Credit: Wikipedia

All prokaryotic organisms are unicellular (i.e consisting of only one cell), so every prokaryotic cell is itself an individual prokaryotic organism. All prokaryotic cells reproduce asexually through cellular binary fission. A prokaryote will create an extra copy of its DNA and literally split itself in two, with each part containing a full genome of DNA. The resulting offspring is essentially a genetically identical clone of the parent organism, though errors in DNA copying and cell fission can result in genetic mutations.

Prokaryotes do engage in a form of DNA mixing that is analogous to sexual genetic recombination in eukaryotes. Prokaryotes are able to take a section of their DNA and introduce it into the DNA of another prokaryote—a process called “horizontal” gene transfer in contract to the “vertical” transmission of genes from parent to offspring in sexually reproducing species. Sharing of DNA is a complicated process that has mostly been studied in bacteria. A number of bacteria show complex genetic adaptions that allow it to directly insert sequences of its DNA into the DNA of another organism. The horizontal sharing of DNA is an adaptation that prokaryotes have evolved over millions of years.

Though they are considered strictly unicellular, some prokaryotes are known to group together in cellular colonies that behave somewhat like a singular entity. For example, prokaryotic organisms will sometimes aggregate while being suspended in a colloidal matrix (i.e. “biofilms”) These biofilms can be highly heterogeneous in structure, containing phenotypical variations through time and space and an ability to adapt to different environments. The different prokaryotes will even signal to each other in a manner similar to intercellular communication in multicellular eukaryotes. The relatively complex behavior of these kinds of colonies has led some biologists to argue that there are, in fact, multicellular prokaryotic organisms. Whether or not these colonies constitute distinct individual lifeforms could change our understanding prokaryotes; specifically how we deal with them in medicine. Prokaryotic colonies can be much more difficult to treat than individual cells as the colony as a whole could react to and subvert treatment.

Prokaryotic organisms are divided into two main kinds of organisms: bacteria and archaea. Incidentally, the categories of bacteria and archaea constitute 2 of the 3 fundamental domains of life, the other category being the domain Eukarya which contains all single and multicellular eukaryotic organisms. Bacteria and archaea are differentiated in virtue of their cell structure and specific evolutionary history.

Bacteria

Before the advent of modern cellular biology, bacteria were classified as a kind of plant. It was only after the introduction of microscopes powerful enough to observe individual cells that bacteria was discovered to have a different cell structure than plants. As they are prokaryotic organisms, bacteria are unicellular, lacking a membrane-bound nucleus and specialized organelles. Bacteria come in a wide range of geometric shapes, from sphere to rod to cone-shaped. Many have specialized flagella which they use for locomotion and to interact with the environment.
Many humans view bacteria as a dangerous nuisance. While it is true that many types of bacteria cause disease in humans, the vast majority of existing bacteria are either completely harmless or in some cases beneficial. The human body contains a number of beneficial bacteria, most existing in the gut where the help with digestion and absorption of nutrients. In fact, it is estimated that there are actually ~30% more bacterial cells in a human body than human cells; 39 trillion bacteria to 30 trillion human cells.

The cell wall of Gram-positive vs Gram-negative bacteria. Source: WikiCommons

One of the defining features of bacteria is their thick cell walls. Bacterial cell walls are composed primarily of a chemical called peptidoglycan, a polymer made from sugars and amino acids. Bacteria are classified into two type, depending on the structure and composition of their cell wall; Gram-positive and Gram-negative. The designation of Gram-positive and Gram-negative is related to the bacterium’s appearance when stained. Gram-positive bacteria retain the staining agent and so appear a bright violet color. Gram-negative bacteria do not retain the staining agent and appear a translucent pink.
Gram-positive bacteria have a thick multi-layered cell wall that contains teichoic acids and relatively low concentrations of phospholipids. Gram-positive bacteria have an average cell wall thickness of approximately 100-120 Å. Gram-positive bacteria are more susceptible to antibiotics due to the dense complex structure of their cell wall. Most classes of antibiotics will interfere in the structure and maintenance of the cell wall, thus making the bacterium more susceptible to antibodies and environmental damage.
In contrast, Gram-negative bacteria have a relatively thin cell wall surrounded by a phospholipid membrane. Due to their thinner cell walls, Gram-negative bacteria are more prone to mechanical breakage, but the presence of a phospholipid membrane makes it more difficult for antibiotics to interact with the cell wall. Thus, Gram-negative bacteria tend to be more resistant to traditional antibiotics whose mechanism of action involves disrupting the structure of the bacterial cell wall.

Archaea

The existence of archaea is actually a rather new development in biology. For the longest time, archaea were considered as a particular kind of bacteria, called archaebacteria. Archaea were first identified as separate from bacteria in 1977 by the biologists Carl Woese and George Fox. Although very similar in shape and size to bacteria, archaea have a unique phylogenetic lineage and contain cellular mechanisms that are more analogous to those found in eukaryotic organisms. Additionally, there is currently no know pathogenic archaea, meaning that archaea do not make people sick like bacteria do.

Archaea are known for being extremophiles, favoring condition and environments that most organisms would be destroyed in. Archaea have been observed living in deep ocean thermal vents where average temperatures are greater than 100° C and in bodies of water with a salt content too high for other kinds of organisms. It was originally thought that all archaea were extremophiles, but archaea have been found living in relatively mundane environments like mud, grasslands, and freshwater lakes. A handful of archaea also live in the human body, mostly to aid with digestion.

Most archaea have a cell wall, but they tend to lack peptidoglycan. The actual cell wall composition of archaea varies greatly from species to species, but the structure of most archaea cell walls is similar to that of Gram-positive bacteria. Archaea also show great metabolic diversity. Some archaea are aerobic, others anaerobic. Some subsist primarily off of inorganic materials like sulfur or ammonia and others eat methane or carbon dioxide. On species of archaea get their energy from the sun, though this process is different than the oxygen creating photosynthesis seen in plant cells.