PROKARYOTIC CELL: CHARACTERISTICS, CELL STRUCTURE, TYPES - BIOLOGY - 2025

The prokaryotic cells are simple and without a nucleus bounded by a plasma membrane structures. The organisms associated with this cell type are unicellular, although they can group together and form secondary structures, such as chains.

Of the three domains of life proposed by Carl Woese, prokaryotes correspond to Bacteria and Archaea. The remaining domain, Eucarya, is made up of larger, more complex eukaryotic cells with a delimited nucleus.

Prokaryotic cell. Source: By Ali Zifan, from Wikimedia Commons

One of the most important dichotomies in the biological sciences is the distinction between the eukaryotic and the prokaryotic cell. Historically, a prokaryotic organism is considered simple, without internal organization, without organelles, and lacking a cytoskeleton. However, new evidence is destroying these paradigms.

For example, structures have been identified in prokaryotes that can potentially be considered as organelles. Likewise, proteins homologous to the proteins of eukaryotes that form the cytoskeleton have been found.

Prokaryotes are very varied in terms of their nutrition. They can use the light from the sun and the energy contained in chemical bonds as an energy source. They can also use different sources of carbon, such as carbon dioxide, glucose, amino acids, proteins, among others.

Prokaryotes divide asexually by binary fission. In this process, the organism replicates its circular DNA, increases its volume and finally divides into two identical cells.

However, there are mechanisms for the exchange of genetic material that generate variability in bacteria, such as transduction, conjugation and transformation.

General characteristics

Prokaryotes are relatively simple single-celled organisms. The most striking characteristic that identifies this group is the absence of a true nucleus. They are divided into two large branches: true bacteria or eubacteria and archaebacteria.

They have colonized almost every imaginable habitat, from water and soil to the interior of other organisms, including humans. Specifically, archaebacteria inhabit areas with extreme temperatures, salinity, and pH.

Structure

Average prokaryotic cell.

The architectural scheme of a typical prokaryote is, without a doubt, that of Escherichia coli, a bacterium that normally inhabits our gastrointestinal tract.

The cell shape is reminiscent of a rod and is 1 um in diameter and 2 um in length. Prokaryotes are surrounded by a cell wall, composed mainly of polysaccharides and peptides.

The bacterial cell wall is a very important characteristic and, depending on its structure, it allows establishing a classification system into two large groups: gram positive and gram negative bacteria.

Followed by the cell wall, we find a membrane (a common element between prokaryotes and eukaryotes) of a lipid nature with a series of prosthetic elements embedded in it that separates the organism from its environment.

DNA is a circular molecule located in a specific region that does not have any type of membrane or separation with the cytoplasm.

The cytoplasm exhibits a rough appearance and has approximately 3,000 ribosomes - structures responsible for protein synthesis.

Types of prokaryotes

Current prokaryotes are made up of a wide diversity of bacteria that is divided into two large domains: Eubacteria and Archaebacteria. According to the evidence, these groups appear to have diverged very early in evolution.

Archaebacteria are a group of prokaryotes that generally live in environments with unusual conditions, such as temperatures or high salinity. These conditions are rare today, but may have been prevalent in early earth.

For example, thermoacidophiles live in areas where the temperature reaches a maximum of 80 ° C and a pH of 2.

Eubacteria, for their part, live in environments common to us humans. They can inhabit soil, water, or live in other organisms - like bacteria that are part of our digestive tract.

Morphology of prokaryotes

Bacteria come in a series of highly varied and heterogeneous morphologies. Among the most common we have the rounded ones called coconuts. These can occur individually, in pairs, in a chain, in tetrads, etc.

Some bacteria are morphologically similar to a rod and are called bacilli. Like coconuts, they can be found in different arrangements with more than one individual. We also find spiral-shaped spirochetes and those with a comma or grain shape called vibrios.

Each of these described morphologies can vary between different species - for example, one bacillus can be more elongated than another or with more rounded edges - and are useful when identifying the species.

Reproduction

Asexual reproduction

Reproduction in bacteria is asexual and occurs by means of binary fission. In this process the organism literally "splits in two", resulting in clones of the initial organism. Sufficient resources must be available for this to occur.

The process is relatively simple: circular DNA replicates, forming two identical double helices. Later, the genetic material accommodates itself in the cell membrane and the cell begins to grow, until it doubles in size. The cell eventually divides and each resulting part has a circular DNA copy.

In some bacteria, the cells can divide the material and grow, but they do not divide completely and form a kind of chain.

Additional sources of genetic variability

There are gene exchange events between bacteria that allow genetic transfer and recombination, a process similar to what we know as sexual reproduction. These mechanisms are conjugation, transformation, and transduction.

Conjugation consists of the exchange of genetic material between two bacteria through a structure similar to fine hairs called pili or fimbriae, which acts as a “bridge”. In this case, there must be a physical closeness between both individuals.

Transformation involves the taking of naked DNA fragments found in the environment. That is to say, in this process the presence of a second organism is not necessary.

Finally we have translation, where the bacterium acquires the genetic material through a vector, for example bacteriophages (viruses that infect bacteria).

Nutrition

Bacteria need substances that guarantee their survival and that give them the energy necessary for cellular processes. The cell will take up these nutrients by absorption.

In a general way, we can classify nutrients as essential or basic (water, carbon sources, and nitrogenous compounds), secondary (such as some ions: potassium and magnesium) and trace elements that are needed in minimal concentrations (iron, cobalt).

Some bacteria need specific growth factors, such as vitamins and amino acids and stimulant factors that, although not essential, help in the growth process.

The nutritional requirements of bacteria vary widely, but their knowledge is necessary to be able to prepare effective culture media to ensure the growth of an organism of interest.

Nutrition categories

Bacteria can be classified according to the carbon source they use, either organic or inorganic and depending on the source of energy production.

According to the carbon source we have two groups: the autotrophs or lithotrophs use carbon dioxide and the heterotrophs or organotrophs that require an organic carbon source.

In the case of the energy source, we also have two categories: phototrophs that use energy from the sun or radiant energy, and chemotrophs that depend on the energy of chemical reactions. Thus, combining both categories, bacteria can be classified into:

Photoautotrophs

They obtain energy from sunlight - which means that they are photosynthetically active - and their carbon source is carbon dioxide.

Photoheterotrophs

They are capable of using radiant energy for their development but they are not capable of incorporating carbon dioxide. Therefore, they use other sources of carbon, such as alcohols, fatty acids, organic acids and carbohydrates.

Chemoautotrophs

They obtain energy from chemical reactions and are capable of incorporating carbon dioxide.

Chemoheterotrophs

They use energy from chemical reactions and carbon comes from organic compounds, such as glucose - which is the most used - lipids and also proteins. Note that the energy source and the carbon source is the same in both cases, therefore the differentiation between the two is difficult.

Generally, microorganisms that are considered human pathogens belong to this last category and use amino acids and lipid compounds from their hosts as a carbon source.

Metabolism

Metabolism comprises all the complex, enzyme-catalyzed chemical reactions that take place inside an organism so that it can develop and reproduce.

In bacteria, these reactions do not differ from the basic processes that occur in more complex organisms. In fact, we have multiple pathways that are shared by both lineages of organisms, such as glycolysis for example.

The reactions of metabolism are classified into two large groups: biosynthetic or anabolic reactions, and degradation or catabolic reactions, which occur to obtain chemical energy.

Catabolic reactions release energy in a staggered manner that the body uses for the biosynthesis of its components.

Fundamental differences from eukaryotic cells

Prokaryotes differ from prokaryotes primarily in the structural complexity of the cell and the processes that occur within it. Below we will describe the main differences between the two lineages:

Size and complexity

In general, prokaryotic cells are smaller than eukaryotic cells. The former have diameters between 1 and 3 µm, in contrast to a eukaryotic cell that can reach 100 µm. However, there are some exceptions.

Although prokaryotic organisms are unicellular and we cannot observe them with the naked eye (unless we are observing bacterial colonies, for example) we should not use characteristic to distinguish between both groups. In eukaryotes we also find unicellular organisms.

In fact, one of the most complex cells are unicellular eukaryotes, since they must contain all the structures necessary for their development confined in a cell membrane. The genera Paramecium and Trypanosoma are notable examples of this.

On the other hand, there are highly complex prokaryotes, such as cyanobacteria (a prokaryotic group where the evolution of photosynthetic reactions took place).

Core

The word "prokaryote" refers to the absence of a nucleus (pro = before; karyon = nucleus) while eukaryotes have a true nucleus (eu = true). Thus, these two groups are separated by the presence of this important organelle.

In prokaryotes, the genetic material is found distributed in a specific region of the cell called the nucleoid - and it is not a true nucleus because it is not delimited by a lipid membrane.

Eukaryotes have a defined nucleus and surrounded by a double membrane. This structure is extremely complex, presenting different areas inside, such as the nucleolus. In addition, this organelle can interact with the internal environment of the cell thanks to the presence of nuclear pores.

Organization of genetic material

Prokaryotes contain from 0.6 to 5 million base pairs in their DNA and it is estimated that they can code for up to 5,000 different proteins.

Prokaryotic genes organize themselves into entities called operons - like the well-known lactose operon - whereas eukaryotic genes do not.

In genes we can distinguish two "regions": introns and exons. The former are portions that do not code for the protein and that are interrupting the coding regions, called exons. Introns are common in eukaryotic genes but not in prokaryotes.

Prokaryotes are generally haploid (a single genetic load) and eukaryotes have both haploid and polyploid loads. For example, we humans are diploid. Similarly, prokaryotes have one chromosome and eukaryotes more than one.

Compaction of genetic material

Within the cell nucleus, eukaryotes exhibit a complex DNA organization. A long DNA chain (approximately two meters long) is capable of twisting in such a way that it can integrate into the nucleus and, during the division processes, it can be visualized under the microscope in the form of chromosomes.

This DNA compaction process involves a series of proteins that are able to bind to the strand and form structures that resemble a pearl necklace, where the strand is represented by DNA and the beads by pearls. These proteins are called histones.

Histones have been widely conserved throughout evolution. In other words, our histones are incredibly similar to those of a mouse, or to go further to that of an insect. Structurally, they have a high number of positively charged amino acids that interact with the negative charges of DNA.

In prokaryotes, certain proteins homologous to histones have been found, which are generally known as histones-like. These proteins contribute to the control of gene expression, recombination and DNA replication and, like histones in eukaryotes, participate in the organization of the nucleoid.

Organelles

In eukaryotic cells, a series of highly complex subcellular compartments can be identified that perform specific functions.

The most relevant are the mitochondria, responsible for the processes of cellular respiration and generation of ATP, and in plants chloroplasts stand out, with their three-membrane system and with the machinery necessary for photosynthesis.

Likewise, we have the Golgi complex, smooth and rough endoplasmic reticulum, vacuoles, lysosomes, peroxisomes, among others.

Ribosome structure

Ribosomes

Ribosomes comprise the machinery necessary for protein synthesis, so they must be present in both eukaryotes and prokaryotes. Although it is an indispensable structure for both, it differs predominantly in size.

Ribosomes are made up of two subunits: a large and a small one. Each subunit is identified by a parameter called the sedimentation coefficient.

In prokaryotes the large subunit is 50S and the small subunit is 30S. The entire structure is called the 70S. Ribosomes are scattered throughout the cytoplasm, where they perform their tasks.

Eukaryotes have larger ribosomes, the large subunit is 60S, the small subunit is 40S, and the entire ribosome is designated as 80S. These are located mainly anchored in the rough endoplasmic reticulum.

Cellular wall

The cell wall is an essential element to face osmotic stress and serves as a protective barrier against possible damage. Almost all prokaryotes and some groups of eukaryotes have a cell wall. The difference lies in the chemical nature of it.

The bacterial wall is composed of peptidoglycan, a polymer made up of two structural elements: N-acetyl-glucosamine and N-acetylmuramic acid, linked together by β-1,4 type bonds.

Within the eukaryotic lineage there are also wall cells, mainly in some fungi and in all plants. The most abundant compound in the wall of fungi is chitin and in plants it is cellulose, a polymer made up of many glucose units.

Cellular division

As discussed earlier, prokaryotes divide by binary fission. Eukaryotes have a complex division system that involves different stages of nuclear division, either mitosis or meiosis.

Phylogeny and classification

Generally we are used to defining a species according to the biological concept proposed by E. Mayr in 1989: "groups of interbreeding natural populations that are reproductively isolated from other groups".

Applying this concept to asexual species, as is the case with prokaryotes, is impossible. Therefore, there must be another way of approaching the concept of species in order to classify these organisms.

According to Rosselló-Mora et al. (2011), the phylo-phenetic concept fits well with this lineage: “a monophyletic and genomically coherent set of individual organisms that show a high degree of general similarity in many independent characteristics, and is diagnosable by a discriminating phenotypic property”.

Previously, all prokaryotes were classified in a single "domain", until Carl Woese suggested that the tree of life should have three main branches. Following this classification, prokaryotes include two domains: Archaea and Bacteria.

Within the bacteria we find five groups: proteobacteria, chlamydia, cyanobacterial spirochetes and gram positive bacteria. Similarly, we have four main groups of archaea: Euryarchaeota, TACK Group, Asgard and DPANN Group.

New insights

One of the most widespread concepts in biology is the simplicity of the prokaryotic cytosol. However, new evidence is suggesting that there is a potential organization in prokaryotic cells. Currently, scientists are trying to break down the dogma of the absence of organelles, cytoskeleton and other characteristics in this single-celled lineage.

Organelles in prokaryotes

The authors of this very novel and controversial proposal assure that there are levels of compartmentalization in eukaryotic cells, mainly in structures delimited by proteins and intracellular lipids.

According to the defenders of this idea, an organelle is a compartment surrounded by a biological membrane with a specific biochemical function. Among these "organelles" that fit this definition we have lipid bodies, carboxy-somes, gas vacuoles, among others.

Magnetosomes

One of the most fascinating compartments of bacteria are magnetosomes. These structures are related to the ability of certain bacteria - such as Magnetospirillum or Magnetococcus - to use magnetic fields for orientation.

Structurally they are a small 50-nanometer body surrounded by a lipid membrane, the interior of which is composed of magnetic minerals.

Photosynthetic membranes

Furthermore, some prokaryotes have "photosynthetic membranes", which are the most studied compartments in these organisms.

These systems work to maximize the efficiency of photosynthesis, increasing the number of photosynthetic protein available and maximizing the membranous surface that is exposed to light.

Compartments in

It has not been possible to trace a plausible evolutionary path from these previously mentioned compartments to the highly complex organelles of eukaryotes.

However, the genus Planctomycetes has a series of compartments inside that are reminiscent of organelles themselves and can be proposed as the bacterial ancestor of eukaryotes. In the genus Pirellula there are chromosomes and ribosomes surrounded by biological membranes.

Components of the cytoskeleton

Similarly, there are certain proteins that were historically considered unique to eukaryotes, including the essential filaments that are part of the cytoskeleton: tubulin, actin, and intermediate filaments.

Recent investigations have succeeded in identifying proteins homologous to tubulin (FtsZ, BtuA, BtuB and others), to actin (MreB and Mb1) and to intermediate filaments (CfoA).

References

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