What is a Prokaryotic Cell and Its Functions?

Also known as prokaryotes, a basic definition of a prokaryotic cell is that it is the most basic functional unit of unicellular living beings. Examples of such unicellular beings are archaea and bacteria.

The prokaryotic cell is distinguished from other types of cell because it lacks a true nucleus that groups together all of its genetic material. In fact, the word prokaryote means ‘before the nut’, with the nut referring to the nucleus. Prokaryotes are also peculiar because they have a cell wall, something that not all other cells possess. They also do not have internal organelles bounded by membranes. We will address the structure of prokaryotes in detail later.

Shapes of prokaryotic cells

Single-celled organisms can be solitary or form colonies. Likewise, they can have a wide variety of shapes. For example, in bacteria there may be:

• Bacilli: they are elongated shapes. They can be lined up in a row (called streptobacillus) or one against another (called a palisade). If they are solitary they are known as a bacillus.
  
  
• Cocci: they have a spherical shape. When they form in a chain they are called streptococcus. They are called diplococcus if they are grouped in pairs, tetrad in fours or staphylococcus if they are multiple cocci grouped together. If it is individual, it is called a coccus.
  
  
• Coccobacilli: those that have a shape somewhere between a bacillus and a coccus.
  
  
• Special shapes: some bacteria do not fit inside these shapes. For example, some are are coiled in spirals (spirilla) such as spirochetes, others are filaments such as Actinobacteria if they are similar to elongated threads and some are wavy in the shape of a comma (vibrios), to name only a few.

In archaea we find more varied forms than in bacteria. In addition to being spirilla, cocci or bacilli, they can be:

• Lobed
• Rod shaped
• Flat
• Irregular

Finally, prokaryotic cells are microscopic since they measure 0.1 to 10 micrometers. Learn more about cells which do have a nucleus in our article on what is a eukaryotic cell?

The prokaryotic cell has multiple functions to fulfill to keep an organism alive. These functions are fulfilled thanks to its various constituent parts which are structured in harmony. Although they are internally considered to be simple cells, they all have complex mechanisms.

Let's see what the structure of the prokaryotic cell is to learn more about them.

• Cell wall: the cell is delimited from its environment and is supported by this cell wall, made of peptidoglycan, polysaccharides or glycoproteins, depending on whether it is an archaea or bacteria.
  
  
• Plasma membrane: also known as the cellular membrane, it is located under the cell wall. It is a plasma membrane made of phospholipids. Also not all types of cells have a cell wall, all have a cell membrane.
  
  
• Mesosomes: this plasma membrane can have folds to increase surface area and therefore efficiency. They are formed via a process of invagination which uses the cell membrane to create them.
  
  
• Glycocalyx: as additional protection, some prokaryotic cells may have a waterproof capsule that protects them against extreme environmental conditions, such as desiccation. It also serves to easily adhere to hosts. It is not always present in all prokaryotes. When it is closely adhered to the cell wall it is called a capsule and when it is diffuse it is called a slime layer.
  
  
• Nucleoid: the genetic material in the form of DNA is free within the cytoplasm crowded at one of the poles, which is called the nucleoid. Learn more about types of genetic material with our guide to the difference between DNA and RNA.
  
  
• Plasmid: small circular fragments or additional chromosomes of DNA, used for sexual reproduction of bacterial conjugation type, which we will review later.
  
  
• Cytoplasm: it is the medium in which all the elements contained in the cell are arranged. This is fluid and lacks a cytoskeleton. Learn more about the cytoplasm and its functions.
  
  
• Ribosomes: they are made up of RNA and proteins.
  
  
• Inclusion bodies: serve to store nutrients as a reserve.
  
  
• Thylakoids: are not present in all prokaryotic cells, only in photosynthetic cyanobacteria. These thylakoids have pigments with which they capture and transform solar energy to produce metabolic energy.
  
  
• Adaptive organelles or appendages: they are accessory organelles that serve cells to have advantages in their environment. Such may be sexual pili to transfer genetic information from one cell to another, flagella to move or fimbriae that are like hairs made useful for adhering to host surfaces.

Now that you know all the organelles and parts that make up the prokaryotic cell, we can address the functions of prokaryotes. The main function of every cell is to keep itself alive, meaning that it can reproduce, feed, grow and respond to its environment.

Let's see in detail how and what these functions of the prokaryotic cell are:

• Transcription and translation: the messenger RNA will be transcribed and then translated by ribosomes to synthesize proteins. These are processes that are necessary for the encoded genetic information to be expressed.
  
  
• Exchange substances with the environment: this occurs thanks to the plasma membrane, in order to remain in communication with its environment.
  
  
• Obtaining energy: prokaryotic cells carry out respiration processes through biochemical reactions to obtain the energy necessary to complete all internal processes.
  
  
• Nutrition: this occurs through metabolic processes to obtain nutrients. The metabolism of prokaryotes is incredibly varied, as a result of the great variety of habitats in which they can live. For example, we can find autotrophic organisms which use atmospheric carbon dioxide to produce their energy. In this case they produce their own carbon. Heterotrophic organisms need to obtain carbon from outside.

In turn, these last organizations are divided into:

• Organotrophs: use organic compounds, such as carbon, for their energy.
  
  
• Phototrophs: use solar energy to produce their energy.
  
  
• Photosynthetic: unlike phototrophs, these organisms produce oxygen as a byproduct of their use of sunlight to produce energy.
  
  
• Lithotrophs they use inorganic matter, such as ammonia, nitrites or sulfur, to produce their energy. We tell you the differences between organic and inorganic compounds with examples in our related post.
  
  
• Mycotrophs: also known as myco-heterotrophs, they are parasitic plants which gain their sustenance from fungi. They do so instead of getting their energy from photosynthesis.

Learn more about one of the most important types of autotrophy with our related article on how photosynthesis works.

The reproduction of the prokaryotic cell can occur in different ways. Among them, we find the following:

Binary fission

The prokaryotic cell, both archaea and bacteria, divides mainly by binary fission. This is a type of asexual reproduction since it does not require another cell to reproduce. In this process, a division of an initial cell occurs to form two new cells with information equal to the initial one, which in turn will divide again and so on.

Budding

Budding can also occur, another type of asexual reproduction. Budding is the production of buds that will eventually separating from their mother cell, forming a new daughter cell.

Bacterial conjugation

We previously mentioned the presence of sexual pili. This is a structure present in some bacteria that serves to exchange genetic information. This is called bacterial conjugation and it is a very useful form of sexual reproduction in which genetic information is exchanged to be better able to respond to the environment.

Bacterial conjugation occurs when a donor bacteria connects with a recipient bacteria through the pili. A part of the plasmid is then transferred. This plasmid is the DNA independent of the chromosomal DNA and allows for reproduction.

Learn more about cellular reproduction with our article on the difference between mitosis and meiosis in cell division.

Prokaryotic cells exist so that certain organisms fulfill their functions. Knowing the different types of prokaryotic cells can help us to better understand these functions. For this reason, we will review the classification of prokaryotic cells.

Organisms with prokaryotic cells are all grouped in the kingdom Monera, the kingdom of organisms that includes bacteria and archaea. Other classifications may use the two-empire system of Prokaryota to refer to prokaryotic bacteria and archaea.

These are the characteristics that define both groups of prokaryotic cell types:

Bacteria

Prokaryotic cells are exclusively which have the presence of a peptidoglycan cell wall. Bacterial prokaryotic cells can be pathogenic, but they can also be part of the biota of healthy organisms. In this way, they can help complete certain physiological processes, especially in animal and human metabolism^[1].

Archaea

Archaea are very primitive cells that are adapted to survival in the extreme conditions that existed in primitive times. During these times in Earth history, there was not yet an atmosphere with oxygen. Despite this, they were still able to thrive due to being anerobic organisms.

These extreme environments also had very high temperatures, such as in the previously abundant hydrothermal vents that connect to the center of the earth. They can live in other places that are not so extreme, such as oceans or on the ground.

Their metabolism is chemoautotrophic and this is one of the important differences with bacteria. Although they are very similar, archaea are closer to eukaryotes due to their metabolic routes. In addition, the DNA and RNA sequences are more similar to eukaryotes than to prokaryotes. Another important difference with bacteria is that their cell wall is made of polysaccharides, glycoproteins or pseudopeptidoglycan.

Some examples of prokaryotic cells are:

• Escherichia coli: a common bacteria and is an important part of the intestinal biota for the manufacture of vitamins K and B, as well as for proper digestion. However, some strains can be pathogenic, causing diarrhea. It is the most common type of pathogen leading to colitis in humans^[2].
  
  
• Halobacterium salinarum: an archaea that grows in marine environments or hypersaline lakes. When there is an abundance of them, the water can turn purple or red.

Learn about saline water bodies with our article on whether there are salt water lakes.

Prokaryotic cells are extremely important. The great variety of living organisms that make up the flora and fauna of the world's ecosystems originated from them. Prokaryotes were the first types of cell to exist. From them, two groups were formed, the already mentioned archaea and bacteria. Over time, eukaryotic cells developed from these groups into cells such as the animal cell. Simply put, we humans would not exist without prokaryotes.

Prokaryotic cells have also been used the development of human civilization. Bacteria have been used since ancient times for the production of food products such as yogurt, wine and cheese. In recent times, they have been able to be introduced into technology. For example, as archaea withstand very extreme temperatures, they can be introduced into industrial processes thanks to their thermostable enzymes.

They have been used for oil extraction, wastewater treatment, solvent production, and even in medicine for the development of antibiotics, among many other applications.

To discover more about prokaryotic and eukaryotic cells, take a look at our explanation on the differences between plant and animal cells.

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