Prokaryotic cell

What are prokaryotic cells?

Prokaryotic cells are the ones that form unicellular organisms that we know as bacteria and archaea. They are very common in nature and surprisingly abundant and important to all ecosystems.

This type of cell is among the smallest that exist on our planet and only form single-celled organisms. Each prokaryotic cell forms a complete organism, so small that it is not possible to see it with our own eyes, but we always need some instrument to help us, such as a microscope, for example..

Prokaryotic cells represent one of the two types of cells found in nature, the second type being that of eukaryotic cells..

While our bodies are made up of millions of eukaryotic cells, just like the bodies of animals and plants, prokaryotic cells make up bacteria and archaea..

Where are prokaryotic cells found?

Prokaryotic cells can be found in all ecosystems on Earth, even in those most inhospitable or challenging where we would think that life could not exist: in glacial ice, in hot springs, in salt flats, in volcanic lands, in the stomach of an animal, on the floor of a desert, etc..

The surface of our skin, the inside of our mouth and stomach, the surface of the fruits and vegetables we eat every day, the hair of our pets, the water we drink, the sea where we bathe during the summer and the leaves that we trample during the fall, everything is covered by some kind of prokaryotic organism.

In fact, some recent estimates have shown that the body of an average adult human, 1.70 meters tall and 70 kg in weight, has, inside and out, between 30 and 39 million bacteria with which it lives daily, representing more than the total number of cells that make up your body.

Some prokaryotic organisms are very dangerous to our health, as is the case with bacteria such as Staphylococcus aureus, better known as "staphylococci"; as they cause severe infections in the skin, mucous membranes and other organs of human beings.

Others, on the other hand, are very useful, either because we use them to produce food -such as yogurt, for example- or because some scientists use them in their laboratories to produce medicines that help us cure diseases..

Characteristics of prokaryotic cells

Prokaryotic cells are normally considered "inferior" or "simple" relative to eukaryotic cells. Its main characteristics are:

• The main characteristic of a prokaryotic cell is that it does not have a nucleus. The term "prokaryote" derives from the Greek roots pro, which means "before", and karyon, which translates to "core". For this reason, the word prokaryotic describes organisms that have a "primordial" or "primitive" nucleus, different from the "true" membranous nucleus that eukaryotic cells have.
• The irregularly shaped region of prokaryotic cells that contains DNA is called the nucleoid..
• Its size is much smaller than that of eukaryotic cells; in fact, an average prokaryotic cell is considered to be up to 15 times smaller than the smallest animal cell.
• They are formed by a membrane that covers them and encloses everything that is inside. This membrane, in turn, is usually covered by a wall that serves to protect cells from certain external conditions..
• In the interior of prokaryotic cells there are no membranous compartments, that is, internal structures delimited by membranes. Its interior is like a kind of "soup" where we find different types of suspended molecules.
• There are many prokaryotic cells that associate with other types of cells to receive benefits from them (symbionts), but others are free-living or live in colonies of clonal organisms, that is, one equal to the other.
• As all prokaryotic cells are unicellular organisms, they have the capacity that any living being has: they feed, grow, reproduce, interact with the environment that surrounds them, communicate with each other and with other organisms around them and die.
• The reproduction of prokaryotic cells is exclusively asexual, that is, a prokaryotic cell, when it divides, gives rise to two identical cells.

Parts of prokaryotic cells and their functions

Prokaryotic cells, despite appearing much simpler in appearance than eukaryotic cells, are very interesting and have many different parts, which they use for particular purposes..

Outer, outer, or extracellular region

Just as our body is covered by the skin, which is the largest and most extensive organ that we have and which protects us from a great number of dangers, prokaryotic cells have an outer covering where different structures are distinguished. Let's see some of them:

• The capsule

It is a kind of gelatinous substance or layer that covers many prokaryotic cells; It is made up of sugars (carbohydrates) that associate with other molecules called lipids and proteins. Its main function is to protect the cell, but it can also participate in the displacement.

• The fimbriae

These are a kind of elongated appendages, very numerous and that are attached to the cell wall of some prokaryotes. They work for the mobility and adherence of cells to almost any surface. In bacteria, the fimbriae are made up of a protein called pilina.

• The sexual pili

It is a structure that bacteria have and that consists of a modified fimbria that functions as a “pipe” that some species use to transfer or exchange genetic information with other cells (bacterial conjugation). This type of exchange is considered a form of bacterial “sexual reproduction”..

• Flagella

They are very important structures for many prokaryotes, as they help them move from one place to another. These are filaments that are longer than the fimbriae and are made up of proteins; they have a "tail" appearance. A single cell can have between one and several hundred flagella on its surface.

Middle or “coverage” region

If we continue the journey of a prokaryotic cell from the outside to the inside, then inevitably we are going to find two structures (with their exceptions, that is, not all prokaryotic cells have them): the cell wall and the cell membrane..

• Cell membrane

The prokaryotic cell membrane is very similar to that of the eukaryotic cell. In fact, it is the structure that define the cell, because it encloses it, gives it the shape and size it has.

The cell membrane, also known as the cytosolic membrane or plasma membrane, is made up of a double layer of molecules called lipids, associated with protein molecules that fulfill many essential functions for the communication of the prokaryotic cell with the environment that surrounds it..

The cell membrane works as a selective filter, that is, it lets some molecules pass from the outside to the inside and vice versa..

• Cell wall

Not all prokaryotic cells have a cell wall, but bacteria, which are among the most abundant prokaryotes, are often differentiated from each other by the characteristics of the cell wall..

The cell wall serves as a protective envelope, as it is a rigid structure, composed of different types of molecules, which protects the cell from dehydration, from the action of some chemical compounds, among many other things..

Internal or intracellular region

The interior of prokaryotic cells is not very complicated:

• Much of the cytosol, which is the "fluid" that cells are filled with, is occupied by the genome in the form of highly coiled deoxyribonucleic acid (DNA) molecules, forming a structure known as chromosome. Genomic DNA is what contains all the information that makes a prokaryotic cell a prokaryotic cell, with all its characteristics.
• In addition to the chromosome, the cytosol also contains smaller DNA fragments known as plasmids, which usually provide valuable information for metabolism or related to resistance to some adverse conditions.
• There is also a large amount of protein, all derived from the "reading" and "translation" of the information contained in the chromosomal or plasmid DNA, which are used for prokaryotic cells to fulfill their functions: feed, grow, communicate and reproduce.
• Other important elements in the bacterial cytosol are the ribosomes, which are the complexes responsible for the translation of the message contained in DNA to form cellular proteins.

Prokaryotic cell types

Prokaryotic cells have highly variable sizes and shapes, which generally depend on both the species we consider and the conditions in which they grow..

Bacteria are among the most studied prokaryotes, so we will say as a generality that prokaryotes can be found in two types or special forms: cocci and bacilli.

Coconuts

Cocci-shaped prokaryotes are organisms whose cells are practically spherical. It is very common to find them in nature forming groups or aggregates, very similar to a bunch of grapes, for example.

Bacilli

Bacillus-shaped prokaryotes, on the other hand, have a rod-like (rod-shaped) appearance that can occur in very different lengths and widths depending on the species.

When associated with each other, this type of cell can be found in nature as a chain or "string" of sausages.

Some authors propose that there are also prokaryotes with intermediate forms between cocci and bacilli, and these are called coccobacilli; since they are neither completely spherical nor completely elongated.

Examples of prokaryotic cells

Since there are a vast number of prokaryotes in the world, we could say that there are countless examples of these organisms, but here we will only name a few of them:

• Escherichia coli (E. coli): a resident bacterium of the human intestine; some strains are pathogenic and cause diarrhea or other gastrointestinal conditions when they grow out of control.
• Salmonella enterica (S. enterica): another pathogenic bacteria, a gram-negative bacillus that causes disease in humans and other animals.
• Haloarcula quadrata (H. quadrata): an arch discovered in Egypt that is shaped like a box or cube.
• Nitrosocaldus yellowstonii -(N. yellowstonii): another archaea that lives in one of the hottest places in the world, in Yellowstone National Park, USA.

Themes of interest

Cell types.