Prokaryotic Cell – Concept, Classification, Parts and Function

We explain what a prokaryotic cell is, its component parts and their functions. Also how it differs from a eukaryotic cell.

What is a prokaryotic cell?

Prokaryotic or prokaryotic cells form unicellular living organisms, belonging to the superkingdom or empire Prokaryota or the domains Archaea and Bacteria, depending on the biological classification that is preferred.

The main characteristic of prokaryotic cells is that do not have a membrane that delimits the cell nucleus and, on the other hand, they present their genetic material dispersed in the cytoplasm, barely gathered in an area called the nucleoid.

Prokaryotic organisms (pro- means “before” and karyo which refers to “core”) are evolutionarily prior to eukaryotes, that is, those that do have a cell nucleus. While prokaryotic cells arose in the very remote past, that does not mean that they have disappeared from Earth. In fact, the simplest life forms are still prokaryotic organisms, such as bacteria and archaea.

This simplicity that characterizes prokaryotic organisms has allowed their great diversification, which translates into extremely diverse metabolisms (the same is not the case with eukaryotes) and enormous diversity in terms of adaptation to different environments, types of nutrition or even cellular structure. .

Mechanisms of nutrition

Prokaryotic cells can be autotrophic (they make their own food) or heterotrophs (they feed on organic matter produced by another living being), both aerobic (they require oxygen to live) and anaerobic (they do not require oxygen to live), which translates into several nutrition mechanisms:

• Photosynthesis. Like plants, some prokaryotes can use the energy in sunlight to synthesize organic matter from inorganic matter, both in the presence and in the absence of oxygen. There are two types of photosynthesis: oxygenic photosynthesis (which produces oxygen) and anoxygenic photosynthesis (does not produce oxygen).
• Chemosynthesis. Similar to photosynthesis, cells undertake the oxidation of inorganic matter as a mechanism to obtain their energy and obtain their own organic matter to grow. Chemosynthesis differs from photosynthesis in that the latter uses sunlight as an energy source.
• Saprophytic nutrition. It is based on the decomposition of organic matter left by other living beings, either when they die or as remains of their own diet.
• Symbiotic nutrition. Some prokaryotes associate with other living beings, obtain their organic matter to exist from them and a mutual benefit is generated.
• Parasitic nutrition. There are prokaryotic organisms (parasites) that feed on the organic matter of another larger (host or host), which they harm in the process (although they do not kill it directly).

Finally, the reproduction of prokaryotic cells can be of two types: asexual (by the mechanism of mitosis) or parasexual (three processes involved in the exchange and incorporation of changes in the genetic material: DNA conjugation, transduction and transformation).

Prokaryotic cell types

Prokaryotic cells can have very varied shapes and often even the same species can adopt changing forms, which is called pleomorphism. However, three main types of morphology can be distinguished:

• Coconut. It is a typical morphological type of bacteria, which has a more or less spherical and uniform shape. Bacteria can also occur in cocci in groups of two (diplococcus), cocci in groups of four (tetracoccus), cocci in chains (streptococcus), and cocci in irregular or clustered groups (staphylococcus). For example: Streptococcus pneumoniae, one of the causative agents of bacterial pneumonia.
• Bacillus. Rod-shaped with rounded ends, it includes a vast array of bacteria and other free-living saprophytic organisms. Bacilli can also be found in groups of two or forming filaments. For example: Escherichia coli and Clostridium botulinum.
• Spirilum. Helical in shape, they are usually very small and range from pathogenic to autotrophic bacteria. For example: species of the genus Campylobacter, such as Campylobacter jejuni, a foodborne pathogen, which causes campylobacteriosis.
• Spirochaete. They also have helical shapes but are very elongated and flexible. For example: the species of the genus Leptospira that cause leptospirosis.
• Vibrions. They are comma-shaped rods. This group includes those of the vibrio type, a genus of proteobacteria responsible for most infectious diseases in man and higher animals, especially those typical of the digestive tract. The best known is Vibrio cholerae, the causative agent of cholera.
• Some variants of these forms they are coccobacilli (ovals) and coryneform bacteria, irregular bacilli with a widened end.

Parts and functions of a prokaryotic cell

The prokaryotic cell has the following structures:

• Plasma membrane. It is the border that divides the interior and exterior of the cell and that serves as a filter to allow the entry and / or exit of substances (such as the incorporation of nutrients or the exit of waste).
• Cellular wall. It consists of a strong and rigid layer that is outside the cell membrane, which gives the cell a defined shape and an additional layer of protection. The presence of a cell wall is a shared trait among plants, algae and fungi, although the composition of this cell structure is different in each of these groups of organisms.
• Cytoplasm. It is a very fine colloidal substance that makes up the cell “body” and is found inside the cell.
• Nucleoids. It does not become a nucleus, it is a very dispersed region that is part of the cytoplasm, where there is usually a single circular DNA molecule that can be associated with a small amount of RNA and non-histonic proteins This DNA molecule is essential for reproduction .
• Ribosomes. They are complexes of proteins and pieces of RNA that allow the expression and translation of genetic information, that is, they synthesize the proteins required by the cell in its various biological processes, as stipulated in DNA.
• Prokaryotic compartments. They are unique to prokaryotic cells. They vary according to the type of organism and have very specific functions within your metabolism. Some examples are: chlorosomes (necessary for photosynthesis), carboxysomes (to fix carbon dioxide (CO₂), phycobilisomes (molecular pigments to collect sunlight), magnetosomes (allow orientation according to the Earth’s magnetic field), etc.

In addition, these cells can present other structures such as:

• Flagellum. It is a whip-shaped organelle used to mobilize the cell, as a propellant tail.
• Outer membrane. It is an additional cellular barrier that characterizes gram-negative bacteria.
• Capsule. It is a layer formed by organic polymers that is deposited outside the cell wall. It has a protective function and is also used as a food storage and waste disposal site.
• Periplasm. It is a space that surrounds the cytoplasm and separates it from the outer membranes, which allows greater effectiveness in different types of energy exchange.
• Plasmids. They are non-chromosomal forms of DNA, circular in shape, which in certain bacteria accompany bacterial DNA and replicate independently, which gives them essential characteristics for greater adaptability to the environment.

Eukaryotic cell

Eukaryotic cells are distinguished from prokaryotes in that have a defined nucleus in their cytoplasm (where most of the cell’s DNA is contained) and in which they have the presence of membranous organelles (which have specific functions within the cell, such as mitochondria and chloroplasts).

Although this difference may seem subtle, it underlies a gigantic change in reproduction and other vital processes that led to a higher level of cellular complexity, without which multicellular beings with complex and superior organizations would not have been able to develop.