Prokaryotic Cell : Structure Functions, Feature

Prokaryotic Cell Structure, Functions, Feature, Shape, Diagram

Prokaryotes are single-celled organisms that are the oldest and most primitive life forms on Earth. As organized in the Three Domain System, prokaryotes include bacteria and archaea. Some prokaryotes, like cyanobacteria, are photosynthetic organisms and are capable of photosynthesis.

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments, including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori). Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, on your body, and on the everyday objects in your environment.

Bacteria (as representatives of the prokaryotes) are evolutionarily less developed organisms. The bacterial cell cytosol is surrounded by a membrane, cell wall, and mucus capsule. The absence of the nucleus helps to increase the rate of transcription and translation. binary rate of cell division is also increased: a colony of bacteria can double their numbers every 20 minutes.

What is a prokaryotic cell?

Prokaryotic or prokaryotic cells are single-celled living organisms, belonging to the Prokaryota empire or Monera kingdom, depending on the preferred biological classification. These cells are characterized by not having a cell nucleus, but rather having their genetic material dispersed in the cytoplasm, just gathered in an area called the nucleoid.

Prokaryotic organisms are evolutionarily earlier than eukaryotes, that is, those with a nucleus. The simplest forms of life were and still are prokaryotes, such as bacteria and archaea.

Origin of prokaryotic cell

Prokaryotes were the first life forms on earth, so it stands to reason that the prokaryotic cell was the first type of organized cell to exist.

The complexity of life is difficult to trace back to its beginnings. Exactly how it was possible to pass from inanimate matter to the first forms of life proper, capable of nourishing, growing, and reproducing, is unknown.

Prokaryotic cell characteristics / Feature of Prokaryotes.

1. Among the main feature of prokaryotic cells we can highlight the following:
2. Their diet can be autotrophic (they make their own food) or heterotrophic (they eat other people’s food).
3. They conform to organisms belonging to the monera kingdom.
4. Their reproduction can be asexual (by binary fission) or parasexual (by exchange of genetic material between prokaryotic cells).
5. They can come in various forms:
6. Coconuts: spherical cell.
7. Bacillus: rod-shaped or elongated cell.
8. Espirito: cell with a zinc-zac shape.
9. Vibrios: cell shaped like a curve.
10. Irregular: cell without a defined shape.

Prokaryotic Cell Size

Prokaryotic cells lack organelles found in eukaryotic cells, such as mitochondria, endoplasmic reticulums, and Golgi complexes. According to the Endosymbiotic Theory, eukaryotic organelles are thought to have evolved from prokaryotic cells that live in endosymbiotic relationships with each other. Like plant cells, bacteria have a cell wall. Some bacteria also have a layer of polysaccharide capsule that surrounds the cell wall.

It is in this layer that bacteria produce biofilms, a slimy substance that helps colonies of bacteria adhere to surfaces and each other for protection against antibiotics, chemicals, and other dangerous substances. Like plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments allow photosynthetic bacteria to obtain nutrition from light.

Composition of Prokaryotic Cells

A prokaryotic cell is primarily made up of a plasma membrane, cell wall, cytoplasm, genetic material in the nucleoid, and the ribosome. They are unicellular and are much smaller compared to eukaryotic cells. They exist in different shapes, including spherical, rod, flat, coconut, spirochete, and some also have shapes, and they do not have a consistent shape.

Only some of them can move, swim, twist or turn with the help of a helically shaped membrane called flagella. The ways prokaryotes receive nutrients include synthesizing their own food by using energy from the light of the atmosphere, preparing their own food through the process of chemosynthesis, or depending on other substances for nutrition when they cannot synthesize their own food.

Prokaryotic cell structure

Prokaryotic cells share a basic structure made up of:

Plasma membrane : Selective, permeable lipid barrier that distinguishes the inside of the cell from the outside. This membrane is also responsible for regulating the entry and exit of various substances found between the cytoplasm and the extracellular medium.

Cellular wall : A rigid and external barrier that provides support and toughness to the cell, although it hinders its growth. The cellular wall is a resistant layer, which in turn is rigid, since it is capable of withstanding the forces of growth of a living being. This is located outside the plasma membrane of the prokaryotic cells of bacteria and archaea. The cell wall is in charge of the internal protection of the cell, giving it, in turn, a certain rigidity.

Cytoplasm :  The wet interior of the cell, which is a kind of internal gel. It is a liquid that allows easy movement of the different structures. This is made up of: water, lipids, proteins, carbohydrates, and ions. This fluid is present in both eukaryotic and prokaryotic cells.

Nucleoid : A region of the cytoplasm in which the genetic material of the cell usually accumulates, and which serves as the nucleus, although it has a very irregular shape.

Ribosomes : Cellular factories of proteins and other substances that the content of genes is synthesized, expressed, and made a reality by the cell. The main function of ribosomes is the synthesis of proteins through genetic material that comes from DNA.

The ribosomes in the prokaryotic cells possess 66% of messenger ribonucleic acid (rRNA) and are characterized subdivided into two: major subunit and minor subunit; the first with approximately 31 proteins, and the second with approximately 21 proteins.

Prokaryotic compartments : Segments of the cytoplasm that seem to be in charge of unique tasks in prokaryotic life, such as cytosomes, carboxysomes, magnetosomes, etc.

Additionally, certain prokaryotes may have:

Flagella : Cellular organs that allow the cell to move. Flagella are long, thin structures that are made up of a certain protein known as flagellin. These structures are responsible for the movement present on the surface of some bacteria. Bacterial flagella are structures where the filament maintains a rotation like a propeller, receiving impulses through a microscopic rotating motor.

Pilli : They are structures, also known as fimbriae, present on the surface of various bacteria, and that maintain the shape of hairs. These structures are responsible for transferring genetic information.

Periplasm. Or periplasmic space : a compartment that surrounds the cytoplasm of prokaryotic cells and that has a key role in energy metabolism (since prokaryotes lack mostly mitochondria).

Capsule or Glycocalyx : Depending on the type of prokaryote, they are external structures of the cell membrane that serve as a food deposit and defense against phagocytosis. The capsules are rigid and defined, while the glycocalyx is ​​diffuse as a mucous layer.

Mesosoma : Invaginations of the plasma membrane towards the cytoplasm are very frequent in prokaryotes and have been recognized as malformations, even though they were initially believed to have some type of function.

Plasmids Or circular and extrachromosomal (non-coding) DNA : molecules that replicate independently of the cell cycle, and that contain various genetic information to conserve, such as antibiotic resistance in the case of certain bacteria.

Prokaryotic Cell Nutrition

Prokaryotic cells present an enormous diversity of nutrition methods, depending on the type of organism and the habitat in which it evolves. This simplicity has allowed its great diversification, which translates into extremely diverse metabolisms (the same does not happen with eukaryotes) and enormous diversity in terms of environment, nutrition, or even structure.

Autotroph : Prokaryotic cells can be autotrophic (they make their own food).

Heterotroph : Heterotrophic (they feed on foreign organic matter), both aerobic (require oxygen) and anaerobic (do not require oxygen), which results in various nutritional mechanisms :

Photosynthesis : Like plants, some prokaryotes can synthesize chemical energy from sunlight, both in the presence and in the absence of 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.

Saprophytic Nutrition : Based on the decomposition of organic matter left by other living things, either when they die or as remains of their own diet.

Symbiotic nutrition : Some prokaryotic Organisms obtain their organic matter to exist from other living beings, generating a benefit and therefore collaborating to exist.

Parasitic nutrition : The opposite of symbiotics: the organism is nourished by the organic matter of another, which is harmed in the process even if it does not directly kill it.

Finally, the reproduction of prokaryotic cells is also usually very varied, both asexual (mitosis) and parasexual (conjugation, transduction, and transformation of DNA for adaptive purposes).

Types of Prokaryotic cell

Prokaryotic cells can be of four types, according to their morphology:

Coconut : A typical morphological type of bacteria, they have a more or less spherical and uniform shape.

Bacillus : Rod-shaped, they include a vast array of free-living bacteria and other saprophytic organisms.

Vibrio : A genus of proteobacteria responsible for most infectious diseases in man and higher animals, especially those typical of the digestive tract, such as cholera.

Spirals : They are helical or spiral-shaped, are usually very small, and range from pathogenic bacteria to autotrophs.

Pleomorphic : That is, in a changing way, mainly referring to the archaea.

Rectangular : An also typical form of archaea like the Haloquadratum.

Functions of a prokaryotic cell

The prokaryotic cell usually has the following structures:

Plasma membrane. That border that divides the interior and exterior of the cell, in turn serving as a filter to allow the entry of nutrients or the exit of waste.

Cellular wall. A trait shared with vegetables and mushrooms, it consists of a strong and rigid fiber that gives the cell a defined shape and an additional layer of protection.

Cytoplasm. The interior of the cell itself, that is, a very fine colloidal substance that makes up the cell “body”.

Nucleoid. Without becoming a nucleus, but rather a very dispersed region, it is the part of the cytoplasm where the genetic material is usually found within the prokaryotic cell. This genetic material is obviously indispensable for reproduction.

Ribosomes. A complex of proteins and pieces of RNA are present in all cells (except sperm), and 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. Exclusive to this cell type, they vary according to the type of organism and have very specific functions within its metabolism. Some examples are: chlorosomes (for photosynthesis), carboxysomes (to fix CO2), phycobilisomes (molecular pigments to collect sunlight), magnetosomes (allow orientation according to the Earth’s magnetic field), etc.

Additionally, these cells can present:
Flagellum. An organelle is used to mobilize the cell, as a propellant tail.

Outer membrane. An additional cellular barrier that characterizes gram-negative bacteria.

Periplasm. A space that surrounds the cytoplasm and separates it from the external membranes, thus allowing greater effectiveness in different types of energy exchange.

Plasmids Non-chromosomal forms of DNA, circular in shape, which in certain bacteria accompany bacterial DNA and replicate independently, conferring essential characteristics for greater adaptability to the environment.

Reproduction of prokaryotic cells

Lacking a cell nucleus, prokaryotic cells divide in two ways only:

Asexual : By binary fission or bipartition, a fast and simple way in which the cell grows to divide into two new ones of identical genetic material.

Binary fission
Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell divides into two identical cells.

Steps of binary fission

1. Binary fission begins with DNA replication of the single DNA molecule.
2. Both copies of DNA adhere to the cell membrane.
3. Next, the cell membrane begins to grow between the two DNA molecules.
4. Once the bacterium nearly doubles its original size, the cell membrane begins to pinch inward.
5. Then a cell wall is formed between the two DNA molecules that divide the original cell into two identical daughter cells.
6. Although E.coli and other bacteria reproduce more commonly by binary fission, this mode of reproduction does not produce genetic variation within the body.

Read : DNA Replication

Parasexual : A DNA exchange method that allows obtaining genetic variability between prokaryotic cells, and that can be done in three ways:

1. Conjugation. When a prokaryotic cell donates another part of its genetic material and the recipient incorporates it into its own.

2. Transduction. When a virus or other microscopic mechanism carries DNA fragments from one prokaryotic cell to another, it “infect” it with new genetic material.

3. Transformation. When prokaryotic cells incorporate free genetic material (plasmids) that come from other degraded cells or from their own cytoplasm.

Importance of the prokaryotic cell

Prokaryotic cells are the first to have formed, that is, they were the first forms of life. They allow us today to imagine what life on our planet would have been like in its earliest appearances, before more complex forms of life emerged.

In fact, the most accepted theory proposes that there was a process of serial endosymbiosis, that is, the beneficial coexistence of some prokaryotes within others of a larger size. After a slow evolutionary process, the first eukaryotic cells emerged from this coexistence, endowed with cytoplasmic organelles.

Examples of prokaryotes:

1. Escherichia Coli Bacterium (E. coli)

It is a rod-shaped bacterium commonly found in the lower intestine of warm-blooded organisms. Most strains of E. coli are harmless, but some can cause food poisoning and are sometimes responsible for food recalls. The harmless E. coli can be beneficial in producing vitamin K2 and preventing the intestine from colonizing with pathogenic bacteria.

2. Streptococcus Bacterium
This prokaryote is responsible for strep throat. It is an infection of the back of the throat that includes the tonsils. Symptoms include fever, red tonsils, sore throat, and enlarged lymph nodes in the neck. Cell division in this bacterium occurs along a single axis and they grow in chains or pairs.

3. Streptomyces Soil Bacteria
More than 500 of these types of bacteria have been described. They are found predominantly in the soil and in decomposing vegetation, with most of the producing spores. They have a distinctive earthy odor that results from the production of a volatile metabolite, geosmin.

4. Archaea :
The archaea subclasses are prokaryotic and can survive in very harsh environments. An example of archaea can be found in geothermally active areas and they live in extremely acidic mud pots, which is known as archaebacterium sulfurous acidocaldarius.

Eukaryotic cell and Prokaryotic cell 

( Prokaryotic cells vs eukaryotic cells)

Eukaryotic cells are distinguished from prokaryotes in that they have a defined nucleus in their cytoplasm, where all of the cell’s DNA is contained. This difference seems subtle but it justifies a gigantic change in reproduction and other vital processes that led to a higher level of cellular complexity, without which multicellular beings could not have been born.

1. The differences between prokaryotic and eukaryotic cells are not just the absence or presence of the cell nucleus where the DNA is contained. Other notable differences are:
2. The form of DNA. That in eukaryotes it is linear, while prokaryotes are circular.
3. The number of chromosomes. That in eukaryotes it is multiple and in prokaryotes one.
4. The size of the cell. That in eukaryotes it amounts to 10-100 µm, and in prokaryotes only between 0.2-2.0 µm.
5. The size of the genome. That in eukaryotes it covers between 2.2 and 150,000 million base pairs, and in prokaryotes only between 0.5 and 10.
6. The presence of organelles. Like the mitochondria, the nucleus, the lysosomes, the Golgi apparatus, etc. Present in eukaryotes and absent in prokaryotes.
7. The complexity of the cytoskeleton. Which is barely structural proteins in prokaryotic cells and complex (microtubules and microfilaments) in eukaryotes.

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