We explain what a prokaryotic cell is, how it originated and its structure. Also, what are its characteristics, reproduction and classification.
What is a prokaryotic cell?
A type of cell that does not have a defined cell nucleus is called prokaryote , so its genetic material is free in the cell cytoplasm. In this they differ from eukaryotic cells , which are much larger and more complex.
Organisms whose cells are prokaryotic are known as prokaryotes and are usually primitive, single-celled, and smaller organisms . On the contrary, organisms (unicellular or multicellular) of cells with a defined nucleus are called eukaryotes and are the most complex forms of life: animals , plants , fungi , etc.
The division between prokaryotic cells and eukaryotic cells is fundamental in the evolutionary understanding of life . The passage from the former to the latter represents an important leap in biological complexity without which organic life as we know it would not exist.
Origin of the prokaryotic cell
Prokaryotes were the first life forms on earth , so it is logical to assume 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 . It is unknown exactly how it was possible to pass from inanimate matter to the first forms of life proper, capable of nourishing, growing and reproducing.
Prokaryotic cell structure
Prokaryotic cells share a basic structure composed of:
- Plasma membrane . Selective and permeable lipid barrier that distinguishes the inside of the cell from the outside.
- Cellular wall. A rigid and external barrier that provides support and tenacity to the cell, although it hinders its growth.
- Cytoplasm . The moist interior of the cell, which is a kind of internal gel.
- Nucleoid. A region of the cytoplasm in which the genetic material of the cell usually accumulates, and which acts as the nucleus, although it has a very irregular shape.
- Ribosomes . Cellular factories of proteins and other substances that the cell synthesizes, expresses and realizes the content of genes.
- 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 Cell organs that allow cell movement.
- 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 mitochondria for the most part).
- Capsule or Glycocalyx. Depending on the type of prokaryote, they are external structures of the cell membrane that serve as a food store and defense against phagocytosis. The capsules are rigid and defined, while the glycocalyx is diffuse as a mucous layer.
- Mesosome. Invaginations of the plasma membrane into the cytoplasm are very frequent in prokaryotes and have been recognized as malformations, despite the fact that 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 diverse genetic information to be conserved, such as resistance to antibiotics in the case of certain bacteria .
Nutrition of prokaryotic cells
\Prokaryotic cells present an enormous diversity of nutritional methods, depending on the type of organism and the habitat in which it evolves. Thus, it is possible to speak of:
- Autotrophic prokaryotes . Those that generate their own food from inorganic elements and energies, using methods such as:
- Photosynthesis. The use of light sun to synthesize utilizable sugars from water and CO 2 .
- Chemosynthesis. The conversion of carbon molecules and other inorganic nutrients into organic matter, using oxidation processes as an energy source .
- Heterotrophic prokaryotes . Those that require the use of organic matter from other living beings (or dead) to survive. They can be of type:
- Parasite. When they enter the organism of a larger living being and feed on its body, causing damage in the process.
- Saprophyte. When they take advantage of the organic waste of other living beings or the decomposing organic matter that remains after their deaths.
- Symbiotic When they benefit from the organic matter of another living being, without causing harm but benefiting it and achieving mutual gain.
Reproduction of prokaryotic cells
Having no cell nucleus, prokaryotic cells divide in only two ways:
- Asexual . By binary fission or bipartition, a quick and easy way in which the cell grows until it divides into two new ones of identical genetic material .
- Parasexual. A method of DNA exchange that allows obtaining genetic variability between prokaryotic cells, and that can occur in three ways:
- Conjugation. When a prokaryotic cell donates to another part of its genetic material and the recipient incorporates it into her own.
- Transduction. When a virus or other microscopic mechanism carries DNA fragments from one prokaryotic cell to another, “infecting” it with new genetic material.
- Transformation. When prokaryotic cells incorporate free genetic material (plasmids) that come from other degraded cells or from their own cytoplasm.
Prokaryotic cell types
Prokaryotic cells are very diverse, which is why they are usually classified based on two different criteria:
- Based on its morphology (shape) :
- Coconuts Spherical, irregular in shape.
- Bacilli In the form of rods.
- Vibrios. With comma shapes, that is, curved.
- Spirilos. Helical or spiral in shape, like a screw.
- Irregular Those that do not have a defined shape, such as arches.
- Based on its cell envelope :
- Gracilicutes. They have a thin peptidoglycan skin .
- Firmicutes. They have a thick wall of peptidoglycan.
- Mendosicutes. Like archaea, whose cell membranes are of different molecular composition from bacteria.
- Tenericutes. They do not have a cell wall, as they are endoparasites.
Importance of the prokaryotic cell
Prokaryotic cells are the first to have been established, that is, they were the first forms of life . They allow us today to imagine what life would have been like on our planet in its first appearances, before more complex forms of life emerged.
In fact, the most accepted theory proposes that there was a serial endosymbiosis process , that is, the beneficial coexistence of some prokaryotes within larger ones. After a slow evolutionary process, from that coexistence the first eukaryotic cells emerged, endowed with cytoplasmic organelles.
Differences with eukaryotic cells
The differences between prokaryotic and eukaryotic cells are not only the absence or presence of the cell nucleus where the DNA is contained. Other notable differences are:
- The shape of DNA. That in eukaryotes is linear, while prokaryotes circular.
- The number of chromosomes . That in eukaryotes it is multiple and in prokaryotes only one.
- The size of the cell. That in eukaryotes it amounts to 10-100 µm, and in prokaryotes only between 0.2-2.0 µm.
- 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.
- The presence of organelles. Like mitochondria, the nucleus, lysosomes , the golgi apparatus , etc. Present in eukaryotes and absent in prokaryotes.
- The complexity of the cytoskeleton. That is hardly structural proteins in prokaryotic cells and complex (microtubules and microfilaments) in eukaryotes.
Domains or super realms of life
In the classification of known life, we usually speak of kingdoms , such as that of plants or animals. But there is an even higher category, known as domain, empire or super kingdom, which allows us to distinguish between two unique forms of life based on their cells: prokaryotes and eukaryotes.
- Prokaryota. Of cells lacking a cell nucleus, they are the most elementary unicellular organisms: bacteria and archaea, sometimes referred to as moneras .
- Eukarya. Of cells endowed with a cell nucleus, it encompasses the rest of known life: plants, fungi, animals and microscopic organisms , unicellular or not, endowed with cell nuclei and organelles.
LUCA is the acronym with which it is known to the Last Common Ancestor Universal , for its acronym in English: Last Universal Common Ancestor. It is the hypothetical living being from which all of us who have ever existed in biological history would descend.
It is estimated that it lived 3.5 billion years ago , and that it is the starting point of life as such. However, it is impossible to rule out that other beings, the same or earlier, have existed at the time.
Examples of prokaryotic cells
Examples of prokaryotic cells are those of bacteria (both gram-negative and gram-positive) or archaebacteria or archaea. Some are non-pathogenic and live outside our bodies, often in habitats inaccessible to humans. Others are the ones we fight on a daily basis, since they are capable of causing us diseases.
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