Prokaryotic Cell Definition
Prokaryotic cells are unicellular entities that are primitive in structure and function, as they lack a membrane-bound nucleus and other organelles. Prokaryotes are considered the first living organisms on earth as they are the simplest form of life.
Characteristics of prokaryotic cells
The general characteristics of prokaryotic cells are listed below:
- In general, prokaryotic cells range in size from 0.1 to 5.0 µm and are considerably smaller than eukaryotic cells.
- The shape of prokaryotic cells varies from cocci, bacilli, spirilla, and vibrio. However, prokaryotic cells with modifications of these forms are also found in nature.
- The cellular organization of prokaryotic cells is primitive since they lack a membrane-bound nucleus and other membrane-bound cellular organelles.
- The genetic material of prokaryotic cells on a single chromosome is made up of a single strand of DNA.
- A critical protein, the histone protein, which is found attached to the chromosomes of eukaryotes is absent in prokaryotic cells.
- Prokaryotic cells also lack the nucleolus and the mitotic apparatus.
- The cell wall of prokaryotic cells is not cellulosic and is made up of carbohydrates and lipids.
- Prokaryotic cells are asexual and therefore reproduce by asexual means without the formation of gametes.
Structure of a prokaryotic cell
The structure of a prokaryotic cell is not as complex as eukaryotic cells since they have primitive cellular organelles. In general, most prokaryotic cells have the following components/parts:
- This is an additional outer covering on some prokaryotic cells that serve to protect the cell against foreign invaders.
- The capsule is made of polysaccharides, which allows cells to adhere to various surfaces and preserves moisture in the cell.
- The cell wall is a resistant nucleus of prokaryotic cells present within the capsule.
- The cell wall of most prokaryotes is made up of a carbohydrate and lipid polymer called peptidoglycan.
- However, in Archaeal cells, the cell wall does not contain peptidoglycan, but some other structure called pseudopeptidoglycan. It is made up of proteins and other polymers.
- The cell wall provides shape to the cell while protecting the cell organelles present in the cell’s cytoplasm.
Cell membrane/ Plasma membrane/ Cytoplasmic membrane
- Beneath the cell wall is a cell membrane made up of phospholipids.
- The phospholipid forms a bilayer consisting of a lipid composed of glycerol attached to a hydrophobic phosphate head and two tails of hydrophilic fatty acids.
- In archaea, the phospholipid tails are generally connected, forming a monolayer rather than the bilayer structure.
- The plasma membrane in prokaryotic cells provides protection for the cell while allowing the transport of essential molecules in and out of the cell.
- The cytoplasm is the complete space of the cells presents within the cell membrane.
- It contains a gel-like cytosol and a water-based solution that contains minerals and other essential ions for the cell.
- Furthermore, the cytoplasm also contains other cellular structures such as chromosomes and ribosomes.
- All prokaryotic cells have 70S ribosomes. The 70S ribosomes are made up of two subunits, 30S, and 50S.
- Here, the 50S subunit contains 23S, and 5S rRNA and the 30S subunit contains 16S rRNA.
- The ribosome is the most commonly observed internal structure in prokaryotic cells.
- The size and number of ribosomes differ in different prokaryotic cells.
- The ribosome is responsible for the formation of polypeptides and, in turn, proteins.
- The nucleoid region of the cytoplasm in prokaryotic cells contains a single circular chromosome and small rings of extrachromosomal DNA called plasmids.
- The single circular chromosome is present as a single copy of genetic material in contrast to the two copies of DNA in eukaryotes.
- Prokaryotic genomes are also smaller than eukaryotic genomes.
- Plasmids, in turn, independently copy outside of the chromosomes. These plasmids can carry some non-essential genes.
- Many prokaryotic cells have cellular appendages that protrude from the cell surface such as flagella, pili, and fimbriae.
- Flagella are the most common appendages in many prokaryotic cells.
- These are tail-like structures that help the cell move.
- Fimbriae are thin, filamentary structures that are used to stick cells to various surfaces.
- The pills, in turn, are longer filaments that have different roles in different cells. An example of this is the sex pill that holds two cells together while transferring DNA molecules through the conjugation process.
Division of prokaryotic cells (Reproduction)
As previously mentioned, prokaryotic cells reproduce asexually without gamete formation. Some asexual ways of reproduction in prokaryotes are:
- Binary fission is a type of asexual reproduction in which a single living cell or organelle grows twice its size and then divides into two identical daughter cells, where each of these daughter cells has the potential to grow to the size of the original cell or organelle.
- Binary fission is the mode of reproduction in many prokaryotes, such as archaea, cyanobacteria, and eubacteria.
- During this process, the genetic material of the stem cell is divided equally into two daughter cells. As a result, no genetic variation is observed in newly formed prokaryotic cells.
Steps of binary fission
- The cell’s DNA divides to form two identical DNA molecules, which move to the cell membrane.
- The cell then doubles in size, and the cell membrane slowly begins to divide, each having a copy of the DNA.
- Once the division of the cell membrane is complete, the cell wall forms between the two strands of DNA that divide the stem cell into two identical daughter cells.
- Another mode of asexual reproduction in prokaryotic cells is by recombination.
- In this case, the genetic material of one cell is incorporated into the cell of another prokaryote by transduction, transformation, and conjugation.
- In conjugation, two cells are connected through sex pills where genes are transferred through the pills.
- In transformation, the prokaryotic cell takes the genetic material from the environment and incorporates it into the bacterial chromosome.
- In transduction, gene exchange occurs through a viral infection. The bacteriophage first infects a bacterium and takes the target gene and transfers it to another cell.
Prokaryotic cell examples
- Bacteria are single-celled organisms found in all ecosystems in the world.
- The cell wall of the bacterial cell is made of peptidoglycan which makes it resistant and thick.
- The capsules are exclusive to some bacteria and therefore may not be present in other prokaryotic cells.
- The genetic material of the bacteria is present in the form of circular coils of chromosomes.
- Examples of bacterial cells are E. coli, Streptomyces spp, Pseudomonas spp, etc.
Archaeal cell (Archaea)
- Archael cells are similar to bacterial cells in that they are also primitive single-celled organisms.
- Archaeological cells are found mainly in extreme environments such as hot springs, oceans, and marshes.
- The capsule is not present in archaic cells, and the cell wall is made up of pseudopeptidoglycan, made up of proteins.
- Similarly, the cell membrane of archaic cells has a phospholipid monolayer that protects the cell against hostile environments.
- Examples of archaic cells are Halobacterium spp, Thermoplasma spp, Sulfolobus spp, etc.
How do Prokaryotic Cells Divide?
Prokaryotic cells divide through the binary fission process. Unlike mitosis, this process does not involve DNA condensation or organelle duplication. Prokaryotic cells have only a small amount of DNA, which is not stored on complex chromosomes. Also, there are no organelles, so there is nothing to divide.
When a prokaryote grows to a large size, the binary fission process occurs. This process duplicates DNA, then separates each new DNA strand into individual cells. This process is simpler than mitosis, and as such bacteria can reproduce much faster.