Prokaryotic cells Definition, Structure, Characteristics, Division.

History of Prokaryotic cells

It is important to note that the distinction between prokaryotes as well as eukaryotes was clearly established by microbiologists Roger Stanier and C. B. van Niel in their 1962 paper on the concept of bacteria (though they used the words procaryote as well as eucaryot in the paper). The paper refers to Edouard Chatton’s 1937 work Titres et Travaux Scientifiques for the use of those terms and for recognizing the distinction. One of the reasons for this classification was that what was at the time known as blue-green algae (now known as the cyanobacteria) was not classified as plants, but rather with bacteria.

Prokaryotic cells Definition

Prokaryotic cells can be described as single-celled structures which are primitive in structure and function because they do not have a membrane-bound nucleus as well as other organelles. The word “prokaryote” is derived from two Greek words “pro” which means “before” and ‘karyon’ meaning “nucleus’. Prokaryotes are thought as the first organism that lived to inhabit the earth since they are the most basic form of life.

Prokaryotic cells Characteristics

The main properties of the prokaryotic cell can be found in the following table:

• Prokaryotic cells are sized between 0.1 up to 5.0 um. They are also significantly smaller than cells from eukaryotic species.
• The form of prokaryotic cells varies between cocci, Bacilli vibrio, and spirilla. Yet, prokaryotic cells that have variations of these forms are also present in nature.
• The cellular structure in prokaryotic cells are primitive because they do not have a nucleus that is membrane bound and various membrane-bound organelles of cells.
• The DNA of prokaryotic cells within one chromosome is composed of a single DNA strand.
• A vital protein, the histone protein, found in the chromosomes eukaryotes is not present in prokaryotic cells.
• Prokaryotic cells are also devoid of the nucleolus, as well as Mitotic apparatus.
• Cell wall in prokaryotic cells are non-cellulosic, and is composed of lipids and carbohydrates.
• Prokaryotic cells can be described as asexual, and therefore reproduce by non-sexual methods without the creation of gametes.

Structure of a prokaryotic cell

The prokaryotic cells is not so complex as cells of eukaryotic origin because they are cellular organelles. Most prokaryotic cells contain the following componentsor parts:

Capsule

This is an extra external covering found in certain prokaryotic cells, which serves to guard the cells against foreign invaders. The capsule is composed of polysaccharides. This allows cells to adhere to different surfaces and keep the moisture inside the cell.

Cell wall

It is the cell’s wall. This wall forms a hard layer of cells that are that is found within the capsule. Its cell walls of many prokaryotes is composed of carbohydrates in polymer form and lipids known as peptidoglycan. In the Archaeal cell, the cell wall isn’t made up of the peptidoglycan, but a different structure known as pseudopeptidoglycan. It is composed of polymers, including proteins. Cell walls provide form to the cell and protects the organelles of the cell located in the cytoplasm of the cell.

Cell membrane/ Plasma membrane/ Cytoplasmic membrane

The cell wall lies beneath the cell membrane, which is composed of phospholipid. The phospholipid makes up an enveloping bilayer comprised of glycerol, which is joined to a hydrophobic head, and two hydrophilic fatty acid tails. In archaea the phospholipid tails are typically connected, creating one monolayer rather than an bilayer. The plasma membrane found in prokaryotic cells offers protection to the cell , while also allowing transportation of crucial molecules inside as well as out.

Cytoplasm

The cytoplasm covers the whole area of cells that is within cell membrane. It is a gel-like cytosol and a water-based solution that is brimming with minerals as well as other ions that are essential for cell. Additionally, the cytoplasm has other cell structures such as the chromosomes, ribosomes and chromosomes.

Ribosomes

Every prokaryotic cell has 70S Ribosomes. The 70S ribosomes are comprised of two parts, 30S and 50S. In this case, the 50S subunit is comprised of 23S and 5S rRNA. The 30S subunit has 16S rRNA. The ribosome is one of the most frequently observed internal structure inside prokaryotic cells. The size and the number of ribosomes are different between prokaryotic cells. The ribosome is responsible the creation of polypeptides and, consequently, proteins.

Nucleoid region

The nucleoid prokaryotic cells’ cytoplasm contains one circular chromosome as well as tiny rings of extrachromosomal DNA , known as plasmids. One circular chromosome appears in a singular copy genetic material, unlike the two DNA copies found in the eukaryotes. Prokaryotic genomes are smaller than genomes of eukaryotes. The plasmids in turn are copied in a separate fashion from outside the chromosomes. The plasmids could carry non-essential genes.

Appendages

A large number of prokaryotic cells possess cell appendages that extend from the cell’s surface in the form of pili, flagella and fimbriae. Flagella are among the most frequent appendages found in many prokaryotic cells. These are structures resembling tails which aid the cell in moving about. Fimbriae are tiny filaments used to attach cells to different surfaces. Pilli are, in turn are larger filaments which play different roles in various cells. A good example is the sexual pilli, which keeps two cells in sync in the process of transferring DNA molecules via this process called conjugation.

Division of prokaryotic cells (Reproduction)

As previously mentioned Prokaryotic cells reproduce sexually without the creation of gametes. Asexual reproduction methods within prokaryotes include:

Binary fission

Binary fission is a form of asexual reproduction in which one living cell, or organelle doubles its size before splitting into identical twin cells in which each of the daughter cells can be expected to increase in size of the organelle or cell. Binary fission is the method of reproduction in a variety of prokaryotes, including archaea, cyanobacteria, as well as Eubacteria. In this process, the genetic material from cells that are parented is evenly split into 2 daughter cells. In the end, there is there is no genetic variation in the newly created prokaryotic cells.

Steps of binary fission

1. The DNA in the cell divides to create two DNA molecules identical to each other and both are transported toward the cell’s membrane.
2. The cell will then double in size as the cell’s membrane gradually begins to divide , with each cell getting a copy of DNA.
3. When that division in the cell membrane has been complete the cell wall gets created between two DNA strands, which divides cells from the same parent into identical daughters cells.

Recombination

In this process, the genes from one bacteria are transferred into the genome of another bacteria. This process occurs in three ways: conjugation and transformation, as well as transduction.

• Conjugation is a process by where genes are exchanged between two bacteria via the structure of a protein tube known as pilus.
• Transformation is the process of sexual reproduction where the DNA of the environment is taken up by the bacterial cells and integrated into its DNA.
• Transduction is the procedure in where the genetic material gets transferred to the bacterial cells by using viruses. Bacteriophages are the virus which starts the process.

Prokaryotic cell examples

Bacterial cells

Bacteria are single-celled organisms which are found in all ecosystems around the globe. Cell wall in a bacteria’s cell is made of peptidoglycan which creates a tough and dense cell. Capsules are unique to certain bacteria and may not be found on other prokaryotic cell types. The genetic material of bacteria can be found by way of spindle-shaped chromosomes. The most common bacteria include E. coli, Streptomyces spp, Pseudomonas spp, etc.

Archaeal cell (Archaea)

Archaeal cells are akin to bacteria in that they are unicellular primitive organisms. Archaeal cells tend to be found in extreme environments such as the hot springs of oceans and marshlands. The capsule isn’t present in archaeal cells, however cells’ walls are composed of pseudopeptidoglycanthat is comprised of proteins. The surface of cells of archaea is an phospholipid monolayer that shields the cell from extreme conditions. Archaeal cells include Halobacterium Spp. Thermoplasma Spp Sulfolobus spp, and so on.

Prokaryotic Cell size

The typical prokaryotic cell ranges between 0.1 up to 5.0 millimeters (mm) in diameter . They are considerably smaller than eukaryotic cells with diameters that range between 10 and 100 millimeters.

The following figure shows the size of prokaryotic bacteria, and eukaryotic cells, both animal and plant, and other organisms and molecules on the scale of logarithms. Each increment on an logarithmic scale signifies an increase of 10 times the amount being measured, which means these are massive dimensions that we’re discussing!

There are a few interesting exceptions, check at the single-celled marine algae Caulerpa–cells have to remain relatively small, whatever their prokaryotic Eukaryotic. Why is this the situation? The answer lies in the fact that as cells get bigger they become more difficult in their ability to transfer sufficient food and waste with their surroundings. To see how this works, let’s look at a cell’s surface-area-to-volume ratio.

Imagine, for the sake to keep things simple it’s an organelle that is shaped like cubes. Certain plant cells are actually cube-shaped. In the event that the width of one cube’s edges is and the size of the cube would be 6l2 which is l3. This means that , as the size of l increases and bigger, the area of the cube will grow rapidly since it increases with the square of the cube of. The volume will grow even more rapidly since it changes in proportion to the cube of.

Thus, as a cell gets bigger, its surface-area-to-volume ratio drops. For example, the cube-shaped cell on the left has a volume of 1 mm^3 and a surface area of 6 mm^2 with a surface-area-to-volume ratio of six to one, whereas the cube-shaped cell on the right has a volume of 8 mm^3 and a surface area of 24 mm^2 with a surface area-to-volume ratio of three to one.

Surface-area-to-volume ratio is important because the plasma membrane is the cell’s interface with the environment. If the cell wants to absorb nutrients, it has to take them through the membrane. Likewise, if it is required to get rid of pollutants, the membrane is the only way to go.

Each membrane patch can exchange a certain amount of a particular substance over the time frame of For instance, since it has a small amount of channels. If the cell becomes to be too big, its membrane won’t have enough capacities for exchange (surface area and the square function) to sustain the amount of exchange needed to support its metabolic growth (volume and cubic function).

The surface-area-to-volume problem is just one of a related set of difficulties posed by large cell size. As cells grow they also take longer to move materials within of them. This puts an upper limit on the size of cells as eukaryotic cells are capable of surpassing prokaryotic cells because of their metabolic and structural characteristics, which we’ll look at in the next section.

Some cells also use geometric tricks to get around the surface-area-to-volume problem. For instance, certain cells are thin and long or have numerous protrusions on their surfaces, which enhance the surface area relative to volume^2.

Morphology/Shape of Prokaryotic cells

Prokaryotic cells come in a variety of shapes. The four most common designs that bacteria have are

• Cocci – A bacterium that is spherical or ovoid is called a coccus (Plural, cocci). e.g. Streptococcus, Staphylococcus.
• Bacilli – Bacilli is a bacterium having a cylindrical shape , also known as a rod, or a Bacillus (Plural, Bacilli).
• Spiral bacteria – Some rods twist into spiral shapes and are called spirilla (singular, spirillum).
• Vibrio – comma-shaped

The archaeon Haloquadratum is composed of flat square-shaped cells.

Components of Prokaryotic Cells

The prokaryotic cells contain four primary components:

• Plasma Membrane- It’s an outer covering made of phospholipid molecules that separates the cell from the environment.
• Cytoplasm–  it is the jellylike substance that exists within the cell. Cell organelles of all kinds are suspended within it.
• DNA– It is the genetic material of the cell. Every prokaryote has the circular DNA. It determines the type of proteins the cells produce. Also, it regulates actions by cells.
• Ribosomes– Protein synthesis occurs here.

Some prokaryotic cells contain cilia and flagella that assist in locomotion.

Frequently Asked Questions

Q1. what is the difference between prokaryotic and eukaryotic cells?

The primary distinction between these two types of organisms is that eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not.

Q2. what are prokaryotic cells?

Prokaryotic cells are the cells that do not have a true nucleus and membrane-bound organelles.

Q3. what is found in both prokaryotic and eukaryotic cells?

Ribosome. Prokaryotes and eukaryotes are the two different types of cells.  Eukaryotic cells contain membrane-bound organelles, such as the nucleus, endoplasmic reticulum, mitochondria while prokaryotic cells do not but the ribosome is the only organelle that can be seen in both prokaryotic and eukaryotic cells.  Prokaryotic and eukaryotic ribosomes perform the same functions that is protein synthesis, however, eukaryotic ribosomes are much larger than prokaryotic ones.

Q4. what do eukaryotic cells have that prokaryotic cells do not?

The primary distinction between these two types of organisms is that eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not. The nucleus is where eukaryotes store their genetic information.

Q5. which cell structures are seen in prokaryotic and eukaryotic cells?

Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane, or cell membrane, is the phospholipid layer that surrounds the cell and protects it from the outside environment.

Q6. which of the following attributes is common to both prokaryotic cells and eukaryotic cells?

Plasma membrane, ribosomes, cytoplasm, and DNA. 

Q7. which of the following statements is incorrect regarding prokaryotic cells?

A) Their DNA is not enclosed within a membrane.

B) They lack membrane-enclosed organelles.

C) They typically have a circular chromosome.

D) They reproduce by binary fission.

E) They lack a plasma membrane.

Ans: E) They lack a plasma membrane.

Q8. how do prokaryotic cells divide?

The usual method of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicate and original chromosomes are separated.

Q9. where do transcription and translation occur in prokaryotic cells?

Prokaryotic transcription occurs in the cytoplasm alongside translation.

Q10. which of the following is a major difference between prokaryotic cells and eukaryotic cells?

The primary distinction between these two types of organisms is that eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not.

Q11. what are examples of prokaryotic cells?

Examples of prokaryotes are blue-green algae, bacteria and mycoplasma. Among prokaryotes, bacteria are the most common and multiply very fast.

Q12. what are the similarities between prokaryotic and eukaryotic cells?

Three similarities between prokaryotic and eukaryotic cells are that both have vesicles, vacuoles, and the ability to carry out the eight functions of life. Prokaryotes do not have organelles.

Q13. which of the following is not found in prokaryotic cells?

Prokaryotes lack a defined nucleus (which is where DNA and RNA are stored in eukaryotic cells), mitochondria, ER, golgi apparatus, and so on. In addition to the lack of organelles, prokaryotic cells also lack a cytoskeleton.

Q14. which of the following involves metabolic cooperation among prokaryotic cells?

A) Biofilms

B) Binary fission

C) Endospore formation

D) Endotoxin release

E) Photoautotrophy

Q15. what do prokaryotic cells lack?

Prokaryotes lack a defined nucleus (which is where DNA and RNA are stored in eukaryotic cells), mitochondria, ER, golgi apparatus, and other membrane bound organelles. In addition to the lack of organelles, prokaryotic cells also lack a cytoskeleton.