Prokaryotic Cells

What are Prokaryotic Cells?

A prokaryotic cell is a single-celled organism without a true nucleus or other membrane-bound organelles, usually found in bacteria and archaea. Prokaryotic cells are defined by their simplicity, their genetic material being put not inside a nuclear envelope but into a nucleoid area.

They have a cell membrane, cytoplasm, ribosomes, and sometimes a cell wall. The DNA is generally one circular chromosome. Some may also contain plasmids, small, circular DNA molecules that can be transferred from one cell to another.

Characteristics of Prokaryotic Cells

The major characteristics of prokaryotic cells include:

Lack of True Nucleus

A prokaryotic cell lacks a nucleus. In place of the nucleus, it is characterised by the presence of a nucleoid region that does not have a limiting membrane, and in prokaryotes, the DNA is generally one singular, circular chromosome that occurs in free space within the cell.

Absence of Membrane-Bound Organelles

One of the crucial distinguishing features of prokaryotic cells is that they lack membrane-bound organelles. Such membrane-bound organelles, typical in eukaryotic cells, such as mitochondria, endoplasmic reticulum, and Golgi apparatus, do not exist in prokaryotic cells. They carry out all their cellular activities either in the cytoplasm or at the membrane.

Small Size and Simple Structure

Generally, the prokaryotic cells are very small compared to the eukaryotic cells and range in size from 0.1 to 5.0 µm in diameter. This small size permits a greater surface area-to-volume ratio that speeds up nutrient uptakes and waste removal.

Prokaryotic cells are the very basic and simple forms of life. All the complex structures found in eukaryotic cells are absent here, which makes the prokaryotic cells simple and small. Although simple, prokaryotic cells work well and are skilled in performing all life functions.

Structure of Prokaryotic Cells

The structure of prokaryotic cell includes:

Cell Wall

• The cell wall is primarily composed of peptidoglycan (in bacteria).

• It provides structural support and protection.

• It also maintains the shape of the cell.

Plasma Membrane

• The plasma membrane consists of a lipid bilayer with embedded proteins.

• It controls the passage of substances in and out of the cell

• It is the site of many metabolic activities.

Cytoplasm

• The cytoplasm contains the cytosol, ribosomes, and genetic material.

• It is the site of metabolic processes and biochemical reactions.

• It suspends cell structures and has a role in moving materials within the cell.

Nucleoid

• Nucleoid is the region in the cytoplasm where the cell's DNA is located.

• It contains a single, circular chromosome.

• It is responsible for storing and transmitting genetic information.

Plasmids

• Plasmids are small, circular DNA molecules found in the nucleoid.

• It carries additional genes—genes for antibiotic resistance are an example.

• Can be transferred between cells; they often carry non-crucial genes.

Ribosomes (70S)

• Prokaryotic ribosomes are composed of an RNA and several proteins.

• They are smaller in size compared to eukaryotic ribosomes (70S).

• They are the sites of protein synthesis

Pili and Fimbriae

• These hair-like appendages are found on the surface of the cell.

• They are involved in DNA transfer from one cell to another during conjugation.

• They are involved in attachment to surfaces and other cells

Flagella

• Flagella is a long, whip-like structure.

• It provides motility through a rotary motion.

• It also allows the cell to move in a favourable environment and away from a potentially harmful one.

Types of Prokaryotic Cells

The types of prokaryotes are:

Bacteria

Bacteria are one of the two super kingdoms of prokaryotic cells. They can be found in almost every habitat on Earth. Bacteria are of tremendous importance to most ecosystems because they participate in the nutrient cycling, such as nitrogen, carbon cycles, and elements such as phosphorus, sulfur, and iron. They occur in several shapes, such as cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped). Some examples of prokaryotic cells include Salmonella, Cyanobacteria, Streptococcus, etc.

Archaea

Archaea is the other half group of prokaryotic cells, apart from bacteria. They are similar to bacteria in that they are unicellular, and neither have a membrane-bound nucleus nor organelles. However, a significant difference between archaea and bacteria is genetic makeup and biochemistry. Archaea usually resides in extreme conditions like very high or low temperatures, high salinity, high pressure, and low oxygen. They are known to thrive under diverse environmental conditions.

Reproduction in Prokaryotes

Since prokaryotes are structurally simple organisms, their mode of reproduction is also simple.

Binary Fission

• Prokaryotic cells reproduce primarily through binary fission, an asexual process where a single cell divides into two genetically identical daughter cells.

• This simple and efficient process of reproduction permits fast growth in this kind of cell, especially when conditions are right.

• Binary fission takes place in several stages: the cell enlarges, the DNA is replicated, and the two identical copies are attached to different regions of the cell membrane.

• The membrane grows inwards at the centre, dividing the cell into two. Finally, the cell membrane and wall divide the original cell in half, and two daughter cells result, each having the complete genetic material of the original cell.

Genetic Exchange – Conjugation, Transformation, Transduction

The mechanism of genetic exchange is described below:

Conjugation: It is a method in which there is a direct transfer of DNA from one prokaryotic cell to another such that a pilus physically connects the two. This gives one cell the ability to offer its plasmid or a small piece of chromosomal DNA to another cell, thus enhancing genetic variation.

Transformation: This mechanism involves the uptake of free DNA fragments from within the environment by a prokaryotic cell. This DNA is of lysed cells and may be integrated into the recipient cell's genome, leading to new genetic traits.

Transduction: This transfer of genetic material between bacteria results from an accident by bacteriophages, a set of viruses infecting bacteria. Bacteriophages take up a fragment of the host bacterium's DNA during an infection and then bring such genetic material to another bacterial cell, thus promoting genetic exchange.

Functions of Prokaryotic Cells

The functions of prokaryotes are:

Decomposers

Prokaryotic cells, especially bacteria, are the chief decomposers in the ecosystem. They break down dead organic materials, thus recycling nutrients back into the environment. This in effect helps nutrients be retained in the soil, which maintains soil fertility and promotes plant growth, which is good for the whole ecosystem. Some examples of prokaryotic cells are, E.coli, streptococcus, etc.

Nitrogen Fixation

Certain prokaryotes, such as cyanobacteria and rhizobia, can fix nitrogen. They change the atmospheric nitrogen to forms that are useful to plants. Nitrogen is one of the principal components of proteins and nucleic acids of many plants. The use of nitrogen-fixing prokaryotes is critical in agriculture to improve the health of soils besides improving productivity.

Pathogens

Several prokaryotic cells are pathogenic and they cause various diseases in mammals, plants, and others. Escherichia coli and Mycobacterium tuberculosis are some of the bacteria that cause severe infection. This knowledge is essential in the development of antibiotics, vaccines, etc., to counter diseases caused by various pathogenic micro-organisms and to ensure human health.

Symbiotic Relationships

Prokaryotes are involved in many different types of symbiotic relationships: mutualism, commensalism, and parasitism. The interactions bring out the different roles and contributions to ecosystems that are very diverse and very crucial.

Differences Between Prokaryotic and Eukaryotic Cells

Prokaryotic cells are far simpler than eukaryotic cells, lacking most of the structural features that define eukaryotes. The differences between these two different cell forms highlight the increasing complexity through evolution and the differentiation of higher organisms. Prokaryotic and eukaryotic cells are differentiated on the following basis:

Applications of Prokaryotic Cells

The applications of prokaryotic cells includes:

Biotechnology (Insulin, Antibiotics, Vaccines)

In biotechnology, prokaryotic cells have uses in genetic engineering for the production of proteins, enzymes, and other valuable products. Bacteria, for example, are genetically engineered to manufacture insulin and other medicinal products. Conversely, in the production of antibiotics, their derivation from the natural metabolism of some bacteria comes in handy.

Environmental Applications (Bioremediation, Waste Treatment)

Environmental applications of prokaryotic cells in bioremediation modified treatment processes. Non-prokaryotic cells degrade the dangerous components to less harmful ones. Prokaryotes also come in handy in waste treatment processes that deal with the degradation of the organic matter in sewage. Thus contributing to the development of more sustainable ways of taking care of waste.

Medical Research

Prokaryotic cells have a crucial role in modified treatment methods. They can be the basis for developing vaccines against bacterial agents and used to develop new treatments. For example, pathogenic bacteria, by being studied, can help scientists devise vaccines for bacterial infections or even explain bacterial apparatuses for developing new treatments. This is basically how important prokaryotic cells are for medical science and the general health benefit.

Prokaryotic Cells NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Characteristics of prokaryotic cells

• Structure of prokaryotic cells

• Prokaryotic vs Eukaryotic Cells

Practice Questions for NEET

Q1. In prokaryotes, as there is no well-defined nucleus, protein synthesis occurs in

1. Nucleoid

2. Cell membrane

3. Cytoplasm

4. Capsule

Correct answer: 3) Cytoplasm

Explanation:

In prokaryotes, the synthesis of proteins transpires within the cytoplasmic compartment. The absence of a distinct nucleus results in the localization of their genetic content, DNA, in the nucleoid area of the cytoplasm. The ribosomes, which are 70S in size, are the primary sites for this synthesis. These organelles are not confined to a particular area, unlike in eukaryotes, and are free-floating. A notable feature is the coupling of transcription, converting DNA to mRNA, and translation, which involves the synthesis of proteins from mRNA, in the same cytoplasmic environment. This concurrent process is a key characteristic of prokaryotic protein synthesis.

Hence, the correct answer is option 3) Cytoplasm

Q2. Infoldings of the plasma membrane in the prokaryotic cell are called

1. Chromosomes

2. Ribosomes

3. Peroxisomes

4. Mesosomes

Correct answer: 4) Mesosomes

Explanation:

In prokaryotic cells, the plasma membrane folds into mesosomes.

They donate to the cell membrane's expanded surface area. This encourages the cell's capacity to execute necessary functions including cell division and breathing. In the past, mesosomes were supposed to be important in these activities. Their exact role is yet unknown to scientists, though. Mesosomes are a fascinating aspect of prokaryotic cells despite this.

Hence, the correct answer is option 4)Mesosomes.

Q3. DNA is held with some proteins in a region termed as nucleoid in:

1. Prokaryotes

2. Eukaryotes

3. Mesokaryotes

4. All of these

Correct answer: 1) Prokaryotes

Explanation:

The prokaryotic cells lack a membrane-bound and defined nucleus. Their genetic material is localized in the cytoplasm as the nucleoid. The cells are surrounded by the cell envelope that is made up of glycocalyx, cell wall, and plasma membrane. For example, for E. Coli, the DNA is not scattered throughout the cell. DNA is held with some proteins in a region termed a nucleoid.

The DNA in nucleoids is organized in a larger loop held by proteins.

Hence, the correct answer is option 1) Prokaryotes.

Also Read:

• MCQs on Prokaryotic Cells

• Semi autonomous Organelles

• Endoplasmic Reticulum

Recommended video for "Prokaryotic Cells"