Prokaryotic Cells: Definition, Structure, Components, Diagram

The cell which lacks a true nucleus and membrane-bound organelles is called a prokaryotic cell. The cells of bacteria and archaea are prokaryotic, hence called true prokaryotes. A prokaryotic cell has a single membrane, and all the reactions occur within the cytoplasm.

• Escherichia coli and Cyanobacteria are the two most common examples of prokaryotic cells.
• These cells are smaller in size as compared with the eukaryotic cells.  
• They possess a cell wall that is made up of peptidoglycan.

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What is a Prokaryotic Cell?

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A prokaryotic cell is one that lacks a nucleus and other membrane-bound organelles. Microorganisms containing prokaryotic cells are called prokaryotes such as bacteria and archea. These were the first organisms to ever exist on the face of Earth. Due to lack of compartmentalisation in prokaryotic cells, some major cell organelles such as mitochondria, chloroplasts, and lysosomes are not present. 

• They have a free-floating circular chromosome.
• These cells have a small range of organelles, generally, a plasma membrane and ribosomes.
• Prokaryotes lack a separate nucleus and other organelles due to the absence of internal membranes.
• All prokaryotic organism have nucleoids, DNA, RNA, ribosomes, and cytosol.
• Prokaryotic cells use DNA as the basis for their genetic information. 
• They reproduce via a cell division process called binary fission.

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Characteristics of Prokaryotic Cell 

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Prokaryotic cells have a variety of characteristics, a few of which are mentioned below: 

• They are devoid of a nuclear membrane.
• There is an absence of mitochondria, Golgi bodies, chloroplasts, or lysosomes. 
• A single chromosome contains all of the genetic material.
• They lack histone proteins, which are essential components of eukaryotic chromosomes.
• Carbohydrates and amino acids make up the cell wall.
• The plasma membrane serves as the respiratory enzyme-carrying mitochondrial membrane.
• They divide by the asexual mode of reproduction called binary fission. Conjugation is the sexual mode of reproduction.

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Prokaryotic Cell Structure

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All prokaryotes have a cell wall and a genetic material but lack a nuclear membrane. They come in a variety of shapes, including spherical, rod-shaped, and spiral. In this section, we will discuss the prokaryotic cell structure in detail.

• Capsule– In addition to the cell wall, bacterial cells have an outer protective coating called a capsule. It aids in the retention of moisture, the protection of cells when ingested, and the adhesion of cells to nutrients and surfaces.
• Cell wall– The outermost layer of the cell is the cell wall that provides structure to the cell. 
• Cytoplasm– The cytoplasm is a gel-like component that consists of enzymes, salts, and cell organelles.
• Cell Membrane– This membrane surrounds the cytoplasm and controls the flow of chemicals into and out of the cells.
• Pili– These are hair-like protrusions on the surface of other bacterial cells. It helps in adhesion and transfer of genetic material during reproduction.
• Flagella– These are elongated whip-like structures that aid in the movement of cell. 
• Ribosomes– Protein synthesis is carried out by ribosomes.
• Plasmids– These are DNA structures that are not found on chromosomes. These do not have a role in reproduction.
• The nucleoid region– It is the part of the cytoplasm that contains the genetic material.

Organelles such as mitochondria, endoplasmic reticulum, and Golgi bodies are absent in prokaryotic cells.

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Prokaryotic Cell Diagram

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The prokaryotic cell diagram is given below. It is mainly composed of a capsule, cell wall, plasma membrane, cytoplasm and ribosomes.

Prokaryotic Cell Diagram  

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Components of Prokaryotic Cell

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The Plasma Membrane, Cytoplasm, DNA and Ribosomes are the four important components of a prokaryotic cell. 

Plasma Membrane

A plasma membrane is a protective layer of phospholipid molecules that segregates the cell from its surroundings. It allows the movement of only certain molecules in and out of the cell as it is a selectively permeable membrane

Cytoplasm

Inside the cell, there is a jelly-like fluid called cytoplasm. It holds all of the cell organelles in suspension. It is made up of salts, water, and other organic molecules. 

DNA

The genetic material of the cell is DNA. Circular DNA is found in all prokaryotes. It controls which proteins the cell produces and also controls the actions of the cell. 

Ribosomes

Protein synthesis takes place in ribosomes. They are composed of RNA and proteins. Cilia and flagella are seen on some prokaryotic cells, which aid in movement.

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Reproduction in Prokaryotes

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Reproduction in prokaryotes can occur either asexually or sexually. The asexual mode of reproduction is binary fission, while conjugation is the sexual reproduction mode. 

Binary Fission

Binary fission is a type of asexual reproduction where the cell divides into two daughter cells.

• The cell wall begins to expand and move inwards.
• The cell is subsequently divided into two daughter cells by forming a cell wall between each DNA strand.

Recombination 

Prokaryotic cells reproduce sexually via conjugation, transformation, and transduction.

• Conjugation is a process where two organisms (lower species) like bacteria exchange genetic material with each other. This is done via a protein tube called pilus.
• Transformation is a type of sexual reproduction in which the prokaryotes take the DNA of other prokaryotes from their surroundings and incorporate it into their DNA.
• The process of genetic material being transmitted into a bacterial cell with the help of viruses is known as transduction. Bacteriophages are the virus that begins this process.

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Examples of Prokaryotic Cells

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The following are some examples of prokaryotic cells:

Bacterial Cells

These are single-celled organisms that can be found anywhere on Earth. They come in a variety of shapes and sizes.

• The cell wall of bacterial cells is made up of peptidoglycan, which gives the cell wall its structure.
• Bacteria have several distinct structures, including pili, flagella, and capsules.
• They also have plasmids, which are extrachromosomal DNA.
• They have the ability to generate endospores, which are robust, dormant structures that let them survive in harsh environments.
• When the conditions are favorable again, the endospores become active.

Archaeal Cells

Archaebacteria are unicellular creatures similar in structure and size to bacteria.

• They can be found in a variety of habitats, including hot springs, as well as dirt, marshes, and even inside humans.
• They have a flagella and a cell wall.
• Peptidoglycan is not found in the cell walls of archaea.
• Distinct lipids with completely different stereochemistry are found in the membranes of archaea.
• Archaea, like bacteria, have a single circular chromosome. Plasmids are also present.

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Things to Remember

• Prokaryotic cell wall is made up of peptidoglycan layer in bacteria.
• Archael cell walls are composed of pseudo peptidoglycan. 
• These cells are very small in size ranging between 0.2 μm and 2.0 μm in diameter.
• The prokaryotic cells reproduce via both asexual and sexual modes. 
• Mycoplasm, cyanobacteria, and bacteria are other examples of prokaryotes.