Labeled Prokaryotic Cell Diagram, Definition, Parts and Function

Definition of Prokaryotic Cell

Unicellular organisms of the domains Archaea and Bacteria are classified as prokaryotes. Prokaryotic cells lack membrane-bound cellular organelles like lysosomes, nucleus, mitochondria, Golgi complex, and endoplasmic reticulum.

Prokaryotic microorganisms are found to be more primitive than eukaryotes.

The cells of higher organisms (eukaryotes) evolved from prokaryotic cells. Thus, these unicellular organisms are considered the earliest living forms that appeared on earth, four billion years ago.

Prokaryotes include both archaea and bacteria. The archaea include inhabitants of some of the most extreme environments, such as thermophiles, methanogens, halophiles, etc. Bacteria include both pathogens and beneficial microbes, such as antibiotics, fermented products, and essential vitamins.

Labeled Prokaryotic Cell Diagram

Parts of prokaryotic cell and their functions

1. Pilus (plural Pili)

• Pili are tiny, thread-like protein tubes found on the surface of prokaryotic cells.
• At the cellular level, they originate from the plasma membrane of prokaryotic cells.
• They are mostly made of a protein called pilin.
• Pili are responsible for the movement of bacterial cells and are also involved in adherence to different surfaces, which is an important virulence characteristic of bacterial pathogens and promotes infection.

2. Cytoplasm

• The cytoplasm of prokaryotes does not possess membrane-bound organelles.
• However, there is an exception: some prokaryotic cells contain specialized membrane-bound organelles in their cytoplasm, which include magnetosomes and acidocalcisomes, as well as protein-based organelles, such as carboxysomes.

3. Ribosomes

• The ribosome (70S) of bacteria is a composite of two asymmetric subunits, the 50S and the 30S subunits, which assemble at the specific site on the mRNA during the initiation of translation.
• This binding site is a sequence of nucleotides located upstream of the start codon on the mRNA transcript.
• Each ribosomal subunit can contribute to specific functions for the synthesis of bacterial proteins.
• Moreover, ribosomes give the cytoplasm of prokaryotes a granular appearance.

4. Capsule

• The bacterial capsule is a polysaccharide coat that covers the cell walls of most bacteria.
• They protect bacterial cells from harsh environmental conditions.
• Capsules are found to have a role in the infection and colonization of several pathogenic bacteria. 
• In most cases, capsules are synthesized by bacteria in response to harsh conditions like higher temperatures and higher glucose concentrations. 
• For example, the virulence of Aeromonas species is due to the production of the capsule when they enter the host blood system, where the environment is rich in glucose.
• Moreover, the capsule is also involved in transmission, adhesion, and survival and also provides bacterial resistance to the host immune defense.

5. Cell Wall

• The cell wall is thin and flexible in nature and extends when bacteria grow.
• The cell wall is a stress-bearing layer that regulates the shape of the bacteria.
• The bacterial cell wall is mostly made up of peptidoglycan, or murein.
• Peptidoglycan is a composite of both sugars and amino acids.
• Peptidoglycan is important for cell wall structural integrity.
• Cell wall structural integrity is crucial for bacterial viability.

6. Plasma membrane

• The plasma membrane of bacteria is composed of phospholipids that form a double-layered structure called a phospholipid bilayer.
• The plasma membrane is amphipathic in nature.
• The interior part of the membrane contains hydrophobic fatty acid side chains (non-polar tails), while the exterior part of the membrane contains hydrophilic phosphate head groups (polar heads). 
• It provides overall protection for the cell and maintains a favorable environment inside the cell.
• The phospholipid bilayer is a medium for different embedded proteins which carry out several functions, including selective transport of molecules, cell-to-cell communication, and virulence.
• The proteins embedded in the plasma membrane are also assigned to perform multi-drug resistance.

7. Plasmid

• A plasmid is a small circular DNA molecule found along with bacterial chromosomal DNA. However, they are both completely different at the gene level.
• Plasmids replicate independently of bacterial nucleoids.
• Plasmids are assigned to perform specialized functions in the host cell. However, in general, plasmids can not replicate without a host cell.
• The genes of the plasmid are involved in several activities, including enhancement of host survival, production of bacterial toxins, and degradation of antimicrobials.
• A single cell can host more than one plasmid, each with different genes and functions.

8. Nucleoid

• A nucleoid represents a nucleus-like structure (not a true nucleus) and occupies a specific location in the prokaryotes.
• It is a compact and poorly organized structure of the genetic material of the prokaryotes.
• Nucleoid does not surround by the nuclear membrane. The prokaryotic chromosome is circular in nature and possesses most of the prokaryotic genes.
• It regulates cell functions and cell reproduction. Interestingly, the length of the prokaryotic chromosome is very large than the dimensions of the cell.

9. Flagellum

• The flagellum is a thread-like filamentous organelle of both bacteria and archaea.
• The flagellum is a motile organelle that enables movement and chemotaxis.
• It is a composite of about 30 types of proteins with copy numbers ranging from a few to a few thousand and is synthesized by self-assembly of these proteins.
• Flagella are also found to be involved in adhesion to different environment surfaces.

Prokaryotic cell examples

Archaea: Methanogens, Halophiles, Thermococci, Paleococcus, Pyrococcus, etc.

Eubacteria: Cyanobacteria, Chlamydiae, Bacilli, Acedobacteri, Fusobacteria, Deinococcus, Fibrobacters, etc.

Frequently asked questions:

Q1. Do prokaryotes have a nucleus?

Answer: No

Q2. Are bacteria prokaryotes?

Answer: Yes

Q3. Do prokaryotes make their own food like plants?

Answer: The prokaryotes include both photosynthetic and non-photosynthetic microbes.

Q4. Cell wall of bacteria is made up of?

Answer: Peptidoglycan or murein.

Q5. What are the functions of the cell wall of bacteria?

Answer: Maintains the shape and structural integrity of the bacteria and cell shape and is very important to cell viability.

References:

1. Chapter Five – Aeromonas Flagella and Colonisation Mechanisms. Advances in Microbial Physiology.
2. Chapter 1 – Capsular Polysaccharides in Escherichia coli. Advances in Applied Microbiology.
3. Imada, Katsumi. “Bacterial flagellar axial structure and its construction.” Biophysical reviews vol. 10,2 (2018): 559-570. doi:10.1007/s12551-017-0378-z
4. Johanna Haiko, Benita Westerlund-Wikström. The role of the bacterial flagellum in adhesion and virulence. DOI: 10.3390/biology2041242.
5. Hiroyuki Terashima, Seiji Kojima, Michio Homma. Flagellar motility in bacteria structure and function of flagellar motor. Int Rev Cell Mol Biol. DOI: 10.1016/S1937-6448(08)01402-0.
6. C J Jones 1 , S Aizawa (1991). The bacterial flagellum and flagellar motor: structure, assembly and function. Adv Microb Physiol. DOI: 10.1016/s0065-2911(08)60007-7.
7. Bonnie Chaban, H Velocity Hughes, Morgan Beeby (2015). The flagellum in bacterial pathogens: For motility and a whole lot more. DOI: 10.1016/j.semcdb.2015.10.032.