A prokaryotic cell is a type of cell that lacks a defined nucleus and other membrane-bound organelles. These cells are structurally simpler and smaller than their eukaryotic counterparts, the cells that make up fungi, plants, and animals. The genetic material in a prokaryotic cell is not housed within a nucleus; instead, it is contained in a region called the nucleoid.
Prokaryotes are the earliest forms of life on earth, with fossil evidence dating them back to about 3.5 billion years. They are a crucial part of the kingdoms of life and fit into two domains: Bacteria and Archaea.
Examples of Prokaryotes
here’s a list of some prokaryotic species, including both bacteria and archaea:
Bacteria
- Escherichia coli: E. coli is a common bacterium found in the human gut. Some strains cause food poisoning.
- Streptococcus pneumoniae: This is a bacterium responsible for pneumonia and other respiratory tract infections.
- Staphylococcus aureus: This species lives on the skin. It can cause a variety of infections, from minor skin infections to more severe diseases like pneumonia and meningitis.
- Salmonella typhimurium: This is a bacterium that causes foodborne illnesses, including diarrhea, fever, and abdominal cramps.
- Mycobacterium tuberculosis: This prokaryotic species causes tuberculosis.
Archaea
- Methanogens: These are a group of archaea that produce methane as a metabolic byproduct. An example is Methanobrevibacter smithii, found in the human gut.
- Halobacteria: These are a group of archaea that thrive in highly salty environments. An example is Halobacterium salinarum.
- Thermophiles: These archaea survive in extremely hot environments. Thermoplasma volcanium and Pyrolobus fumarii are examples.
- Sulfolobus solfataricus: This archaea species lives in volcanic hot springs and thrives in highly acidic conditions.
Prokaryotes: Bacteria and Archaea Domains
Bacteria
Bacteria are the best-known domain of prokaryotes. They live in various environments, including soil, water, and even the human body. Bacteria are incredibly diverse, with thousands of species displaying different shapes, sizes, metabolic capabilities, and environmental preferences. They can be pathogenic, causing diseases, or beneficial, aiding digestion or nutrient cycling in ecosystems.
Archaea
Archaea, like bacteria, are single-celled organisms without a nucleus. While they look similar to bacteria, they are genetically and biochemically distinct. Scientists discovered archaea in extreme environments, such as hot springs or salty lakes, but they inhabit a wide range of environments. Notably, Archaea have unique metabolic processes and biochemical pathways that allow their survival in harsh conditions.
Prokaryotic Cell Characteristics
Prokaryotic cells typically range in size from 0.1 to 5 micrometers, making them much smaller than eukaryotic cells. They display several unique characteristics:
- Nucleoid: Prokaryotic cells lack a defined nucleus. Their genetic material, usually a single circular DNA chromosome, is within the nucleoid.
- Cell Wall: Prokaryotic cells typically have a protective cell wall that provides structural support and protection.
- Plasmids: These small, circular pieces of DNA often carry genes beneficial for the cell’s survival in certain environments. Transferring plasmids between prokaryotes promotes genetic diversity.
- Ribosomes: Prokaryotic ribosomes, the sites of protein synthesis, are smaller than those in eukaryotes.
- Flagella and Pili: These structures facilitate movement (flagella) and adherence to surfaces or other cells (pili).
The Components of a Typical Prokaryotic Cell
All prokaryotic cells have a plasma membrane, cytoplasm, nucleoid, and ribosomes. Other components vary by species.
Cytoplasm
The cytoplasm is a gel-like substance inside the cell that surrounds all other cell components, like ribosomes and DNA. The cytoplasm is primarily water but also includes enzymes, salts, and organic molecules.
Plasma Membrane
The plasma membrane is a semi-permeable layer that separates the interior of the cell from the outside environment. It is a lipid bilayer that regulates the movement of substances in and out of the cell.
Cell Wall
The cell wall, located outside the plasma membrane, provides protection and structural integrity to the cell. In bacteria, the cell wall contains peptidoglycan. It has a different chemical composition in archaea.
Capsule
Some prokaryotes have an additional layer called a capsule made of polysaccharides. The capsule helps in adherence to surfaces and offers protection against host immune responses.
Nucleoid
The nucleoid is the region that contains the prokaryotic cell’s genetic information.
Plasmids
Plasmids are small, circular pieces of DNA separate from the cell’s main chromosome. They often carry genes that provide a survival advantage, like antibiotic resistance.
Ribosomes
Ribosomes perform protein synthesis. Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
Flagella
Flagella are long, whip-like structures that prokaryotic cells use for movement.
Pili
Pili are short, hair-like structures that function in adhesion to surfaces and DNA transfer during conjugation.
Prokaryotic Cell Morphology
Bacteria and archaea exhibit several distinct shapes:
- Cocci: These cells are spherical and exist as single cells, in pairs (diplococci), chains (streptococci), or clusters (staphylococci).
- Bacilli: These cells are rod-shaped. They are solitary (bacilli), occur in pairs (diplobacilli), or are in chains (streptobacilli).
- Spirilla: These cells are spiral-shaped and either rigid (spirilla) or flexible (spirochetes).
- Vibrio: Vibrio are comma-shaped cells.
- Square: Some archaea have flat, square cells.
Prokaryotic Cell Reproduction
Prokaryotes primarily reproduce asexually through a process known as binary fission. In binary fission, a single cell divides into two identical daughter cells. Some prokaryotes also exchange genetic material in processes like conjugation, transformation, and transduction.
Comparisons Between Eukaryotic and Prokaryotic Cells
While both cell types share some common features, like the presence of a plasma membrane, cytoplasm, and DNA, they have key differences:
- Nucleus: Prokaryotic cells lack a defined nucleus, while eukaryotic cells contain a membrane-bound nucleus.
- Size: Prokaryotic cells are generally smaller (0.1–5 µm) than eukaryotic cells (10–100 µm).
- Organelles: Eukaryotic cells contain membrane-bound organelles, such as the endoplasmic reticulum, Golgi apparatus, and mitochondria. Prokaryotic cells lack membrane-bound organelles. Instead, different regions within the cytoplasm perform these functions.
- Cell Division: Prokaryotes reproduce through binary fission, while eukaryotes undergo a more complex process of mitosis and meiosis.
- Genetic Material: Prokaryotes typically have a single circular chromosome and plasmids, while eukaryotes have multiple linear chromosomes and lack plasmids.
References
- Campbell, N.A.; Williamson B,; Heyden, R.J. (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 9780132508827.
- Gribaldo, S.; Brochier-Armanet, C. (January 2020). “Evolutionary relationships between archaea and eukaryotes”. Nature Ecology & Evolution. 4 (1): 20–21. doi:10.1038/s41559-019-1073-1
- Maton, A. (1997). Cells: Building Blocks of Life. New Jersey: Prentice Hall. ISBN 9780134234762.
- Murat, Dorothee; Byrne, Meghan; Komeili, Arash (2010). “Cell Biology of Prokaryotic Organelles”. Cold Spring Harbor Perspectives in Biology. 2 (10): a000422. doi:10.1101/cshperspect.a000422
- Stoeckenius, W. (1981). “Walsby’s square bacterium: fine structure of an orthogonal procaryote”. Journal of Bacteriology. 148 (1): 352–60. doi:10.1128/JB.148.1.352-360.1981