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Prokaryotic Cells: Definition, Structure and Characteristics

Illustration of Salmonella bacteria, a prokaryote with flagella, under magnification.
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What is a prokaryotic cell? 

Prokaryotes are small, single-celled organisms with a simple structure. Unlike other cells, they don’t have a nucleus or other membrane-bound organelles. Instead, most prokaryotes store their genetic information as a single circular DNA molecule, or chromosome, that floats freely within the cytoplasm.1 However, some prokaryotes have been discovered with linear chromosomes.2 Additionally, many prokaryotes possess extra circular DNA molecules called plasmids, which can provide advantageous traits for survival in specific environments.3


Despite their apparent relative simplicity, prokaryotes have substantially greater phylogenetic and functional diversity than eukaryotes.4, 5 For instance, although only about 17,000 species of prokaryotes have been described,6 molecular techniques indicate the existence of millions or even billions of prokaryotic species worldwide.7, 8 This immense diversity allows prokaryotes to thrive in a wide range of environments.


Contents

Structure of prokaryotic cells

Characteristics of prokaryotic cells

    - Do prokaryotes have membrane-bound organelles?

    - Do prokaryotes have ribosomes?

    - Do prokaryotes have a cell wall and a cell membrane?

    - Do prokaryotes have cytoplasm?

    - Do prokaryotes have mitochondria?

    - Do prokaryotic cells have DNA?

Examples of prokaryotes

    - Prokaryotic bacteria cell

    - Archaea

Why are prokaryotes important?

Structure of prokaryotic cells

While prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions (Figure 1). In prokaryotic cells, DNA bundles together in a region called the nucleoid. Primitive organelles, such as micro-compartments found in some bacteria, help organize cellular processes by concentrating specific enzymes and substrates needed for biochemical reactions.9


Prokaryotic cells also feature various external structures that enhance their functionality. The cell wall provides structural support and protection, while the capsule, found in some bacteria, plays a crucial role in resisting host immune attacks, adaptation to new niches and pathogenesis.10 Surface appendages like pili, also known as fimbriae, aid in attachment and genetic exchange, while flagella enable movement, allowing prokaryotes to seek nutrients or avoid harmful conditions.11


These efficient and specialized features make prokaryotic cells highly adaptable and capable of thriving in diverse and often extreme environments. Table 1 below provides an overview of the key features of a prokaryotic cell:


Table 1. The structure and function of key prokaryotic cell structures.

Structure

Function

Nucleoid

A central region of the cell that contains its DNA.

Ribosome

Responsible for protein synthesis

Cell wall

Provides structure and protection. In bacteria, it is often made of peptidoglycans, a mix of carbohydrates and proteins.

Cell membrane

Separates the cell from its environment and regulates the movement of substances in and out of the cell.

Capsule

Found in some bacteria, this carbohydrate layer surrounds the cell wall and helps the cell attach to surfaces.

Pili (or fimbriae)

Rod-shaped structures involved in attachment, DNA transfer and other roles.

Flagella

Tail-like structures that enable the cell to move.

Diagram of a prokaryotic cell showing labeled structures, including nucleoid, ribosomes, and plasma membrane.

Figure 1: The structure of a prokaryotic cell. Credit: Technology Networks.

Characteristics of prokaryotic cells

Prokaryotic cells are defined by their simplicity, lacking a distinct nucleus and membrane-bound organelles.

Do prokaryotes have membrane-bound organelles?

Prokaryotes lack membrane-bound organelles like the mitochondria, golgi apparatus and the nucleus.

Do prokaryotes have ribosomes?

Prokaryotes have ribosomes but they are smaller and structurally simpler than those in eukaryotic cells. These non-membrane-bound organelles play a crucial role in protein synthesis, enabling the translation of genetic information into functional proteins.

Do prokaryotes have a cell wall and a cell membrane?

Prokaryotes typically have both a cell wall and a cell membrane, but exceptions exist. For example, mycoplasmas do not have a cell wall.12 These organisms rely on their cell membrane for structural integrity and often exhibit unique adaptations to survive without the rigidity of a cell wall.

Do prokaryotes have cytoplasm?

Prokaryotes have cytoplasm, which is defined as the total inner-cellular volume, and serves as the site for most cellular processes. The cytoplasm is a gel-like substance that contains water, enzymes, nutrients and molecules like proteins, RNA and metabolites. It also includes cytosol, which is the fluid part of cytoplasm.


Since prokaryotes lack membrane-bound organelles, all biochemical reactions, such as energy production and metabolism, occur within the cytoplasm. This makes it a critical part of prokaryotic cell function.

Do prokaryotes have mitochondria?

Prokaryotes do not have mitochondria.


Interestingly, one widely accepted theory of eukaryotic evolution, known as the endosymbiotic theory, suggests that mitochondria originated from free-living prokaryotic cells.13 According to this theory, an ancestral eukaryotic cell engulfed a prokaryotic organism capable of energy production. Over time, the two organisms developed a symbiotic relationship, eventually functioning as a single entity. This evolutionary milestone led to the formation of modern eukaryotic cells, with mitochondria playing a central role in energy generation.


New research, however, is prompting scientists to refine this theory. The discovery that some prokaryotes, like planctomycetes, exhibit behaviors such as cell consumption, previously thought to be exclusive to eukaryotes, challenges the traditional dichotomy between these domains.14 These findings suggest that prokaryotes may have played a more significant role in early eukaryotic evolution than previously recognized, indicating a need to refine existing theories.

Do prokaryotic cells have DNA?

Prokaryotic cells have DNA, typically a single, circular molecule that contains all the genetic information necessary for the cell's functions and reproduction.


However, recent studies have indicated that some prokaryotes have linear chromosomes. For example, Borrelia burgdorferi, the bacterium that causes Lyme disease, contains a linear chromosome of 910,725 base pairs.15 Unlike circular chromosomes, Borrelia’s linear DNA cannot supercoil into a compact structure within the nucleoid, resulting in diffuse strands throughout the cell instead.16


Additionally, many prokaryotes carry plasmids - small, circular DNA molecules. These plasmids can provide extra traits, such as antibiotic resistance, virulence genes or metabolic pathways required to occupy diverse niches.17

Examples of prokaryotes

Prokaryotes are classified into two domains: bacteria and archaea. Although archaea were once grouped with bacteria, modern research has revealed that bacteria and archaea have distinct biochemical properties and evolutionary histories.18

Prokaryotic bacteria cell

Bacteria are single-celled organisms that thrive in a wide variety of environments, from soil and water to the human gut to extreme habitats like hot springs.19

Bacteria possess specialized cellular structures that carry out essential functions, including energy production and protein transport, enabling them to adapt and thrive in diverse environments. Their simplicity and versatility make bacteria some of Earth’s most resilient and ecologically important organisms.

Archaea

Archaea are single-celled organisms with unique properties that set them apart from the other domains of life, Bacteria and Eukarya. They are the only life forms capable of producing biological methane through methanogenesis, and their membranes contain lipids with a glycerol-1-phosphate backbone, a feature that enhances stability under extreme conditions.18

Why are prokaryotes important?

Prokaryotes are crucial, ubiquitous inhabitants of planet Earth. As the most diverse organismal life forms occupying various ecological niches, prokaryotes play significant roles in most ecological processes and profoundly impact human health, biotechnology, agriculture and the environment.20, 21, 22

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