What Are Prokaryotic Cells? Structure, Function, and Definition

Shigella bacteria, illustration
Shigella bacteria. KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Some prokaryotes, such as cyanobacteria, are photosynthetic organisms and are capable of photosynthesis

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori).

Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, in your body, and on everyday objects in your environment.

Prokaryotic Cell Structure

Bacterial Cell Structure
Bacterial Cell Anatomy and Internal Structure. Jack0m/Getty Images

Prokaryotic cells are not as complex as eukaryotic cells. They have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid.

Prokaryotic organisms have varying cell shapes. The most common bacteria shapes are spherical, rod-shaped, and spiral.

Using bacteria as our sample prokaryote, the following structures and organelles can be found in bacterial cells:

  • Capsule: Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients.
  • Cell Wall: The cell wall is an outer covering that protects the bacterial cell and gives it shape.
  • Cytoplasm: Cytoplasm is a gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
  • Cell Membrane or Plasma Membrane: The cell membrane surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
  • Pili (Pilus singular): Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
  • Flagella: Flagella are long, whip-like protrusions that aid in cellular locomotion.
  • Ribosomes: Ribosomes are cell structures responsible for protein production.
  • Plasmids: Plasmids are gene-carrying, circular DNA structures that are not involved in reproduction.
  • Nucleoid Region: Area of the cytoplasm that contains the single bacterial DNA molecule.

Prokaryotic cells lack organelles found in eukaryoitic cells such as mitochondria, endoplasmic reticuli, and Golgi complexes. According to the Endosymbiotic Theory, eukaryotic organelles are thought to have evolved from prokaryotic cells living in endosymbiotic relationships with one another. 

Like plant cells, bacteria have a cell wall. Some bacteria also have a polysaccharide capsule layer surrounding the cell wall. This is the layer where bacteria produce biofilm, a slimy substance that helps bacterial colonies adhere to surfaces and to each other for protection against antibiotics, chemicals, and other hazardous substances.

Similar to plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments enable photosynthetic bacteria to obtain nutrition from light.

Binary Fission

E. coli Bacterium Binary Fission.
E. coli bacteria undergoing binary fission. The cell wall is dividing resulting in the formation of two cells. Janice Carr/CDC

Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical cells.

Steps of Binary Fission

  • Binary fission begins with DNA replication of the single DNA molecule. Both copies of DNA attach to the cell membrane.
  • Next, the cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward.
  • A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

Although E.coli and other bacteria most commonly reproduce by binary fission, this mode of reproduction does not produce genetic variation within the organism. 

Prokaryotic Recombination

Bacterial Conjugation
False-color transmission electron micrograph (TEM) of an Escherichia coli bacterium (bottom right) conjugating with two other E.coli bacteria. The tubes connecting the bacteria are pili, which are used to transfer genetic material between bacteria. DR L. CARO/Science Photo Library/Getty Images

Genetic variation within prokaryotic organisms is accomplished through recombination. In recombination, genes from one prokaryote are incorporated into the genome of another prokaryote.

Recombination is accomplished in bacterial reproduction by the processes of conjugation, transformation, or transduction.

  • In conjugation, bacteria connect through a protein tube structure called a pilus. Genes are transferred between bacteria through the pilus.
  • In transformation, bacteria take up DNA from their surrounding environment. The DNA is transported across the bacterial cell membrane and incorporated into the bacterial cell's DNA.
  • Transduction involves the exchange of bacterial DNA through viral infection. Bacteriophages, viruses that infect bacteria, transfer bacterial DNA from previously infected bacteria to any additional bacteria that they infect.
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Bailey, Regina. "What Are Prokaryotic Cells? Structure, Function, and Definition." ThoughtCo, Sep. 7, 2021, thoughtco.com/prokaryotes-meaning-373369. Bailey, Regina. (2021, September 7). What Are Prokaryotic Cells? Structure, Function, and Definition. Retrieved from https://www.thoughtco.com/prokaryotes-meaning-373369 Bailey, Regina. "What Are Prokaryotic Cells? Structure, Function, and Definition." ThoughtCo. https://www.thoughtco.com/prokaryotes-meaning-373369 (accessed March 21, 2023).