- Distinguish between prokaryotic cells and eukaryotic cells in terms of structure, size, and the types of organisms that have these cell types.
- Identify structures of bacterial cells in models and diagrams, including details of Gram-positive and Gram-negative cell walls and flagella.
- Describe the functions of the structures found in prokaryotic cells.
Bacteria are Prokaryotic Cells
Cells fall into one of two broad categories: prokaryotic and eukaryotic. Prokaryotic cells do not have a nucleus (membrane-bound structure that surrounds the cell's DNA). Only eukaryotic cells have a nucleus. Bacteria and archaea are the forms of life that are composed of prokaryotic cells, whereas plants, animals, fungi, and protists (including protozoa) are all composed of eukaryotic cells. In addition to prokaryotic and eukaryotic cells differing from each other based on absence or presence of a nucleus, prokaryotic cells are typically much smaller than eukaryotic cells and also have fewer organelle structures inside of their cells.
Figure 1: This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured). The smallest cells in the world are prokaryotic cells (including bacteria) that tend to be between 1 μm and 10 μm long. Eukaryotic cells are usually larger than prokaryotic cells (between 10 μm and 100 μm, but sometimes they can be much larger such as a frog egg that is 1 mm and can be viewed with the naked eye)
All cells (both prokaryotic and eukaryotic) share four common components:
- a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment
- cytoplasm, consisting of a jelly-like cytosol within the cell in which other cellular components are found
- DNA, the genetic material of the cell
- ribosomes, which synthesize proteins (prokaryotic ribosomes differ from eukaryotic ribosomes in several ways)
A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. Prokaryotic DNA is found in a central part of the cell: the nucleoid
Figure 2: This figure shows the generalized structure of a prokaryotic cell. Prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria (flagellum, pili, capsule).
|prokaryotic cell structure||function of this cell structure|
|capsule||enables the cell to attach to surfaces in its environment (including attachment to a host in pathogenic [disease-causing] species)|
|cell wall||acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration|
|cytoplasm||semifluid inside of the cell that holds cell components and is the site of the cell's metabolism (its chemical reactions that keep it alive)|
|flagellum (singular) / flagella (plural)||a whip-like tail that rotates to move a cell; prokaryotic cells can have no flagella, one flagellum, or multiple flagella depending on the species|
|nucleoid||a central region within a prokaryotic cell composed of the cell's chromosome (its DNA); the chromosome is a single DNA molecule that is in a circular and comprises the genome of the cell; this chromosome wraps and twist onto itself to form the nucleoid|
|plasma membrane||a fluid membrane that separates the inside of the cell from the outside of the cell; this structure is semipermeable (some materials can pass through the membrane and others cannot)|
|plasmid||(not shown in the figure) a small DNA molecule (often circular) found in the cytoplasm; plasmids are much smaller than the DNA chromosome in the nucleoid; plasmids can be exchanged by cells or picked up by cells to acquire new traits; plasmids can carry antibiotic resistance genes and therefore create challenges for treating some infections when they are shared between cells; prokaryotic cells can survive without plasmids and do not always contain a plasmid, while some may contain multiple plasmids|
|pilus (singular) / pili (plural)||important for attachment to surfaces or other cells, including host cells for pathogenic (disease-causing) cells; some pili, called "sex pili," can be used to exchange genetic material (DNA) between cells during a process called conjugation|
|ribosome (singular) / ribosomes (plural)||small structures found in the cytoplasm that build proteins; proteins are needed to conduct a cell's metabolism, form structures, and do so many other things within a cell; instructions for building different types of proteins come through chemical messages originating in the DNA|
Bacterial Cell Wall Structures
Most bacteria are divided into two major groups: Gram-positive bacteria and Gram-negative bacteria based on the cell wall composition (can be differentiated by using the Gram stain procedure on the bacteria). Knowing the Gram reaction of a clinical isolate (isolated bacterial species from a patient) can help the health care professional make a diagnosis and choose the appropriate antibiotic for treatment.
Figure 3: (Top) Microscopic images of bacteria stained with the Gram stain. (Top right) Gram-positive bacterial species stain purple with the Gram stain. (Top left) Gram-negative bacterial species stain pink with the Gram stain. (Bottom) Diagrams illustrating major structural components of Gram-positive bacterial cell walls and Gram-negative bacterial cell walls. (Bottom right) Gram-positive bacterial species have a thick layer of peptidoglycan outside of the plasma membrane and a layer of periplasm. (Bottom left) Gram-negative bacterial species have a thin layer of peptidoglycan outside of the plasma membrane surrounded by periplasm with an outer membrane outside of the peptidoglycan layer. Embedded in the outer membrane of Gram-negative cell walls are lipopolysaccharides (LPS) that are not shown in this diagram.
The results of the Gram stain reflect differences in cell wall composition. Gram-positive cells have thick layers of peptidoglycan (a substance composed of carbohydrates and protein subunits) in their cell walls. Gram-negative bacteria have very little peptidoglycan. Gram-positive bacteria also have teichoic acids, whereas Gram-negative bacteria do not. Gram-negative cells have an outer membrane that resembles the phospholipid bilayer of the plasma membrane. The outer membrane contains lipopolysaccharides (LPS), which are released as endotoxins when Gram-negative cells die. This can be of concern to a person with an infection caused by a Gram-negative organism.
Figure 4: Detailed view of the structure of bacterial cell walls for Gram-positive species and for Gram-negative species. (Left) Gram-positive cell walls consist of a thick layer of peptidoglycan (PG) outside of the cell's plasma membrane (PM) with teichoic acid (TA) and lipoteichoic acid embedded across the peptidoglycan layer. (Right) Gram-negative cell walls consist of a thin layer of peptidoglycan (PG) outside of the cell's plasma membrane (PM). Gram-negative cell walls have an outer membrane (OM) beyond the peptidoglycan layer containing lipopolysaccharide (LPS) as well as porin channels that enable some materials to pass across the outer membrane. In Gram-positive and Gram-negative cell wall types lipoprotein (LP) is also found.
Some species are known as Gram-variable, and so both Gram-positive and Gram-negative reactions may occur when a Gram-variable species is stained using the Gram stain. The vast majority of bacteria are either Gram-positive or Gram-negative. However, not all bacteria can be stained with the Gram stain (for example, Mycoplasma sp., which have no cell wall, stains poorly with the Gram stain).
Figure 5: The basic structure of a bacterial flagellum consists of a basal body, hook, and filament. The basal body composition and arrangement differ between gram-positive and gram-negative bacteria. (credit: modification of work by “LadyofHats”/Mariana Ruiz Villareal)
- Carefully examine cell models of bacterial cells found in the laboratory.
- Identify the following structures on the cell models:
- basal body (of flagellum)
- cell wall
- filament (of flagellum)
- Gram-negative cell wall
- Gram-positive cell wall
- hook (of flagellum)
- outer membrane
- peptidoglycan (thick layer)
- peptidoglycan (thin layer)
- periplasm (2 places)
- plasma membrane (in 3 places)
- teichoic acid
- Cell wall peptidoglycan in Mycobacterium tuberculosis: An Achilles’ heel for the TB-causing pathogen by Arundhati Maitra et al. is licensed under CC BY 4.0
- General Biology by OpenStax is licensed under CC BY 4.0
- Laboratory Exercises in Microbiology: Discovering the Unseen World Through Hands-On Investigation by Joan Petersen and Susan McLaughlin is licensed under CC BY-NC-SA 4.0
- Microbiology by OpenStax is licensed under CC BY 4.0
- Prokaryote cell.svg by Ali Zifan is licensed under CC BY-SA 4.0
- Red Mountain Microbiology by Jill Raymond Ph.D.; Graham Boorse, Ph.D.; Anne Mason M.S. is licensed under CC BY-NC 4.0