Prokaryotes
Functional Anatomy of Prokaryotic Cells
Introduction
- All living cells can be classified into two groups: Prokaryotes and Eukaryotes
Section 4-2
PROKARYOTIC CELLS: The Size, Shape, and Arrangement of Bacterial Cells
- The thousands of bacterial species that exist are distinguished by shape, chemical composition, nutritional requirements (metabolism).
- 99% of bacteria that exist in nature occur in
- Bacteria range from 0.2-2.0 µm in diameter (cocci) and 2.0 – 8.0 µm in length (bacillus, spiral)
- Cocci can replicate in many dimensions, creating multiple morphologies:
- Bacilli only divide along their short axis, creating a limited range of morphologies.
- Spiral bacteria have one or more twists, so they are never straight. Their diversity comes from variations in their shape, and not replication.
Section 4-3/4
Prokaryotic Cells: Structures external to the cell wall
- Before we dive into the internal anatomy prokaryotes, let’s discuss the features external to the cell wall.
- Glycocalyx = (meaning sugar coat) – a general term for the stuff that surrounds cells
- Sticky, gelatinous polymer
- Composed of polysaccharide, protein or both
- Differs in composition between different species
- Made inside the cell and secreted to the cell surface
- When nutrients are low, cells can break down glycocalyx into its protein and carbohydrate constituents and use them for energy and resources
- Extracellular polymeric substance (EPS) – a glycocalyx that helps cells in a biofilm attach to their target environment and to each other.
- Protects cells within it
- Facilitates communication among them
- The ability to attach to the environment is very important because it contributes to making some bacteria pathogenic.
- Glycocalyx allows bacteria to attach to intestines, the lungs, and teeth, for example.
- Two basic types of glycocalyces:
- Capsule – a type of glycocalyx with an organized structure attached firmly to the cell wall.
- Determined using negative staining.
- Often protects pathogenic bacteria from phagocytosis by the cells of the host
- Contributes to bacterial virulence
- g. Bacillus anthracis produces a capsule of D-glutamic acid àthis bacterium causes anthrax
- Slime layer – a type of glycocalyx with an unorganized structure that is loosely attached to the cell wall.
- Capsule – a type of glycocalyx with an organized structure attached firmly to the cell wall.
- Flagella = long filaments that propel bacteria, making them motile.
- Atrichous bacteria lack flagella
- Peritrichous bacteria have flagella evenly distributed over the entire cell.
- Monotrichous bacteria have a single flagellum at one pole.
- Lophotrichous bacteria have a tuft of flagella at one pole.
- Amphitrichous bacteria have flagella at both poles.
Anatomy of the Flagellum
- 3 basic parts to the flagellum:
- Filament – long whip-like portion of the flagellum that drives its propulsion; composed of flagellin, a sphere-like protein.
- Hook = connects the filament to the actual cell. Its rotation causes the whipping motion of the filament, which then propels the cell forward.
- Basal Body = Rings that anchors the flagellum to the cell.
- Basal bodies differ in gram-positive and gram-negative bacteria because the cell membrane and cell wall configurations differ as well.
- Gram-positive: large cell wall; no outer membrane; therefore, the only rings anchoring the flagellum are inner membrane rings.
- Gram-negative: have an inner and outer membrane with a strip of cell wall in between. Therefore, the flagellum basal body has 2 sets of rings: one set anchors the inner membrane, and the other set anchors to the cell wall.
- Basal bodies differ in gram-positive and gram-negative bacteria because the cell membrane and cell wall configurations differ as well.
Flagellar Movement
- Taxis - Bacteria move towards positive stimuli and away from negative stimuli. This movement is due to flagella.
- Bacteria have sensors that alert them to chemicals or nutrients in their environment. After detecting the signal, the information is passed to the flagella and the rate of “runs” and “tumbles” are modified to achieve a desired result.
- Runs > Tumbles = moving toward something; Tumbles > Runs = moving away
Axial Filaments
- Axial Filaments – allow corkscrew movements of bacteria called spirochetes.
- Examples of bacteria that are spirochetes are Treponema pallidum, which causes syphilis, and Borrelia burgdorferi, which causes Lyme disease.
- Axial filaments arise at the ends of the cell, beneath an outer sheath, and they spiral around the cell.
Fimbriae and Pili
- Fimbriae = structures straighter and thinner than flagella that have a role in adherence. Fimbriae are made of pilin, a protein that can cover the cell surface in varying densities.
- Fimbriae play an important role in the formation of biofilms, as well as pathogenesis for some bacteria, including Neisseria gonorrhoeae.
- Pili = similar to Fimbriae in basic structure, but are much longer and much more rare on the cell surface. Pili play a role in cell motility; like a retracting arm.
- Pili (singular = pilus) also play a role in the transfer of DNA from one bacteria to another in process called conjugation.
- This is how antibiotic resistance is passed from one bacteria to the next.
- Pili (singular = pilus) also play a role in the transfer of DNA from one bacteria to another in process called conjugation.
Section 4-5/6/7
Prokaryotic Cells: The Cell Wall
- The cell wall of bacteria is a semi-rigid structure that is responsible for the shape of the cell. Unlike the cell membrane, which is quite fluid, the cell wall can only be changed by considerable force.
- Due to the distinct nature of the bacterial cell wall, it is a good target for antibiotics and other drug therapies (because cell walls are distinct from all mammal cells). For example, penicillin interferes with the structural integrity of the cell wall, which eventually leads to lysis of the cell membrane.
- Lysozyme is another agent that can destroy the cell wall of gram positive bacteria. Lysozyme occurs naturally in saliva, tears, and mucus.
The cell wall is composed of peptidoglycan, a dense lattice of proteins and carbohydrates.
Gram Positive Gram Negative
Very thick cell wall |
Thin cell wall |
Cell wall contains teichoic acid |
Does not contain teichoic acid |
Cell wall is the outer most layer of the cell |
Cell wall is located in the periplasm between two cell membranes |
No outer membrane |
Outer membrane contains lipopolysaccharides (LPS) |
Cell wall is susceptible to antibiotics |
Cell wall is protected from antibiotics by outer cell membrane |
- Lipopolysaccharide (LPS) - a complex molecule that contains lipids and carbohydrates. The Lipid A portion is the part that is responsible for the symptoms associated with gram negative bacterial infection. LPS is a strong stimulant for the immune system, and can cause fever, inflammation, and dilation of blood vessels which can lead to septic shock.
- Acid-Fast cell walls - cell walls that contain mycolic acid. Though these bacteria are gram-positive, the waxy nature of mycolic acid does not permit gram staining. Thus, a special type of stain called acid-fast is required to identify these types of bacteria, which include Mycobacterium and Nocardia.