Prokaryotic Cells Flashcards
What are the key features of prokaryotic cells and what do they do?
- Fimbrae/Pili (Attachment)
- Nucleoid (Circular chromosome)
- Plasma membrane (Cell regulation)
- Cell wall (cell integrity)
- Glycocalyx (Extracellular: Cell protection)
- Flagella (Movement)
- Ribosomes (Cell movement)
What are the three types of Prokaryotic cells (shapes)?
- Cocci
- Rods
- Spirals
Describe the bacterial genome
- Typically a single circular chromosome (nucleoid)
- No nuclear membrane, however, the chromosome is restricted to defined region of the bacterial cell known as the nucleoid
- Other small circular self-replicating DNA molecules can be found in the cytosol (separate to the main chromosome) - these are known as ‘plasmids’
Describe the function of the cell wall of bacteria: peptidoglycan
Function:
1. Rigid macromolecular layer that provides strength to cell
2. Protects cell from osmotic lysis and confers cell shape
Peptidoglycan = mesh-like structure
Prokaryotes that lack cell walls = Mycoplasmas (group of pathogenic bacteria)
Describe the structure of peptidoglycan
- Long carbohydrate chains that are formed from alternating units of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid).
- The carbohydrate chains are held together by peptide cross-bridges
- Then sheets/layers of carbohydrate chains held together by peptide-cross bridges are connected together by tetra peptide side chains.
Transpeptidase = the enzyme that cross-links the peptidoglycan chains to form rigid cell walls
Describe the Gram stain - 1877, the most common gram stain reference bacteria (which will be used in Lab 5)
A gram stain of mixed staphylococcus aureus which are gram-positive cocci, in purple. And escherichia coli, gram-negative bacilli, in red, the most common gram stain reference bacteria.
What is the gram stain procedure?
- Application of crystal violet (purple dye: this stains all cells
- Application of iodine (mordant): makes the crystal violet bigger so it can’t come out of the cells as easily as it gets in
- Alcohol wash (decolorisation): in gram-negative cells the colour gets washed out. in gram-positive cells the colour stays
- Application of safranin (counterstain): dyes both cells pink but the purples ones stays purple because its a darker colour
What is the structure of a Gram-positive bacterial cell wall?
Its cell wall is a very thick peptidoglycan layer (20-80 nm)
- Peptidoglycan traps crystal violet, which masks the red safranin die
What is the structure of a Gram-negative bacterial cell wall?
Its cell wall has an outer membrane and then a thin layer of peptidoglycan (5-10nm)
- Crystal violet is easily washed away, revealing red safranin dye
What is the structure and function of bacterial flagella?
Function: some bacteria are capable of movement in liquid medium
How?
- Motile bacteria produce flagella
- Long flexible appendage resembling ‘tails’
- Proteinaceous 10-20nm in diameter, 5-10/cell
- Number of flagella and location on cell surface varies
- Act like a propeller: cell rotates them to move through a liquid medium
Describe a bacterias Tactic response: Chemotaxis
Chemotaxis: bacteria move along a concentration gradient towards a chemical attractant (positive) OR away form a chemical repellant (negative)
Experiment findings:
- Control: all the bacteria are evenly dispersed
- Attractant: bacteria are trying to get into the tube with the attractant (favourable conditions)
- Repellant: bacteria are trying to get away from tube with the repellant (unfavourable conditions)
Describe the first type of bacterial adherence factor: Fimbriae
These can be called Adherence Pili or Adhesins
- Function: structures with adhesive properties that cause bacteria to adhere/stick to surfactse
- Not all bacteria possess fimbriae: it is an inherited trait
- Can be mistaken for flagella but are NOT involved in motility
- Much shorter and more numerous than flagella ‘hair-like’, 100-1000/cell and 2-8nm in diameter. 1 micrometer in length.
Describe the second type of bacterial adherence factor: Pili/Pilus
Can also be called sex Pili or F Pili.
- Attachment to other bacteria
- Transfer genetic material from one cell to another
- The genetic transfer is called Conjugation
- Conjugation is a form of Horizontal Gene Transfer
Define Glycocalyx and what are the two types?
Glyxocalyx = a gelatinous polysaccharide and/or peptide outer covering that forms a sticky meshwork of fibres
- Capsule: Glycocalyx organised into a defined structure attached firmly to cell wall
- Slime Layer: Gycocalyx disorganised without cell shape, attached loosely to cell wall
What are the functions that capsules can serve? (There are many more, but name the two we are focusing on)
- Virulence factors: protecting bacteria from phagocytosis and engulfment by immune cells
- Prevent cell from drying out
