Introduction to prokaryotic cells

Question 1

What is the average size of prokaryotic cells?

Answer 1

1um

Question 2

How much of earths biomass is prokaryotic?

Answer 2

Half

Question 3

How many bacterial species are there?

Answer 3

estimated 10^19

Question 4

What are some characteristics of bacterial cells?

Answer 4

• Do not have organelles
• Single circular chromosomes
• No nuclear envelope of nucleolus
• Peptidoglycan is present in the cell wall
• Cannot grow at more then 100 degrees

Question 5

What is the bacterial cell walls function?

Answer 5

Rigid macromolecular layer that provides strength to cell

Protect cells from osmotic lysis (water movement into the cell - reason why cell doesn’t explode from water coming in) and confers cell shape

Question 6

What are Mycoplasmas?

Answer 6

Prokaryotes that lack cell walls

Question 7

What is the structure of peptidoglycan?

Answer 7

• mesh-like structure that is interconnected
• Composed of strands (various sugars) that are connected together
• NAG subunit is attached to NAM subunit continuously, forming long strands
• Links strands via amino acid cross bridges (this cross-linking is what makes the material very strong)

Question 8

What is Transpeptidase?

Answer 8

The enzyme that cross links peptidoglycan chains to form rigid cell walls

Question 9

What is the procedure of the gram stain?

Answer 9

Gram stain procedure
1. Add crystal violet stain to a sample (positively charged, will bind to the bacteria).

2. Add iodine - fixes the dies, helps it stay. Both bacteria cells are still staining purple
3. Wash swap with ethanol.
4. Add counterstain, called safrain.
   Purple are gram positive, pink/red are gram negative.

Question 10

What is structure of a gram positive cell wall?

Answer 10

Thick peptidoglycan layer (20-80 nm)

• Peptidoglycan traps crystal violet, which masks the red safranin dye
• Can’t wash crystal violet away with alcohol - wall is to thick

Question 11

What is the structure of a gram negative cell wall?

Answer 11

Thin layer of peptidoglycan (5-10nm) with inner (plasma membrane) and outer membranes

• Outer membrane is decorated with lipopolysaccharides
• Crystal violet is easily rinsed away, revealing the red safranin dye

Question 12

What is bacterial flagella?

Answer 12

Allows the bacterium to move through fluid mediums

• Resemble tails
• Made of proteins and 10-20 nm in diameter, 5-10 per cell
• Number of flagella and location on cell surface can vary
• Acts like a propeller
• All rotation of flagella is in the same direction

Question 13

What is the structure of a flagella?

Answer 13

• Long filament (F): extends into surrounding medium. Composed of subunits (protein = flagellin)
• Filament is attached to motor by the Hook (H): curved section connecting the filament to the cell surface (enables the bacteria to put power)
• Driven by the Basal Body (BB): anchors the flagellum into the cell membrane of the bacterium by special disc-shaped - powered by protons

Question 14

What is chemotaxis?

Answer 14

Bacteria moving along a concentration gradient towards a chemical attractant (+) or away from a chemical repellent (-)

Question 15

How do bacteria sense gradients?

Answer 15

Chemoreceptors in the cell wall.

Question 16

What happens during chemotaxis?

Answer 16

Bacteria sense the change in chemical concentration outside the cell over TIME “temporal gradients” and thus respond to the change in concentration as the call moves

• Every time it stops it is sensing the concentration gradient of glucose
• IF concentration gradient keeps increasing, it will keep moving in that direction

Question 17

Why can bacteria not sense a spacial gradient?

Answer 17

They are to small

- Cannot sense that difference in conc between the front and that back

Question 18

Name the two bacterial adherence factors and why are they useful?

Answer 18

Fimbrae and Glycocalyx

• All bacteria live attached to surface so need these to be able to attach
• Bacteria like to colonise skin and mucosal surfaces

Question 19

What is fimbriae?

Answer 19

Protein structures with adhesive properties that cause bacteria to stick/adhere to surfaces

• Bacteria will synthesis particular fimbriae depending on what surface they want to colonise
• Not all bacteria possess fimbriae: it is an inherited trait
• Much shorter and more numerous than flagella “hair-like” (typically 100-1000 per cell)

Question 20

What is Glycocalyx?

Answer 20

Form capsules and slime layers.

• A gelatinous polysaccharide and / or polypeptide outer covering that is used to attach to teeth and various other structures
• Forms a sticky meshwork of fibers

Glycocalyx organized into a defined structure attached firmly to cell wall = CAPSULE

Glycocalyx disorganised without cell shape, attached loosely to cell wall = SLIME LAYER

Question 21

What do capsules do for that bacteria?

Answer 21

• Virulence factors - protecting bacteria from phagocytosis and engulfment by immune cells
• Adherence to cell surfaces and structures such as medical implants, catheters and so on
• Prevent cell from drying out (desiccation)

Question 22

What are bacterial endospores?

Answer 22

Mechanism for long term survival of bacteria

• Bacterial endospores are formed during unfavourable growth conditions and germinate under favourable conditions - protect cells from stress (overcrowding, nutrient deficiency)
• If a bacteria cell can form endospores it can become dormant for millions of years
• Only present in some gram+ bacteria: members of the genus Bacillus and Clostridium (and present in gram negative)

Question 23

What are endospores resistant to?

Answer 23

Heat, harsh chemicals, antibiotics, disinfectants and radiation

Question 24

How do endospores get nutrients?

Answer 24

• Mother cell provides all nutrients to make endospore - grows inside mother cell
• Endospore has everything it needs in it to be able to become a bacterial cell when conditions become favourable

Question 25

What are some factors that influence endospores to be formed?

Answer 25

Nutrient starvation

High cell density