4 - Microbial Membranes And Cell Walls

Question 1

Structures on outer layer of bacterial cells

Answer 1

• capsule, S layer
• gram + - cell wall
• gram - - outer membrane
• periplasmic space - within peptidoglycan in gram pos and neg bacteria
• cell plasma membrane

Question 2

Capsule structure info

Answer 2

• sometimes called slime layer or glycoalyx
• polysaccharide components of cell wall
• usually loose network of polymer fibres extending outward from wall

Question 3

Capsule function

Answer 3

• not required for growth or reproduction
• Carbon store
• protection against desiccation (drying out)
• May be involved in capture of nutrients - acquisition of ions from environment
• confer advantages in vitro such as attachment to surfaces - biofilms, holdfast to eukaryotic cells
• exclude phage, antimicrobials and disinfectants - offer some resistance
• pathogens often capsular can resist phagocytosis, otherwise killed by host when it isn’t

Question 4

Different capsules in pathogens

Answer 4

• most commonly is a polysaccharide structure
• glycolipid capsule
• protein capsule
• extracellular slime

Question 5

S-layer info

Answer 5

• Paracrystalline outer wall composed of protein/glycoprotein
• regularly structured layer external to cell wall
• s layer is only cell wall structure in some archaea
• May protect against ion and pH fluctuations, osmotic stress, and predators (e.g. Bdellovibrio)
• May protect against host defences - sometimes a virulence factor (can combat immune systems)

Question 6

Peptidoglycan structure

Answer 6

• alternating residues of:
  NAG (N-acetylglucosamine)
  NAM (N-acetylmuramic acids)
• arranged in diners which are cross linked by amino acid side chains creating amide bonds
• is a mesh like polymer that retains the gram stain in gram +ve cells

Contains non-protein amino acids:
- D-glutamic acids
- D-alanine
- DAPA

D amino acids used as they protect against degradation by proteases and enzymes

Question 7

Why D amino acids used in peptidoglycan structure in bacterial cell walls

Answer 7

• connected to form a glycan tetrapeptide
• Protect against degradation by proteases

Question 8

Peptidoglycan synthesis - cross links

Answer 8

• chains of linked peptidoglycan subunits joined by cross-links between the peptides
• often carboxyl group of terminal D-alanine connected to amino group of DAPA (meso-diamino-pimelic acid)
• sacs are strong enough to retain shape when isolated yet are porous, elastic, and stretchable

Question 9

Difference between gram +ve and -ve bacterial cell walls

Answer 9

Gram +ve thicker than gram -ve cell walls
- gram +ve contain outer membranes

Question 10

Which type of bacteria is penicillin more effective against

Answer 10

Gram positive

Question 11

Function for techioc acid in bacterial cell walls

Answer 11

• only found in gram +ve cell walls
• role is unclear
• May make membrane -vely charged
• May help with binding if Ca2+ and Mg2+

Question 12

How does penicillin affect bacteria and their cell walls
- briefly describe peptidoglycan synthesis

Answer 12

• penicilin inhibits peptidoglycan synthesis in gram +ve bacterial cells by inhibiting transpeptidation - lyses cells
  PG synthesis:
• linker peptide initially has two D-alanine peptides
• one is cleaved during linkage with DAPA
• called transpeptidation - this step is penicilin sensitive
• penicilin halts cell wall synthesis
• osmotically sensitive cells lyse

Question 13

Lysozyme info

Answer 13

• ‘antibacterial’ enzyme
• degrades the beta, 1,4 glycosidic bond in peptidoglycan backbone
• loss of peptidoglycan makes cells sensitive to changes in osmotic pressure
• important host defences against bacteria
• present in saliva, tears, secreted in airways
• lots in egg whites

Question 14

Where lysozymes found

Answer 14

Saliva
Tears
Secreted in airways

Question 15

Techioc acid structure

Question 16

Which bacteria is techioc acid found in

Answer 16

Only gram +ve

Question 17

Archaea cell walls info

Answer 17

• no peptidoglycan
• some contain pseudomurein
• Contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid in bacteria
• contains B - 1,3 links instead of B-1,4 links
• not degraded by lysozyme, not sensitive to penicilin
• no D-amino acids in linker molecules
• other archaea contain glycoproteins, polysaccharides or S layers (protein or glycoprotein)

Question 18

Basics of a membrane

Answer 18

• Unit membrane comprised of phospholipids
• Hydrophobic groups inside, hydrophilic groups outwards
• Proteins that traverse the bilayer have hydrophobic regions
• Hydrophilic/charged substances may attach to the hydrophilic surfaces

Question 19

Sterols/hopanoids info in membrane structure

Answer 19

• hopanoids in bacteria, sterols in eukaryotes
• rigid planar molecules while fatty acids are flexible
• this stabilises membrane structure
• hopanoids not found in archaea
• this role in archaea filled by isopropene structures

Question 20

What archaea have instead of hopanoids/sterols in membrane

Answer 20

• Isopropene structures
• used to stabilise membrane structure

Question 21

Gram -ve outer cell membrane info

Answer 21

• Outer cell membrane only in gram -ve bacteria
• asymmetric due to insertion of lipopolysaccharide into external layer of outer membrane

Question 22

Two ways outer membrane linked to cell in bacteria

Answer 22

1. Braun’s lipoprotein
2. Adhesion sites - continuum of inner and outer membrane, where two membranes adhere

Question 23

Braums lipoprotein info

Answer 23

• Most abundant protein in outer membrane
• covalently linked to peptidoglycan and embedded in outer membrane by hydrophobic end

Question 24

Adhesion sites of outer membrane info

Answer 24

• continuum of inner and outer membrane where two sites adhere
• Around 400 adhesion sites in E. Coli cell
• allow transport of substances to outer membrane and outer cell
• can be visualised using TEM
• plasmolysis makes cell flaccid
• this increases space between membranes, makes adhesion sites more visible
• immunoglobulin staining of a phage

Question 25

How to make adhesion sites visible in bacteria

Answer 25

- can be visualised using TEM - plasmolysis makes cell flaccid - this increases space between membranes, makes adhesion sites more visible - immunogold staining of a phage - MS2 lysis protein in adhesion sites

Question 26

differences between bacterial and archaeal cell walls

Answer 26

bacteria: - contains peptidoglycan - usually contain outer membrane (gram +ve) - B-1,4 glycosidic bonds between sugar derivatives - contaons some L and some D-isomer amino acids - peptidoglycan broken down by lysozymes and penicilin archaea: - no peptidoglycan - usually have S layer (protein) instead of polysaccharide containing cell wall to prevent osmotic lysis - typically lack outer membrane - some methanogens (methane producing archaea) contain pseudomurein polysaccharides - B-1,3 glycosidic bonds between sugar derivatives - psuedomurein contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid in peptidoglycan - all amino acids are L-stereoisomers in linker molecules - pseudomurein immune to penicilin and lysozymes

Question 27

difference and similarities between bacterial and arachaeal cytoplasmic membranes

Answer 27

differences: bacteria: fatty acid tails are bound glycerol by ester linkages - forms a lipid bilayer archaea: contains isopropenoid tails - hydrophobic isopropenoid tails bound to glycerol by ether bonds (C-O-C) - forms a lipid monolayer - formed from phosphoglycerol diethers, with C20 or C40 side chains similar: both membranes amphipathic in nature (hydrophobic end facing in, hyddrophilic facing out)

Question 28

main functions of cytoplasmic membrane

Answer 28

- cells permeability barrier - prevent passive leakage of solutes into and out of cell - anchors many proteins that catalyse many key cell functions - transport proteins, etc. - role in energy conservation and consumption (AT?)

Question 29

Cytoplasmic membrane as a permeability barrier info

Answer 29

- barrier to diffusion of most substances, especially polar or charged molecules - membrane therefore has specific transport proteins to move required solutes into and out of the cell

Question 30

Cytoplasmic membrane as a site containing key proteins info

Answer 30

- transport proteins usually highly specific, transport only one type of molecule - other transporters may be more general, and transport certain types of molecules, e.g. amimo acids, or sugars - if accumulating solutes against conc. gradient, then ATP required for AT - ensures sufficient amount of nutrients in cell required for chemical reactions - rate of uptake at max. when transporters are saturated - some nutrients transported by low- affinity transporter when nutrient present at high external conc. - nutrients transported by high-affinity transporters when external nutrient conc. is low

Question 31

Cytoplasmic membrane role in energy conservation and consumption info

Answer 31

- involved in respiration - involved in generating and dissipation of the proton motive force, in ATP production, in mitochondria and chloroplasts

Question 32

similarities between bacterial and archaeal cell walls

Question 33

which stage of PG synthesis does penicilin inhibit

Answer 33

transpepidation

Question 34

how and why archeae have a monolayer cytoplasmic membrane

Answer 34

- contains double-headed ether lipids - stabilises membrane at extremes of pH/temp

Question 35

difference between L and D- amino acids

Answer 35

- L amino acids rotate left (levorotation) - D amino acids rotate right (dextrorotation)

Question 36

peptidoglycan cross links structure and info

Answer 36

- Chains of linked peptidoglycan subunits joined by cross-links between the peptides - Often carboxyl group of terminal D-alanine connected to amino group of diamino pimelic acid (DAPA) - Sacs are strong enough to retain shape when isolated yet are porous, elastic and stretchable