1. Prokaryotic Cell Structure and function (Part I)
BIO3124
Lecture #3 (I)

2. Plasma Membrane Properties and Functions
defines the existence of a cell
Made of lipid bilayer
Double layer of phospholipids
Surrounds the cell
approx nm in thickness
Separates exterior environment from interior
Dynamic selective barrier
Concentrates certain components intracellulary
Allows excretion of waste
Sense the outside world
Several metabolic processes
ex. Respiration, photosynthesis

3. Fluid Mosaic Model of Membrane Structure
Lipid bilayer in which proteins float (Singer and Nicholson model)

4. Membrane proteins
Membrane proteins serve numerous functions, including:
- Structural support
- Detection of environmental signals
- Secretion of virulence factors and communication signals
- Ion transport and energy storage
Have hydrophilic and hydrophobic regions that lock the protein in the membrane 4

5. Phosphatidylethanolamine
Membrane lipids
Amphipathic phospholipids
polar ends (hydrophilic)
Glycerol, negative charge (outer leaflet)
Ethanolamine, positive charge (inner leaflet)
Phosphatidylethanolamine
nonpolar ends (hydrophobic)
Tails of fatty acids
Palmitic acid
Oleic acid (kinking) increase fluidity
Cyclopropane conversion
aging cells

6. Bacterial Membranes differ from eukaryotes in lacking sterols
do contain hopanoids, sterol-like molecules
synthesized from similar precursors
Stabilize bacterial membranes
total mass on earth ~1012 tons
a highly organized, asymmetric structure, flexible and dynamic

7. Archaeal membranes Etherglycerol, not ester bond
Terpene derived lipids
some have a monolayer membranes
Tetra-ether glycerol
Cyclopentane: isoprene cyclization
Increased stability

8. Archaeal membranes Moderately thermophilic - Bilayer or mixed
Extreme thermophiles
eg. Solfolobus and
Theromoplasma

9. Role of cell membrane in energy metabolism
bacterial cell membranes involved in ETC
Gradual energy release
forming proton gradient across membrane 9

10. Animation: A bacterial electron transfer system

11. The Proton Motive Force
The transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force.
- It drives the conversion of ADP to ATP through ATP synthase.
- This process is known as the chemiosmotic theory. 11

12. PMF Drives Many Cell Functions
Besides ATP synthesis, Dp drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs 12

13. ATP synthase mechanism
Note: pump also works in reverse to create H+ gradient

14. Cell Transport Transporters pass material in/out of cell
Passive transport follows gradient of material
Pumps use energy
ATP or PMF
Move material against their gradient
Passive diffusion lets small molecules into cell

15. The Bacterial Cell Wall
rigid structure that lies just outside the plasma membrane

16. Functions of cell wall provides characteristic shape to cell
protects the cell from osmotic lysis
may also contribute to pathogenicity
very few procaryotes lack cell walls, ie Mycoplasmas

17. Evidence of protective nature of the cell wall
Lysozyme treatment
Penicillin inhibits peptidoglycan synthesis

18. Gram negative cell wall
few PG layers, defined Periplasmic space
unique outer membrane, LPS, Braun’s lipoprotein
Braun’s

19. Gram positive cell walls
Multiple PG layers, periplasmic space exposed
Teichoic acid

20. Peptidoglycan (Murein) Structure
Mesh-like polymer composed of identical subunits
contains N-acetyl glucosamine and N-acetylmuramic acid and several different amino acids
chains of linked peptidoglycan subunits are cross linked by peptides

21. Cell wall unit structures

22. G- G+ Bacterial cell wall
Top: E. coli peptidoglycan; direct cross-linking, typical of many gram-negative bacteria
Bottom: Staphylococcus aureus peptidoglycan
NAM= N-acetylglucosoamine; NAG= N-acetylmuramic acid

24. Animation: Bacterial peptidoglycan cell walls

25. Wall Assembly
Cleavage by autolysin
Pre-formed subunits added.
Bridges created (transpeptidation)

26. Archaeal cell walls lack peptidoglycan Resemble G+ thick wall
cell wall varies from species to species but usually consists of complex hetero-polysaccharides and glycoproteins eg. Methanosarcina, and Halcoccus have complex polysacharides resembling those of eukaryotic connective tissue extracellular matrix
Methanogens have walls containing pseudomurein

27. Archaeal cell walls: Pseudomurein
NAG
NAT
NAT instead of NAM; links to NAG by β(1→3) glycosidic linkage instead of β(1→4)
no lactic acid between NAT and peptides
NAT connects to tetra-peptide through C6
instead of NAM C3 in eubacteria
in some tetra-peptide consists of L-amino acids instead of D-amino acids

28. The Gram-Positive Envelope
Capsule (not all species)
Polysaccharide
S-Layer (not all species)
Made of protein
Thick cell wall
Teichoic acids for strength
Thin periplasm
Plasma membrane

29. Gram-Positive Cell Walls
CW composed primarily of peptidoglycan
contain large amounts of teichoic acids
polymers of glycerol
or ribitol joined by
phosphate groups
some gram-positive bacteria have layer of proteins on the surface of peptidoglycan
Isolated cell wall from Bacillus megaterium; latex spheres are 0.25 micrometer in diameter

30. The Gram-Negative Envelope
Capsule (not all species)
Polysaccharide
Outer Membrane
LPS (lipopolysaccharide)
In outer leaflet only
Braun lipoprotein
Thin cell wall
one or two layers of peptidoglycan
Thick periplasm
Plasma membrane
Peptidoglycan cell wall

31. Braun (Murein) lipoprotein
Braun lipoprotein
Bridges inner leaflet of outer membrane to peptidoglycan
67 aa protein with
N-terminal Cyc-triglyceride
C-terminal lysine connected to mDAP by peptide bond

32. Porins
more permeable than plasma membrane due to presence of porin proteins and transporter proteins
porin proteins form channels through which small molecules ( daltons) can pass

33. Lipopolysaccharides (LPSs)
consists of three parts
lipid A
core polysaccharide
O-side chain (O antigen)

34. Importance of LPS protection from host defenses (O antigen variation)
contributes to negative charge on cell surface (core polysaccharide)
helps stabilize outer membrane structure (lipid A)
can act as an endotoxin (lipid A)

35. Capsules, Slime Layers, and S-Layers
layers of material lying outside the cell wall
capsules
usually composed of polysaccharides, some have proteins
well organized and not easily removed from cell eg. Klebsiella and Pneumococcus
slime layers
similar to capsules except diffuse, unorganized and easily removed
a capsule or slime layer composed of organized, thick polysaccharides can also be referred to as a glycocalyx

36. Capsules, Slime Layers, and S-Layers
regularly structured layers of proteins or glycoproteins
In bacteria the S- layer is external to the cell wall
common among Archaea, act as molecular sieve letting passage of small molecules
S-layer of Thermoproteus tenax

37. Functions of capsules, slime layers, and S-layers
protection from host defenses (e.g., phagocytosis)
protection from harsh environmental conditions (e.g., desiccation)
attachment to surfaces
protection from viral infection or predation by bacteria
protection from chemicals in environment (e.g., detergents)
facilitate motility of gliding bacteria
protection against osmotic stress

38. Pili and Fimbriae Fimbriae (s., fimbria) Sex pili (s., pilus)
short, thin, hairlike, proteinaceous appendages
up to 1,000/cell
mediate attachment to surfaces
some (type IV fimbriae) required for twitching motility or gliding motility that occurs in some bacteria
Sex pili (s., pilus)
similar to fimbriae except longer, thicker, and less numerous (1-10/cell)
required for mating (conjugation)
Produced by F+ strains
The fimbriae of P. vulgaris