Prokaryotic Cell Structure
and function (Part I)
BIO3124
Lecture #3 (I)
Plasma Membrane Properties and Functions


defines the existence of a cell
Made of lipid bilayer
 Double

layer of phospholipids
Surrounds the cell
 approx. 5-10 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
Fluid Mosaic Model of Membrane Structure

Lipid bilayer in which proteins float (Singer and Nicholson model)
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
Membrane lipids


Amphipathic phospholipids
polar ends (hydrophilic)


Glycerol, negative charge (outer leaflet)
Ethanolamine, positive charge (inner
leaflet)

nonpolar ends (hydrophobic)




Tails of fatty acids
Palmitic acid
Oleic acid (kinking) increase
fluidity
Cyclopropane conversion
 aging cells
Phosphatidylethanolamine
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
Archaeal membranes



Etherglycerol, not ester
bond
Terpene derived lipids
some have a monolayer
membranes



Tetra-ether glycerol
Cyclopentane: isoprene
cyclization
Increased stability
Archaeal membranes
Moderately thermophilic
- Bilayer or mixed
Extreme thermophiles
eg. Solfolobus and
Theromoplasma
Role of cell membrane in energy metabolism
• bacterial cell membranes involved in ETC
• Gradual energy release
• forming proton gradient across membrane
Animation: A bacterial electron transfer system
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.
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
ATP synthase mechanism
Note: pump also works in reverse to create H+ gradient
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
The Bacterial Cell Wall

rigid structure that lies
just outside the
plasma membrane
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
Evidence of protective nature of the cell wall

Lysozyme treatment

Penicillin inhibits
peptidoglycan
synthesis
• few PG layers, defined Periplasmic space
• unique outer membrane, LPS, Braun’s lipoprotein
Braun’s
• Multiple PG layers, periplasmic space exposed
• Teichoic acid
Peptidoglycan (Murein) Structure
Mesh-like polymer composed of identical
subunits
 contains N-acetyl glucosamine and Nacetylmuramic acid and several different
amino acids
 chains of linked peptidoglycan subunits
are cross linked by peptides

Cell wall unit structures
Bacterial cell wall
G-
G+
Animation: Bacterial peptidoglycan cell walls
Wall Assembly



Cleavage by autolysin
Pre-formed subunits added.
Bridges created (transpeptidation)
Archaeal cell walls

lack peptidoglycan

Resemble G+ thick wall

cell wall varies from species
to species but usually
consists of complex heteropolysaccharides and
glycoproteins
eg. Methanosarcina, and
Halcoccus have complex
polysacharides resembling those of
eukaryotic connective tissue
extracellular matrix

Methanogens have walls
containing pseudomurein
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
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

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

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
Braun (Murein) lipoprotein

Braun lipoprotein

Bridges inner leaflet of
outer membrane to
peptidoglycan
67 aa protein with
 N-terminal Cyctriglyceride
 C-terminal lysine
connected to mDAP by
peptide bond

Porins

more permeable than
plasma membrane
due to presence of
porin proteins and
transporter proteins

porin proteins form
channels through
which small molecules
(600-700 daltons) can
pass
Lipopolysaccharides (LPSs)

consists of
three parts



lipid A
core
polysaccharide
O-side chain
(O antigen)
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)

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

Capsules, Slime Layers, and S-Layers

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
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
Pili and Fimbriae

Fimbriae (s., fimbria)
 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 (110/cell)
 required for mating (conjugation)
 Produced by F+ strains
The fimbriae of P. vulgaris