Prokaryotic Cell Structure Prokaryotic Cell Structure and function and function (Part I)(Part I)

BIO3124Lecture #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, photosynthesisex. 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

Extreme thermophileseg. Solfolobus and Theromoplasma

Moderately thermophilic- Bilayer or mixed

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 transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force.

The Proton Motive Force

- It drives the conversion of ADP to

ATP through ATP synthase.

- This process is known as the

chemiosmotic theory.

Besides ATP synthesis, p drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs

PMF Drives Many Cell Functions

ATP synthase mechanism

Note: pump also works in reverse to create H+ gradient

Cell Transport Transporters pass material

in/out of cellPassive transport follows gradient

of material Pumps use energy

ATP or PMFMove 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 N-acetylmuramic 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 autolysinautolysin Pre-formed subunits added. Bridges created (transpeptidationtranspeptidation)

Archaeal cell walls lack peptidoglycan

Resemble G+ thick wall

cell wall varies from species

to species but usually

consists of complex hetero-

polysaccharides and

glycoproteinseg. Methanosarcina, and

Halcoccus have complex

polysacharides resembling those of

eukaryotic connective tissue

extracellular matrix

Methanogens have walls

containing pseudomurein

Archaeal cell walls: Pseudomurein

• 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 C6instead of NAM C3 in eubacteria• in some tetra-peptide consists of L-amino acids instead of D-amino acids

NATNAG

Capsule (not all species)

Polysaccharide S-Layer (not all species)

Made of protein

Thick cell wall Teichoic acids for strength

Thin periplasm Plasma membrane

The Gram-Positive Envelope

Gram-Positive Cell Walls

CW composed primarily of peptidoglycan

contain large amounts of teichoic acids

polymers of glycerolor ribitol joined byphosphate groups some gram-positive bacteria have

layer of proteins on the surface of peptidoglycan

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

The Gram-Negative Envelope

Peptidoglycan cell wall

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

Braun (Murein) lipoprotein

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 (1-10/cell)

required for mating (conjugation) Produced by F+ strains

The fimbriae of P. vulgaris