Structure and Function of Prokaryotes

• Structures External to the Cell Wall

• Cell Walls Biochemistry (Gram +/-)

Prokaryote “Anatomy” Overview

Cell envelope: Collectively all the structures outside from the plasma membrane.

Cell Wall & Osmotic Pressure• Solutes diffuse from high to low concentration.

• Net movement of water is toward higher solute concentrations.

• Cells have semi-permeable membranes (water passes; solutes don’t).

• Intracellular and external environment strive for isosmotic equilibrium.

• Osmotic pressure, refers to that required to hold back the net movement of water.

• Hypotonic environments promote “osmolysis”; unless there’s a rigid cell wall.

• Hypertonic environments promote “plasmolysis”

Two Cell Wall Designs:

Gram StainDifferential staining to distinguish cell wall types.

(Christian Gram 1884)

Cell Wall BiochemistryWhat is peptidoglycan?

Gram -

Gram +

Cell Wall BiochemistryWhat is peptidoglycan?

(Inter-)(gly)

Cell Wall (Gram +)

Cell Wall (Gram -)(w/ Outer Membrane)

Periplasmic space

Braun’s lipoprotein

(LPS = endotoxin)core

Glycocalyx: means sugar coating; often polysaccharide or polypeptide layer external to the cell wall.

• Capsules: organized, consolidated, well attached.

• Slime Layer: unorganized; loose; removed easily.

• Function in attachment; protection; virulence.

Structures External to the Cell Wall

S-layer: extremely well organized layer of protein subunits that forms a rigid mesh, or mail, next to cell wall. Functions in protection and in resisting hypoosmotic stress

Structures External to the Cell Wall

peritrichous

monotrichous

amphitrichous

lophotrichous

Flagella:• Mostly made of flagellin.

• Filament thick (20 nm) & long (10-20 µm).

• Filament possibly sheathed.

• Varied locations on cell:

Fimbriae:• 1000’s of thin (~5 nm) & short appendages of helical proteins.

• Attachment to (specific) surfaces.

Sex Pili:• 1-10 slightly larger than fimbriae.

• Only in cells with a fertility plasmid (F factor), called donors.

• Attaches to like cells without F factor, called recipients.

• Facilitates genetic transfer between cells; with recipient gaining the F factor and possibly other genes.

Flagellar Motility & Chemotaxis

Other kinds of motility:

Spirochete; axial flagella

Gliding motility; no flagella

Random (searching)

Positive Chemotaxis

(“follow that smell”)

Low [nutrient solute] High [nutrient solute]