Cell Wall

Functional Anatomy Of Prokaryotic And Eukaryotic Cells
	1. Overview	2. Prokaryotes: Morphology	3. Prokaryotes: External Structures	4. Prokaryotes: The Cell Wall	5. Prokaryotes: Internal Structures	6. Eukaryotic Cells Endosymbiotic Theory

Objectives:

7. Describe the composition of the bacterial cell wall.

8. Differentiate between gram-negative and gram-positive cell walls.

9. Describe the following in regard to cell walls:

· Mycoplasmas

· L-forms

· Protoplasts

· Spheroplasts

10. Identify the bacteria that normal do not have cell walls.

11. Describe the composition and structure of the plasma membrane.

12. List three functions of the plasma membrane in prokaryotic cells.

13. Identify

· Chromatophores or thylakoids

· Mesosomes

The Cell Wall

Composition And Characteristics

1. The cell wall surrounds the plasma membrane and protects the cell from changes in water pressure.

2. The bacterial cell wall consists of peptidoglycan (or murein), a polymer consisting of NAG and NAM and short chains of amino acids.

3. Penicillin interferes with peptidoglycan synthesis.

N-Acetylglucosamine (NAG) And
N-Acetylmuramic Acid (NAM) Joined As In Peptidoglycan

Structure Of Peptidoglycan In A Gram-Positive Cell Wall

Alternating NAM and NAG molecules form a carbohydrate backbone (the glycan portion).

Rows of NAG and NAM are linked by polypeptides (peptido- portion).

The structure of the polypeptide cross-bridges may vary but they always have a tetrapeptide sidechain, which consists of 4 amino acids attached to NAMs. The amino acids occur in alternating D and L forms.

Gram-positive cell wall

4. Gram-positive cell walls consist of many layers of peptidoglycan and also contain teichoic acids. Teichoic acids may:

bind and regulate movement of cations into and out of the cell
prevent extensive wall breakdown and possible cell lysis during cell growth
provide much of the cell wall's antigenicity

5. Gram-negative bacteria have a lipopolysaccharide-lipoprotein-phospholipid outer membrane surrounding a thin (sometimes one layer) peptidoglycan layer. Gram-negative cell walls have no teichoic acids.

6. The outer membrane protects the cell from phagocytosis and from penicillin, lysozyme, and other chemicals.

7. Porins are proteins that permit small molecules to pass through the outer membrane; specific channel proteins allow other molecules to move through the outer membrane.

Gram-Negative Cell Wall

8. The lipopolysaccharide component of the outer membrane consists of sugars (O polysaccharides) that function as antigens and lipid A, which is an endotoxin. Endotoxin causes fever and shock.

Cell Walls And The Gram Stain Mechanism

1. The crystal violet-iodine complex combines with peptidoglycan.

2. The decolorizer removes the lipid outer membrane of gram-negative bacteria and washes out the crystal violet.

Atypical Cell Walls

1. Mycoplasma is a bacterial genus that naturally lacks cell walls.

2. Archaea have pseudomurein; they lack peptidoglycan.

Damage To The Cell Wall

1. In the presence of lysozyme, gram-positive cell walls are destroyed, and the remaining cellular contents are referred to as a protoplast.

2. Proteus and some other genera can lose their cell walls spontaneously or in response to penicillin and swell into L forms (Lister Institute). L forms can live and divide and/or return to the normal walled state.

3. In the presence of lysozyme, gram-negative cell walls are not completely destroyed, and the remaining cellular contents are referred to as spheroplasts.

4. Protoplasts and spheroplast are subject to osmotic lysis.