The Cytoplasmic Membrane

This elementary membrane, also known as the plasma membrane, is typical of living cells. It is basically a double layer of phospholipids with numerous proteins integrated into its structure. The most important of these membrane

— Basic Bacterial Cell Structure

Nucleus equivalent-

(syn. nucleoid)

Capsul

Plasmid

Cytoplasmic membrane

Flagella

— Basic Bacterial Cell Structure

Nucleus equivalent-

(syn. nucleoid)

Flagella

Bacterial Cell Structure Plasmid

Capsul

Plasmid

Cytoplasmic membrane

Depot substances

- metaphosphates (volutin)

- glycogen (granulose)

Attachment pili

70S ribosomes

Fig.3.7 All bacteria have the same basic structure (not to scale).

Depot substances

- metaphosphates (volutin)

- glycogen (granulose)

Attachment pili

70S ribosomes

Fig.3.7 All bacteria have the same basic structure (not to scale).

proteins are permeases, enzymes for the biosynthesis of the cell wall, transfer proteins for secretion of extracellular proteins, sensor or signal proteins, and respiratory chain enzymes.

In electron microscopic images of Gram-positive bacteria, the mesosomes appear as structures bound to the membrane. How they function and what role they play remain to be clarified. They may be no more than artifacts.

Cell Wall

The tasks of the complex bacterial cell wall are to protect the protoplasts from external noxae, to withstand and maintain the osmotic pressure gradient between the cell interior and the extracellular environment (with internal pressures as high as 500-2000 kPa), to give the cell its outer form and to facilitate communication with its surroundings.

Murein (syn. peptidoglycan). The most important structural element of the wall is murein, a netlike polymer material surrounding the entire cell (sacculus). It is made up of polysaccharide chains crosslinked by peptides (Figs. 3.8 and 3.9).

The cell wall of Gram-positive bacteria (Fig. 3.10). The murein sacculus may consist of as many as 40 layers (15-80 nm thick) and account for as much as

The Murein Building Block ch2oh ch2oh

The Murein Building Block ch2oh ch2oh

Fig. 3.8 The murein (syn. peptido-glycan) of the cell wall is composed of a series of identical subunits. The terminal D-alanine is split off each time a new crosslink is synthesized. Only in staphylococci is a pentagly-cine interpeptide bridge inserted between adjacent peptides.

NH CO CH3 NH CO CH3

CH CH3

NH CO CH3 NH CO CH3

CH CH3

CO I

L-alanine

D-isoglutamine L-lysine—[glycine]g-

D-alanine

D-alanine (COOH)

Fig. 3.8 The murein (syn. peptido-glycan) of the cell wall is composed of a series of identical subunits. The terminal D-alanine is split off each time a new crosslink is synthesized. Only in staphylococci is a pentagly-cine interpeptide bridge inserted between adjacent peptides.

30% of the dry mass of the cell wall. The membrane lipoteichoic acids are anchored in the cytoplasmic membrane, whereas the cell wall teichoic acids are covalently coupled to the murein. The physiological role of the teichoic

— The Structure of Murein

Lipoteichoic Acid
(^Mu^) = N-acetyl muramic acid (= 3-0 lactyl ether of N-acetyl glucosamine) (GlcNAo = N-acetyl glucosamine O = Aminosäure

Fig. 3.9 Soluble murein fragments of Gram-negative and Gram-positive bacteria can stimulate excessive cytokine secretion in macrophages by binding to toll-like receptors and CD14. Cytokines cause the clinical symptoms of sepsis or septic shock syndrome (see under Lipoid A, p. 156).

— The Cell Wall of Gram-Positive Bacteria

Cell wallassociated proteins

Membrane lipoteichoic acid

Cell wall teichoic acid

Cell wall-specific polysaccharide

— The Cell Wall of Gram-Positive Bacteria

Cell wallassociated proteins

Teichoic Acid

Cell wall-specific polysaccharide

Murein (syn. peptid-oglycan)

Cytoplasmic membrane

Fig. 3.10 Note the characteristic thick murein layer, the proteins and teichoic acids anchored in the murein, and the lipoteichoic acid fixed to the membrane by a lipophilic anchor (not to scale).

1 r

Murein (syn. peptid-oglycan)

Cytoplasmic membrane

Fig. 3.10 Note the characteristic thick murein layer, the proteins and teichoic acids anchored in the murein, and the lipoteichoic acid fixed to the membrane by a lipophilic anchor (not to scale).

acids is not known in detail; possibly they regulate the activity of the auto-lysins that steer growth and transverse fission processes in the cell. Within the macroorganism, teichoic acids can activate the alternative complement pathway and stimulate macrophages to secrete cytokines. Examples of cell wall-associated proteins are protein A, the clumping factor, and the fibronec-tin-binding protein of Staphylococcus aureus or the M protein of Streptococcus pyogenes. Cell wall anchor regions in these proteins extending far beyond the murein are bound covalently to its peptide components. Cell wall-associated proteins frequently function as pathogenicity determinants (specific adherence; phagocyte protection).

The cell wall of Gram-negative bacteria. Here, the murein is only about 2 nm thick and contributes up to 10% of the dry cell wall mass (Fig. 3.11). The outer membrane is the salient structural element. It contains numerous proteins (50% by mass) as well as the medically critical lipopolysaccharide.

- The Cell

K antigen

Wall of Gram-Negative Bacteria

O chain Core Lipoid A

cxxxxxxxxxxxoxxraxxrcccccxDcco

Lipopoly-

saccharide

- The Cell

K antigen

Wall of Gram-Negative Bacteria

O chain Core Lipoid A

Lipopoly-

saccharide

Bacterial Murein

cxxxxxxxxxxxoxxraxxrcccccxDcco

Murein

Cytoplasmic membrane

Fig. 3.11 Note the characteristic thin murein layer and the outer membrane connected to it by proteins (OmpA, murein lipoprotein). Many different proteins are localized in the outer membrane. Its outer layer is made up of closely packed lipopolysaccharide complexes (see Fig. 3.12).

Murein

Cytoplasmic membrane

Fig. 3.11 Note the characteristic thin murein layer and the outer membrane connected to it by proteins (OmpA, murein lipoprotein). Many different proteins are localized in the outer membrane. Its outer layer is made up of closely packed lipopolysaccharide complexes (see Fig. 3.12).

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment