Microcheck

Facilitated transport does not require energy but is rarely used by bacteria. Active transport via the major facilitator superfamily uses proton motive force. Active transport via an ABC transporter uses ATP as an energy source. Group translocation chemically modifies a molecule as it enters the cell. Proteins that have a leader sequence are secreted from a cell by the general secretory pathway.

■ Transport proteins may be referred to as what two other terms?

■ Describe the role of binding proteins in an ABC transport system.

■ Can you argue that group translocation is a form of active transport?

The cell wall of most common prokaryotes is a rigid structure, which determines the shape of the organism. A primary function of the wall is to hold the cell together and prevent it from bursting. If the cell wall is somehow breached, undamaged parts maintain their original shape (figure 3.31). The cell wall is composed of unique structures and molecules, some of which are recognized by our immune system as the sign of an invader. Antimicrobial medications target some of the unique structures.

Figure 3.31 The Rigid Cell Wall Determines the Shape of the Bacterium

Even though the cell has split apart, the cell wall maintains its original shape.

Figure 3.31 The Rigid Cell Wall Determines the Shape of the Bacterium

Even though the cell has split apart, the cell wall maintains its original shape.

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58 Chapter 3 Microscopy and Cell Structure

Table 3.5 Comparison of Features of Gram-Positive and Gram-Negative Bacteria

Peptidoglycan and teichoic acids

Cytoplasmic membrane

Cytoplasmic membrane

Outer membrane

Peptidoglycan

Outer membrane

Peptidoglycan

Cytoplasmic membrane

Gram-Negative

Cytoplasmic membrane

Gram-Positive

Gram-Negative

Color of Gram-Stained Cell Representative Genera Distinguishing Structures/Components

Peptidoglycan

Teichoic acids

Periplasm

Outer membrane

Endotoxin (lipopolysaccharide)

Porin proteins

General Characteristics

Sensitivity to penicillin

Sensitivity to lysozyme

Form that results from removal of peptidoglycan

Purple

Bacillus, Staphylococcus, Streptococcus

Thick layer

Present

Absent

Absent

Absent

Absent (unnecessary because there is no outer membrane)

Generally more susceptible (with notable exceptions)

Protoplast

Reddish-pink

Escherichia, Neisseria, Pseudomonas

Thin layer

Absent

Present

Present

Present

Present; allow passage of molecules through outer membrane

Generally less susceptible (with notable exceptions)

No (unless also treated with EDTA)

Spheroplast

The type of cell wall distinguishes two main groups of bacteria—Gram-positive and Gram-negative. A comparison of the features of these groups is presented in table 3.5.

Peptidoglycan

Although the structure varies in Gram-positive and Gram-negative cells, the rigidity of bacterial cell walls is due to a layer of peptidoglycan, a macromolecule found only in bacteria. The basic structure of peptidoglycan is an alternating series of two major subunits, N-acetylmuramic acid (NAM) and N-acetyl-glucosamine (NAG). These subunits, which are related to glucose in their structure, are covalently joined to one another to form a glycan chain (figure 3.32). This high molecular weight linear polymer serves as the backbone of the peptidoglycan molecule.

Attached to each of the NAM molecules is a string of four amino acids, a tetrapeptide chain, that plays an important role in the structure of the peptidoglycan molecule. Cross-linkages can form between tetrapeptide chains, thus joining adjacent glycan chains to form a single, very large three-dimensional molecule. In Gram-negative bacteria, tetrapeptides are joined directly. In Gram-positive bacteria, they are usually joined indirectly by a peptide interbridge, the composition of which may vary among species.

An assortment of only a few different amino acids make up the tetrapeptide chain. One of these, diaminopimelic acid, which is related to the amino acid lysine, is not found in any other place in nature. Some of the others are D-isomers, a form not found in proteins. ■ D-isomer, p. 27 ■ lysine, p. 26

The Gram-Positive Cell Wall

A relatively thick layer of peptidoglycan characterizes the cell wall of Gram-positive bacteria (figure 3.33). Depending on the species, this single component makes up 40% to 80% of the dry weight of the wall. As many as 30 layers, or sheets, of interconnected glycan chains make up the polymer. Regardless of its thickness, peptidoglycan is fully permeable to many substances including sugars, amino acids, and ions.

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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