Cell Membrane Cell Wall and Outer Membrane

The cell membrane or plasma membrane is a flexible semipermeable membrane that surrounds the cytoplasm. The cell membrane contains two different layers (bilayer) that regulate the movement of substances in and out of the cell. The outer layer is hydrophilic (water-loving), while the inner layer is hydrophobic (water-fearing). Together these layers form a protective and regulating barrier between the cytoplasm and the environment.

TABLE 3.1 Significant Differences Between Gram-Negative and Gram-Positive Bacteria

Characteristic Gram-Negative Bacteria Gram-Positive Bacteria

Lipids Much lipopolysaccharide Very little

Peptidoglycan Thin layer Thick layer

Outer membrane Present Absent

FIGURE 3.5 Axial filament in spirochete.

FIGURE 3.5 Axial filament in spirochete.

The cell wall is an extremely porous and rigid structure that lies outside the cell membrane. The cell wall performs three functions. First, it provides protection. Second, it maintains the characteristic shape of the cell. Third, it prevents the cell from bursting when fluids flow into the cytoplasm. Based upon chemical composition, there are two types of cell walls, Gram negative and Gram positive (Table 3.1).

Gram-positive bacteria retain the crystal violet stain after the Gram-staining technique due to their thick cell wall. Gram-negative bacteria cannot retain the crystal violet stain due to their thin cell wall and presence of relatively large quantities of lipoproteins and lipopolysaccharides. Gram negative bacteria are stained red with Safranin at the end of the Gram staining technique.

An outer membrane is found only in Gram-negative bacteria. It is a bilayer membrane and surrounds the cell wall. The outer membrane controls the transportation of some proteins from the environment into the cell.

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