Directed Movement of Molecules Across the Cytoplasmic Membrane

Nearly all molecules that enter or exit a cell must cross the otherwise impermeable cytoplasmic membrane through proteins that function as selective gates. Mechanisms allowing nutrients and other small molecules to enter the cell are called transport

3.5 Directed Movement of Molecules Across the Cytoplasmic Membrane 55

systems. Some of these same mechanisms are used to expel wastes and compounds that are otherwise deleterious to the cell. This function of transport systems is sometimes called efflux. Efflux is medically important because it allows certain bacteria to oust antimicrobial medications and disinfectants that are meant to destroy them. By doing so, these bacteria can resist those antimicrobial chemicals. ■ antimicrobial drugs, p. 521 ■ disinfectants, p. 110

Cells actively move certain proteins they synthesize out of the cell—a process called secretion. Some of these secreted proteins make up structures such as flagella, which are appendages used for motility. Others are enzymes secreted to break down substances such as oligosaccharides and peptides, which would otherwise be too large to transport into the cell.

Transport Systems

Mechanisms used to transport molecules across the membrane employ highly specific proteins. These proteins, sometimes called transport proteins, permeases, or carriers, span the membrane. One end projects into the surrounding environment and the other into the cell. The interaction between the transport protein and the molecule it carries is highly specific. Consequently, a single carrier generally transports only a specific type of molecule. As a carrier transports a molecule, its shape changes, facilitating the passage of the molecule (figure 3.27).

Nutrient molecules

Outside of cell

Nutrient molecules

Outside of cell

Inside of cell

Transport protein

Outside of cell

Inside of cell

Figure 3.27 Transport Protein A transport protein changes its shape to facilitate passage of a compound across the cytoplasmic membrane.

Inside of cell

Transport protein

Outside of cell

Inside of cell

Figure 3.27 Transport Protein A transport protein changes its shape to facilitate passage of a compound across the cytoplasmic membrane.

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