The ability of Prokaryotic microorganisms to move compounds into the cell, and to remove waste products of metabolism out of the cell, is crucial for the survival of the cell. Some of these functions are achieved by the presence of water-filled channels, particularly in the outer membrane of Gram-negative bacteria, which allow the diffusion of molecules through the channel. But this is a passive mechanism and does not involve the input of energy by the bacterium to accomplish the movement of the molecules across the membrane. Mechanisms that depend on the input of energy from the microorganism are active membrane transport mechanisms.
Prokaryotic membrane transport depends on the presence of specific proteins. These proteins are located within a membrane that surrounds the cell. Gram-positive bacteria have only a single membrane surrounding the contents of the bacterium. So, it is within this membrane that the transport proteins reside. In Gram-negative bacteria, the transport proteins are important constituents of the inner of the two membranes that are part of the cell wall. The inner membrane is also referred to as the cytoplasmic membrane.
There are a number of proteins that can participate in transport of molecules across Prokaryotic membranes. Different proteins have different modes of operation. In general, there are three different functional types of protein. These are termed uniporters, antiporters, and symporters.
Uniporters can actually be considered analogous to the water-filled channels of the Gram-negative outer membrane, in that a uniporter is a single protein or collection of like proteins that produces a channel through which molecules can passively diffuse. No energy is required for this process. Some degree of selectivity as to the types of molecules that can pass down a channel is achieved based on the diameter of the channel. Thus, a small channel excludes large molecules.
A uniporter can also function in a process known as facilitated diffusion. This process is governed by the concentrations of the molecule of interest on either side of the membrane. If the concentration on one side of the membrane barrier is higher than on the other side, the movement of molecules through the connecting channel will naturally occur, in order to balance the concentrations on both sides of the membrane.
An antiporter is a membrane protein that can transport two molecules across the membrane in which it is embedded at the same time. This is possible as one molecule is transported in one direction while the other molecule is simultaneously transported in the opposite direction. Energy is required for this process, and functions to allow a change in the shape of the protein or to permit all or part of the protein to swivel upon binding of the molecules to be transported. One model has the molecules binding to the protein that is exposed at either surface of the membrane, and then, by an internal rotation of the transport protein, both molecules are carried to the other membrane surface. Then, each molecule is somehow released from the transport protein.
The third type of transport protein is termed a symport. This type of protein can simultaneously transport two molecules across a membrane in the same direction. The most widely held model for this process has the molecules binding to the transport protein that is exposed on the external surface of the membrane. In an energy-dependent process, these molecules are driven through a central region of the protein to emerge on the opposite side of the membrane. The protein molecule remains stationary.
The energy for prokaryotic membrane transport can come from the breakdown, or hydrolysis, of an energy-containing molecule called adenosine triphosphate (ATP). The hydrolysis of ATP provides energy to move molecules from a region of lower concentration to a region of higher concentration (i.e., transport is against a concentration gradient).
Alternatively, energy for transport in the antiport and symport systems can be provided by the molecules themselves. The fact that the molecules prefer to be associated with the protein, rather than in solution, drives the transport process.
The outer membrane of Gram-negative bacteria does contain proteins that participate in the active transport of specific molecules to the periplasmic space, which separates the outer and inner membranes. Examples of such transport proteins include the FhuA of Escherichia coli and FepA of this and other bacteria. This type of active transport is important for disease processes, as iron can be crucial in the establishment of an infections, and because available iron is normally in very low concentration in the body.
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