Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Prokaryotic membrane transport

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Blue-green algae and some bacteria are able to manu-

facture their own food from sunlight through the process of

photosynthesis. Green plants likewise have this capability.

This type of bacteria are the photoautotrophs. Other bacteria

are able to utilize elements like nitrogen, sulphur, hydrogen, or

iron to make their food. This type of Prokaryote are the

chemoautotrophs. But the bulk of the Prokaryotae exists by

decomposing and using compounds made by other organisms.

This decomposition is a vital process. Without this bacterial

activity, the wastes of other organisms would blanket Earth.

The relative simplicity of the Prokaryotae, as compared

to eukaryotes, extends to the genetic level. The prototypical

bacterial species Escherichia coli contains approximately

5,000 genes. On average, about one in every 200 bacteria is

likely to have a mutation in at least one of the genes. In a 100

ml culturecontaining one million bacteria per milliliter, this

translates to 500,000 mutant bacteria. This ability of members

of the Prokaryotae to mutant and so quickly adapt to a chang-

ing environment is the principle reason for their success

through time.

The ecological distribution of the Prokaryotae is vast.

Bacteria have adapted to live almost everywhere, in environ-

ments as diverse as the thermal deep-sea vents to the boiling

hot springs of Yellowstone National Park, from the soil to the

intestinal tract of man and animals. The diversity of bacteria

led to the design of a classification system just for them. David

Hendricks Bergeyspearheaded this classification scheme in

the first half of the twentieth century. His efforts culminated in

the publication (and ongoing revisions) of the Bergey’s

Manual of Systematic Bacteriology.

See alsoBacterial kingdoms; Evolutionary origin of bacteria

and viruses

PROKARYOTIC CELLS, GENETIC REGULA-

TION OF•seeGENETIC REGULATION OF PROKARYOTIC

CELLS

PProkaryotic membrane transportROKARYOTIC MEMBRANE TRANSPORT

The ability of Prokaryotic microorganisms to move com-

pounds into the cell, and to remove waste products of metabo-

lismout 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 move-

ment 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 pres-

ence 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 pro-

teins are important constituents of the inner of the two mem-

branes 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 gen-

eral, there are three different functional types of protein. These

are termed uniporters, antiporters, and symporters.

Uniporters can actually be considered analogous to the

water-filed channels of the Gram-negative outer membrane,

in that a uniporter is a single protein or a collection of sev-

eral 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 concen-

trations of the molecule of interest on either side of the mem-

brane. 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 trans-

ported in one direction while the other molecule is simultane-

ously 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 rota-

tion 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 mole-

cules 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 mol-

ecules 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-con-

taining 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 concentra-

tion (i.e., transport is against a concentration gradient).

Alternatively, energy for transport in the antiport and

symport systems can be provided by the molecules them-

selves. The fact that the molecules prefer to be associated

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