Microbial Transport - Transport of nutrients by bacteria

The first step in nutrient use is uptake of the required nutrients by the microbial cell. Uptake mechanisms must be specific. The necessary substances alone and not others, must be acquired because a cell need not take in a substance that it cannot use. 

Microorganisms often live in nutrient-poor habitats, so they must be able to transport nutrients from dilute solutions into the cell against a concentration gradient. Also, nutrient molecules must pass through a selectively permeable plasma membrane that prevents the free passage of most substances.

In view of the enormous variety of nutrients and the complexity of the task, it is well-known that microorganisms make use of several different transport mechanisms - passive, active and group translocation, symport, antiport and uniport, electrogenic and electro neutral transport, transport of Iron.

The most important of these are facilitated diffusion, active transport, and group translocation.

Passive Diffusion

A few substances, such as glycerol, can cross the plasma membrane by passive diffusion. Passive diffusion, often called diffusion or simple diffusion, is the process in which molecules move from a region of higher concentration to one of lower concentration. The rate of passive diffusion is dependent on the size of the concentration gradient between a cell’s exterior and its interior. These do not require energy input.

A fairly large concentration gradient is required for adequate nutrient uptake by passive diffusion (i.e., the external nutrient concentration must be high while the internal concentration is low). The rate of uptake decreases as more nutrient is acquired unless it is used immediately. Very small molecules such as H₂O, O₂, and CO₂ often move across membranes by passive diffusion. Larger molecules, ions, and polar substances must enter the cell by other mechanisms.

Facilitated Diffusion

The rate of diffusion across selectively permeable membranes is greatly increased by using carrier proteins, sometimes called permeases, which are embedded in the plasma membrane. Diffusion involving carrier proteins is called facilitated diffusion. The rate of facilitated diffusion increases with the concentration gradient much more rapidly and at lower concentrations of the diffusing molecule than that of passive diffusion.

Some permeases are related to the major intrinsic protein (MIP) family of proteins. MIPs facilitate diffusion of small polar molecules. They are observed in almost all organisms. The two most widespread MIP channels in bacteria are aquaporins, which transport water and the glycerol facilitators, which aid glycerol diffusion.

Once the carrier is saturated The diffusion rate reaches a plateau because the carrier protein is binding and transporting as many solute molecules as possible. The resulting curve resembles an enzyme-substrate curve and is different from the linear response seen with passive diffusion.

Carrier proteins also resemble enzymes in their specificity for the substance to be transported; each carrier is selective and will transport only closely related solutes.

 Although a carrier protein is involved, facilitated diffusion is truly diffusion.

• ·       A concentration gradient spanning the membrane drives the movement of molecules
• ·       No metabolic energy input is required.

If the concentration gradient disappears, net inward movement ceases. The gradient can be maintained by transforming the transported nutrient to another compound.

Simple Diffusion & Facilitated Diffusion

The mechanism of facilitated diffusion is not yet understood completely. The carrier protein complex spans the membrane. After the solute molecule binds to the outside, the carrier may change conformation and release the molecule on the cell interior. The carrier subsequently changes back to its original shape and is ready to pick up another molecule.

                            

The net effect is that a hydrophilic molecule can enter the cell in response to its concentration gradient. The mechanism is driven by concentration gradients and therefore is reversible. If the solute’s concentration is greater inside the cell, it will move outward. Because the cell metabolizes nutrients upon entry, influx is favoured.

Although glycerol is transported by facilitated diffusion in many, facilitated diffusion does not seem to be the major uptake mechanism. This is because nutrient concentrations often are lower outside the cell.

Facilitated diffusion is much more prominent in eucaryotic cells where it is used to transport a variety of sugars and amino acids.

Facilitated diffusion can efficiently move molecules to the interior only when the solute concentration is higher on the outside of the cell. Microbes must have transport mechanisms that can move solutes against a concentration gradient because microorganisms often live-in habitats characterized by very dilute nutrient sources.

Microbes use two important transport processes in such situations: active transport and group translocation. Both are energy-dependent processes.

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