Mechanisms of facilitated diffusion and the role of membrane transport proteins

Facilitated diffusion. Hydrophilic molecules and ions that are unable to pass through the membrane on their own are transported with the help of specific carrier proteins. Therefore, this transport mechanism is called facilitated diffusion. Facilitated diffusion is the transfer of a substance along a concentration and electrochemical gradient with the participation of specific membrane proteins in this process. In other words, facilitated diffusion is a diffusion process coupled with a chemical reaction of the interaction between the transported substance and the carrier protein. This process is specific and occurs at a higher rate than simple diffusion. Each protein transports only a specific molecule or a group of similar molecules; that is, these proteins are relatively specific. This is how the selectivity of nutrient uptake by the cell is ensured.

At the same time, like simple diffusion, facilitated diffusion is also unable to proceed against the concentration gradient or electrochemical gradient. This type of diffusion is characterized by a saturation effect: at a certain significant concentration of the diffusing substance, a further increase in its concentration no longer accelerates diffusion. This indicates the similarity of the mechanisms of facilitated diffusion to the mechanisms of enzymatic catalysis (saturating substrate concentration). Apparently, the transported substance binds to a carrier, which delivers it from one surface of the membrane to the other and then releases it (an analogy with the formation of an enzyme-substrate complex, followed by the release of the reaction product and the enzyme). If there is too much of the diffusing substance, all carrier molecules become occupied, and it is no longer possible to accelerate the transfer by increasing the concentration.

There are two types of membrane transport proteins: carrier proteins and channel-forming proteins. The first type of carriers acts like a ferry. Having loaded the transported molecules ("passengers"), they carry them across the membrane and return either empty or after picking up other "passengers". The second type of carriers does not perform shuttle movements but integrates into the membrane, forming a channel.

Carrier proteins. The action of carrier proteins resembles that of an enzyme, but the transported substance does not change in the process. Like an enzyme, a carrier protein has a special site for binding the transported substance. The transport protein binds to a molecule or ion of the transported substance in the same way an enzyme binds to a substrate, i.e., based on the principle of complementarity. The resulting complex can move across the plasmalemma, which is impermeable to ions. On the inner side of the membrane, the ions separate from the carriers and move into the cell's interior. The carrier molecules again become capable of transporting other ions. The carrier theory can be described by the Michaelis-Menten equation:

Km  C where: V is the rate of enzymatic ion uptake reactions at concentration C;

is the maximum possible rate of ion uptake reached before the full saturation of all carriers;

Km is the Michaelis constant, reflecting the degree of affinity of the carrier for the ions;

C is the molar concentration of the substrate.

This theory allows for the quantitative determination of the parameters of the uptake system.

The mode of movement of a substance through a membrane depends both on the properties of the transported compound and the features of the composition and structural organization of the membrane. Transmembrane transfer can be carried out by uniport, symport, or antiport (Fig. 44; Raven P., Evert R., Eichhorn S., 1990).

Fig. 44. Scheme of transport protein functioning

Uniport is the simplest type of transfer of a single dissolved substance from one side of the membrane to the other, carried out by the mechanism of simple or facilitated diffusion.

If so-called cotransport systems are present in the membrane, it is possible to move two different substances across the membrane. Cotransport systems are transport proteins that jointly transport two different substances by the type of symport or antiport, i.e., the carrier has binding sites for both substances.

Complementarity (from Lat. "completion") is the spatial correspondence of the surfaces of interacting molecules or their parts, leading to the formation of secondary bonds between them (hydrogen, ionic, van der Waals).

• Symport is the transfer of one substance across the membrane due to the energy of the concentration gradient of another substance transferred simultaneously with it in the same direction, as the carrier has binding sites for it as well.
• Antiport is the movement of a substance against the concentration gradient due to the energy of the concentration gradient of another substance moving in the direction of its own concentration gradient.

All transport proteins form a continuous protein passage through the membrane, so the substances they transport do not directly contact the hydrophobic inner part of the lipid bilayer. There are two mechanisms for the passage of substances through the plasmalemma:

• Relay mechanism – when the transported substance is sequentially passed from one carrier molecule to another. In this case, the carrier molecules integrate into the membrane one after another, and the ions or molecules captured by the outermost carrier molecule are passed on by relay.
• Shuttle mechanism – if the carrier moves together with the ions transported into the cell, the transfer mechanism is called a shuttle mechanism.

Channel-forming proteins. These proteins form channels in membranes that penetrate the lipid bilayer and are filled with water. The outer surface of these channels is hydrophobic, while the inner surface is hydrophilic; the channel diameter is 0.5–0.8 nm. Substances pass through the channels without contacting the hydrophobic part of the membrane.

Practically all nutrients enter plant cells as charged particles through specialized protein pores — ion channels. To date, more than 50 types of such membrane structures are known. The most common of these are channels permeable to potassium and calcium ions.

The main feature of these channels is their ability to selectively recognize the necessary elements and transport them into the cell at a colossal speed. The movement of ions occurs passively along a gradient, that is, due to the difference in concentration of the element on both sides of the membrane. This delivery method works thousands of times faster than transport via carrier proteins.

How channel permeability is regulated

The permeability of ion channels does not remain constant; the plant is able to flexibly regulate their operation in response to specific stimuli. This allows cells to adapt the uptake of elements to changing external conditions. The activity of membrane channels is modulated by a number of physiological and external factors:

• magnitude of membrane potential;
• pH level;
• ion concentration;
• intracellular signaling molecules;
• other endogenous and exogenous factors.