MEMBRANE TRANSPORTER PROTEINS: FUNCTIONS AND TYPES - BIOLOGY - 2025

The membrane transporters are integral membrane proteins specialized in carrying out the specific transport of ions and small molecules soluble both sides of the cell membranes.

Since these molecules cannot on their own traverse the hydrophobic heart of lipid bilayers, these proteins enable the cell to: maintain differentially defined environments, ingest nutrients, excrete waste products of metabolism, and regulate concentrations of ions and molecules.

Membrane transporter protein. By Emma Dittmar - Own work, CC BY-SA 4.0, https: //commons.wikimedia.org/w/index.php? Curid = 64036780

Transporter proteins have been classified into two large groups: channels and transporters. Transporters specifically bind the molecule to be transported and undergo conformational changes to be able to mobilize them. In turn, the channels do not bind molecules, but rather form a tunnel from which they move freely, simply excluded by their molecular radius.

In addition to this classification, there are others that take into account the amount of molecules to be transported, the direction in which they are transported, the dependence or not on energy and the source of energy they use.

Transport across the cell membrane

The synthesis of a membrane was the final evolutionary event that gave rise to cells.

Absolutely all cell membranes constitute barriers that oppose the free passage of ions and molecules into and out of cells. However, they must allow the entry of those that are vital for their operation as well as the exit of waste.

Therefore, the trafficking of molecules in both directions is carried out selectively. In other words, the cell decides who to let in or out of it and at what time.

To achieve this, it uses the existence of specialized transmembrane proteins that function as channels or gateways, called membrane transporters.

About 20% of the genes in a cell code for these membrane transporter proteins. This gives us an idea of ​​the relevance that transport has for cell function.

In this sense, the study of these proteins is of great importance both in the identification of chemotherapeutic targets, as well as of possible means of transport for drugs into target cells.

Functions of membrane transporters

Cellular transporters are responsible for transferring solutes of an organic and inorganic nature through cell membranes.

This transfer is carried out specifically only at the times when the cell needs it in order to:

- Maintain cell electrochemical gradients, essential for the performance of vital functions such as the production of energy demanded by the cell and the response to stimuli in excitable membranes.

- Take macro and micronutrients from the medium necessary to provide the cell with the monomers that will constitute the skeletons of its constituent macromolecules (nucleic acids, proteins, carbohydrates and lipids).

- Respond to stimuli and therefore participate in cellular signaling processes.

Types of membrane transporter proteins

Membrane transporters have been classified according to the type of transport they carry out into two broad categories: channels and transporters.

Types of membrane transporter proteins. By LadyofHats (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons.

Channel proteins

Channel proteins mediate the passive transport of water molecules, as well as various specific types of ions. This type of transport does not require energy to be carried out and it occurs spontaneously in favor of the concentration gradient of the molecule to be transported.

The name of channels is due to the fact that the structure that these proteins acquire resembles a tunnel, through which the simultaneous passage of numerous molecules occurs, which are selected based on their molecular radius. It is for this reason that these transporters can be considered a molecular sieve.

Among the functions associated with these transporters are the creation, maintenance and disruption of electrochemical gradients across cell membranes.

However, many other channels alternate between open and closed states in response to the arrival or removal of certain stimuli.

Such stimuli can be electrical in nature in voltage-dependent channels, chemical in ligand-dependent channels, or physical in channels that respond to mechanical changes such as stress or strain.

Conveyors

Transporter proteins are also called carriers or permeases. They use electrochemical gradients to carry out transport to one or the other side of the membrane.

This type of transporter proteins can mediate two types of transport. The facilitated passive transport of a molecule in a single direction and down a concentration gradient or the cotransport of two different molecules.

In turn, the cotransport in the same direction is carried out by symporters and in opposite directions by anticarriers.

On the other hand, unlike channels, which allow the simultaneous passage of many molecules through them, transporters only allow the limited and specific passage of a certain number of molecules. To ensure this, they have specific binding sites.

In this case, once the molecule is bound to the transporter, the latter undergoes a conformational change that exposes the binding site to the other side of the membrane, thus favoring transport.

This dependence on a structural change in carrier proteins slows the rate at which molecules are transported.

Types of conveyors

Based on the dependence or not of energy to carry out the transport, the transporter proteins can be classified into: passive facilitator transporters and active transporters.

- Passive facilitating conveyors

Passive facilitating transporters do not require energy supply and carry out the transport of molecules from a zone of high concentration to one of low concentration.

- Active facilitating transporters

In contrast, active transporters require input of energy to move substances against their concentration gradient. This mechanism responds to an active transport process.

Primary Conveyors (pumps)

The pumps carry out the transport of ions and molecules to the intracellular and extracellular media, using a primary active transport mechanism.

That is, they use the energy from ATP hydrolysis to make the “uphill movement of ions and molecules” become an energetically favorable process.

One of the functions associated with this type of transporter is the generation of the internal acidic medium characteristic of the lysosomes of animal cells, of the vacuoles of plant cells and of the stomach lumen.

Secondary active transporters

These transporters take advantage of the energy released during the cotransport of an ion in favor of its electrochemical gradient to be able to transport another molecule against its concentration gradient. In other words, they carry out the secondary active transport of molecules.

References

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