The term uniport is used in biology to describe the transport of individual molecules in a single direction across a cell membrane and down its concentration gradient (facilitated transport).
This type of transport through membranes, which imposes a selective permeability barrier, supposes the maintenance of a more or less constant intracellular environment, which allows the establishment of many cellular functions that depend on fine molecular and energy balances.
Communication between cells, as well as between cells and the environment that surrounds them, is an essential process for the life of all organisms and depends, to a great extent, on a group of transmembrane proteins known as “transporter proteins”.
These proteins are responsible for transporting those substances that, due to their chemical nature, cannot easily cross membranes, such as ions and water-soluble molecules such as amino acids and glucose.
It should be noted that the transport of this type of molecules from or to the outside of the cell, or from the cytosol to the lumen of an organelle, is mediated by specific transporter proteins, capable of “recognizing” or identifying the substrate they must transport.
Transport across membranes
Some authors consider that there are three types of transporters in cell membranes: pumps, channel proteins, and transporter proteins.
The pumps are proteins that transport small molecules against their concentration gradients or their electrical potential, and make use of the energetic force coming from the hydrolysis of ATP (they are ATPases). These proteins perform what is called “active transport”, since it requires energy.
– Channel proteins
Channel proteins facilitate the transport of different ions and water in favor of their concentration gradient or their electrical potential. They consist of “ducts” formed by proteins that cross the membrane in its entire thickness, through which the molecules travel at high speed.
There are channel proteins that are permanently open, while others can be closed, opening up to special stimuli.
– Transport proteins
The carrier proteins are a class of proteins that facilitates the movement of many ions and molecules across biological membranes.
These proteins interact directly with the substrates they transport and this interaction generates conformational changes in their structure, so that the transport is delicately selective and slower than the other two types described.
Types of carrier proteins
In the scientific literature, it is common to find texts that refer to three types of transporter proteins: symporters, anti-carriers and uni-carriers.
The symport and antisport has to do with the simultaneous movement of two molecules. This couples the movement of one of them against its concentration gradient or electric potential with the movement of the other (or more) in favor of its gradient (usually ions).
Specifically, symport collaborates with the transport of two molecules in the same direction, while anti-support involves the movement of a molecule in one direction and another in the opposite direction.
The uniport is the simplest class of membrane transport, since it consists of the transport of a single molecule at a time and in favor of its concentration gradient, so it can be said that it somehow facilitates simple diffusion.
Unicarrier proteins are, for example, those such as those that transfer sugars, amino acids and nucleotides from the outside to the inside of animal cells.
Some bacteria, plants, and lower eukaryotes possess representatives of a superfamily of carrier proteins, the members of which catalyze both unport, symport, and antport. This superfamily is known as the “major facilitator superfamily.”
Unicarrier proteins accelerate the movement of molecules from one side of the plasma membrane to the other.
This movement is energetically favorable, since the molecules are transported in favor of their concentration gradient, that is, from where there is “more” to where there is “less”. For this reason, uniport is often considered a type of facilitated diffusion or facilitated transport.
Some specific characteristics distinguish this type of transport:
– The speed of the passage of a molecule from one side to the other, in favor of its gradient through a unicarrier protein, is greater than that which would happen by simple diffusion.
– As with all transport catalyzed by transporters (including symport and antisport), uniport is specific, since each protein recognizes a particular molecule.
– Unlike simple diffusion, uniport occurs in special membrane sites (where the transporter proteins are found) and, as there is a limited number of proteins, it has a maximum speed, defined by the number of transporters and the concentration of the substrate being transported.
Unicarrier proteins, according to Woelfersberger (1994), can be classified as channels and as transporters or “carriers” .
As can be understood from the previous statement, channel proteins fall under the classification of unicarrier transporter proteins. These types of proteins are basically hydrophilic pores (related to water) that cross the membrane and through which water and other solutes can move by diffusion, since it occurs in favor of their concentration gradient.
The interior or lumen of each protein channel is organized in the membrane in such a way that it is accessible to either side of the membrane at the same time.
Transporters or carriers
Transporters or carriers are also transmembrane proteins that form a kind of duct through the entire thickness of cell membranes. However, although they have substrate binding sites on both sides of the membrane, they are not simultaneously exposed.
For this reason transporters can facilitate movement in both directions and also countertransport, whereas channel proteins cannot.
Among the most representative examples of the uniport is the case of glucose transport across the plasma membrane of mammalian cells. This transport is catalyzed by a group of proteins known as GLUT ( Glucose Transporters ).
They are transmembrane proteins composed of a peptide chain that crosses the plasma membrane at least 12 times, and that has binding sites for glucose both on the outside and inside.
This type of protein has two conformations, one when it is not bound to glucose and another when it is bound to it. The conformational changes in these proteins are reversible and random and depend on the binding of glucose.
In addition, they catalyze transport in both directions, depending on the concentration of glucose on one or the other side of the membrane.
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