3 Important Types of Active Transport (Explained With Diagram)

During diffusion (passive or facilitated), substances pass through the plasma membrane until some sort of equilibrium is achieved.

The equilibrium may be of the Gibbs-Donnan variety or may be a simple concentra­tion equilibrium. Both involve interplay between the concentrations of soluble solute inside and outside the cell.

Cells can also accumulate solutes in quanti­ties far in excess of that expected by any of the above mechanisms if the solute is rendered insoluble once it has entered the cell, because insoluble materials do not contribute to concentration gradients.

Alterna­tively, once inside the cell, a solute may enter a meta­bolic pathway and be chemically altered, thereby re­ducing the concentration of that particular solute and allowing additional solute permeation.

In all the cases we have so far considered, solute passage through the membrane hinges on the presence of a concentration gradient, with the solute moving in the direction of the gradient.

Substances can also move through the plasma mem­brane into or out of the cell against a concentration gradient. This requires the expenditure of energy on the part of the cell and is called active transport.

Ac­tive transport ceases when cells are:

(1) Cooled to very low temperatures (such as 2-4 °C),

(2) Treated with metabolic poisons such as cyanide or iodoacetic acid, or

(3) Deprived of a source of energy.

The best under­stood and most exhaustively studied cases of active transport arc those that involve the movements of so­dium and potassium ions across the plasma mem­branes of erythrocyes, nerve cells, and Nitella cells and that result in an ionic concentration gradient across the cell membrane. The mechanism that estab­lishes and maintains these gradients appears to be basically similar in all of these cells and can be illustrated with the erythrocyte.

1. The Na⁺/K⁺ Exchange Pump:

The cytoplasm of the erythrocyte contains 0.150 M K⁺, whereas the surrounding blood plasma contains only 0.005 M K⁺. In contrast, the erythrocyte con­tains only 0.030 M Na⁺ and the plasma contains 0.144 M Na⁺. Hence, marked K and Na⁺ concentration gradients exist across the cell membrane. Utilizing radioactive isotopes of Na and K clearly established that these ions are permeable to the erythroctye membrane and are constantly diffusing through it.

Yet, in spite of this permeability, Na⁺ and K⁺ concentration gradients across the membrane are maintained. The gradients are maintained because sodium ions diffusing into the cell from the plasma under the influence of the con­centration gradient are transported outward again, and potassium ions diffusing out of the cell are re­placed by the inward transport of K⁺ from the plasma.

That these movements involve active meta­bolic processes is clearly demonstrated when the tem­perature of a blood sample is reduced from 37 °C (nor­mal human blood temperature) to 4°C, when cyanide is added to the blood, or when plasma glucose con­sumed during erythrocyte metabolism is not resupplied to the blood sample. Under these conditions, cell metabolism is interrupted and is followed by the in­ward diffusion of Na⁺ and the outward diffusion of K ⁺ until the ionic concentrations on both sides of the erythrocyte membrane are in a passive equilibrium.

In the case of red blood cells and nerve cells, the ac­tive transport of Na⁺ and K⁺ appears to be linked, that is, the mechanism responsible for the outward transport of Na⁺ simultaneously transports K⁺ in­ward. An enzyme isolated from nerve cell membranes and believed to be involved in Na⁺ and K⁺ transport has been shown to have two sites that bind one or more of each of these cations. The enzyme is believed to be an integral protein spanning the lipid bilayer.

Active transport of Na⁺ and K⁺ through the mem­branes of nerve cells and erythrocytes requires ATP, and ATP cannot be replaced by other nucleoside tri­phosphates such as GTP, UTP, and ITP. ATP is con­verted to ADP during active transport by a membrane-bound Na⁺ and K⁺ stimulated ATPase. This enzyme and that involved in the transport of Na⁺ and K⁺ may be one and the same. The membranes of cells from many other mammalian tissues seem to possess a similar ATPase activity.

Two K⁺ and three Na⁺ are transported through the membrane for each molecule of ATP dephosphorylated. Transport of Na⁺ and K⁺ through the plasma membrane is believed to occur in the following stages (see Fig. 15-40). Three sodium ions and one molecule of ATP inside the cell are bound to specific sites on the enzyme carrier, while two potassium ions are bound to a site on the same enzyme facing the ex­terior of the cell.

Binding of the substrates results in and is followed by a change in the tertiary structure of the carrier molecule such that the bound sodium and potassium ions are “translocated” across the mem­brane. It is presumed that at some stage during this process, ATP is split, releasing ADP. Translocation is followed by an alteration of the binding sites such that the sodium ions are “released” outside the cell, while the potassium ions are released inside the cell.

Once the ions are released, the carrier undergoes another change in structure, priming it for another round of the transport cycle. This stage, called “recovery,” is accompanied by the release of inorganic phosphate. Although this model is widely accepted, it has also been suggested that the enzyme site that binds Na⁺ on the inside of the cell binds K⁺ oh the outside fol­lowing translocation, while the site that initially binds K⁺ on the outside binds Na⁺ on the inside following translocation. In this manner, the recovery phase would result in an additional movement of ions through the membrane and would be more efficient.

2. Cotransport:

Amino acids, sugars, and other metabolites are also actively transported through the plasma membrane into the cell. In many cells, the transport of these me­tabolites is coupled to the movements of sodium ions, as shown in Figure 15-41. The Na⁺7K⁺ exchange pump creates a steep concentration gradient across the plasma membrane favoring the inward diffusion of Na⁺.

Indeed for every two K⁺ pumped into the cell, three Na⁺ are pumped out. Carrier proteins in the membranes bind both Na⁺ and the metabolite, follow­ing which a change in the carrier’s structure brings both substrates to the cell interior, where they are re­leased. Release of the Na⁺ internally is followed by its active extrusion back through the membrane.

The lat­ter event is coupled to ATP hydrolysis and results in the maintenance of the steep Na⁺ gradient. In a sense, the steep Na⁺ gradient acts as the driving force for the inward transport of metabolites, and the simultaneous movements of Na⁺ together with me­tabolites into the cell constitute cotransport.

Because the ATP-dependent Na⁺/K⁺ pump pumps three Na⁺ for every two K⁺ an electrical gradient is created across the membrane. For this reason, the Na⁺/K⁺ exchange pump is called an electrogenic pump. The potential energy of an electrogenic pump is coupled to ATP synthesis in mi­tochondria.

3. “Simple” Active Transport:

The passage of some substances through membranes against a concentration gradient is unidirectional but not coupled to ionic movement even though ATP is consumed in the process. Such movement is called simple active transport. The carrier enzyme cycli­cally binds the solute at one membrane surface and re­leases it at the other. The cycle is accompanied at some point by the hydrolysis of ATP.