1.3.2: Carrier-mediated Transport

Carrier proteins in the cell membrane assist in two types of transport: facilitated diffusion and active transport. Carrier-mediated transport is characterized by distinctive attributes, which include stereospecificity, saturation, and competition.

Stereospecificity: Each carrier protein exhibits a specific affinity for binding particular solutes. An instance is the facilitated diffusion of D-glucose, the natural isomer, whereas L-glucose is not transported. Simple diffusion, conversely, would not differentiate between these isomers.

Saturation: The extent of transportation hinges on the quantity of available carrier molecules. The term transport maximum (Tmax) denotes the maximum solute amount transportable across a biological membrane at a given time, regardless of the solute's concentration gradient. As an illustration, Tm for glucose at the kidney tubule is 375 mg/dL.

Competition: Similar solutes compete for binding sites on carrier molecules. For example, galactose is a competitive inhibitor in transporting glucose across the wall of the small intestine.

Facilitated Diffusion

Facilitated diffusion is a passive process requiring no cellular energy expenditure. It is a mechanism employed for substances that cannot traverse the lipid bilayer due to their dimensions or polarity. This process might necessitate specialized channels for particular solutes like electrolytes to cross the membrane. For instance, despite the high extracellular concentration of sodium ions, these polarized electrolytes cannot penetrate the nonpolar lipid bilayer. Therefore, membrane proteins form sodium ion channels, enabling sodium ions to move along their concentration gradient from the extracellular space into the cells. Various other solutes, including amino acids, phosphate, and bicarbonate ions, rely on facilitated diffusion for cellular entry or exit.

The process of facilitated diffusion involves specific steps in the carrier protein to facilitate the movement of molecules across the cell membrane (see Figure 12).

Figure 12 | Facilitated Diffusion | ¹⁸ First, the carrier protein recognizes and binds to the specific molecule that needs to be transported. Upon binding, the carrier protein undergoes a conformational change in its structure, allowing it to encapsulate the bound molecule within its structure and shield it from the hydrophobic environment of the lipid bilayer. The protein-molecule complex enables the movement of the molecule from one side of the membrane to the other, where it undergoes another conformational change that releases the transported molecule into the intracellular environment.

The facilitation by carrier proteins allows for a higher molecular transport rate than simple diffusion, especially when the concentration gradient is steep. However, as the concentration gradient decreases and approaches equilibrium, the rate of facilitated diffusion may reach a plateau as the carriers become saturated or all available binding sites on the proteins are occupied. Vmax (maximum velocity) is the maximum rate of transport achieved when all the carrier proteins (transporters) are saturated with the substrate. At this point, increasing the substrate concentration further does not increase the rate of transport because all available carriers are already engaged ¹⁹.

Tmax specifically refers to the maximum rate of substance transport across a membrane and depends on factors like the number of available carrier proteins, their activity, and the physiological conditions. Therefore, Tmax is equivalent to Vmax in the context of carrier-mediated transport. At Tmax, the transport rate becomes constant, and further increases in concentration do not result in a corresponding rise in the rate of facilitated diffusion. In the example of glucose transport, its size and polarity prohibit simple diffusion across the lipid bilayer. A carrier protein, known as the glucose transporter (GLUT), facilitates the entry of glucose molecules by transporting them into the cell. The number of GLUT molecules influences the amount of transported glucose.

While both simple and facilitated diffusion are passive processes that do not require energy input, facilitated diffusion involves specific carrier proteins or channels to enable the movement of certain molecules that cannot freely cross the membrane. In contrast to simple diffusion, where the rate escalates proportionally with the concentration of diffusing substances, facilitated diffusion demonstrates a maximum rate known as Vmax (Figure 13, Table 1).

Figure 13 | Comparison of Simple Diffusion and Facilitated Diffusion | The graph illustrates that facilitated diffusion attains a maximum rate, referred to as Vmax, while simple diffusion persists as long as a concentration gradient exists.

Table 1: Comparison of Simple and Facilitated Diffusion

	Simple Diffusion	Facilitated Diffusion
Carrier proteins	Molecules move directly through the lipid bilayer of the cell membrane. No carrier proteins required.	Molecules move through specific protein channels or carrier proteins embedded in the cell membrane.
Direction of transport based on concentration gradient	High to low concentration	High to low concentration
Examples	Oxygen, carbon dioxide, and lipids (nonpolar substances)	Glucose, amino acids, and ions (polar molecules)
Energy Requirement	No energy input required	No energy input required
Diffusion Rate	Diffusion rate depends on concentration gradient, permeability, temperature, and molecular size.	Diffusion rate depends on the number of available carrier proteins. May reach a saturation point.
Specificity	Not highly specific	Often highly specific. Carrier proteins have selective binding sites.
Vmax	None	Represents the maximum rate of transport when carrier proteins are saturated.

Active Transport

Active transport, also called primary active transport, refers to a cellular transport process necessitating ATP to move a substance across a membrane against its concentration gradient, aided by protein carriers. In this mechanism, specific integral membrane proteins called pumps move molecules across the membrane. These pumps use energy from the hydrolysis of ATP to transfer ions or other molecules against their concentration gradient. One prominent example is the sodium-potassium pump or sodium-potassium ATPase, which actively transports three sodium ions out of the cell while simultaneously bringing two potassium ions into the cell (Figure 14). Calcium ATPase or calcium pump within the sarcoplasmic reticulum of muscle fibers, the hydrogen ATPase located in the plasma membrane, and the hydrogen-potassium ATPase (protein pump) within the parietal cells of the stomach are also examples of primary active transporters. Primary active transport is integral to maintaining proper cellular functions and generating the electrochemical gradients necessary for secondary active transport and other physiological processes.

Figure 14 | Sodium-Potassium Pump | ²⁰ The pump starts in an open conformation with three sodium ions binding to the pump's cytoplasmic side. The binding of sodium ions stimulates the phosphorylation of the pump by transferring a phosphate group from ATP to the pump. The pump undergoes a conformational change upon phosphorylation, exposing sodium ions to the extracellular side. Sodium ions are released into the extracellular fluid. Two potassium ions from the extracellular fluid bind to the pump with the phosphorylated pump in its new conformation. The pump is dephosphorylated, reverting to its original conformation, which triggers the release of potassium ions into the cytoplasm, and the cycle repeats

Ouabain, also referred to as digitalis, is a plant-derived glycoside. It is often discussed in the context of the sodium-potassium pump due to its significant impact on the pump's function and the physiological processes it influences. It functions as an irreversible inhibitor of sodium-potassium pumps ²¹. When the sodium-potassium pump ceases its operation, an increase in intracellular sodium levels and a subsequent rise in intracellular calcium levels occur. Elevated intracellular calcium levels, in turn, can lead to enhanced contractility of cardiac muscle fibers, which is why ouabain and its derivatives have historically been used in treating certain heart conditions, such as congestive heart failure and atrial fibrillation. The use of ouabain as a therapeutic agent has diminished over time, however, due to potential side effects and the development of more targeted treatments.

Secondary active transport permits the movement of multiple solutes, with one solute (typically sodium ion) transported downhill, furnishing the energy needed for the uphill transport of other solutes. This energy is not supplied directly through metabolic means but is indirectly from the sodium ion gradient upheld across cell membranes by the sodium-potassium ATPase ( Figure 15).

Figure 15 | Cotransport | On the left side of the illustration, the cotransport mechanism involves the simultaneous movement of sodium ions (\(\ce{Na+}\)) and glucose molecules across the cell membrane. The arrows indicate the direction of movement, highlighting the coordinated influx of sodium ions and glucose. The sodium and potassium exchange mechanism is portrayed on the right side. \(\ce{Na+}\) are actively transported out of the cell, while potassium ions (\(\ce{K+}\)) are transported into the cell.

Symport is a membrane transport mechanism in which two or more substances are transported across a biological membrane in the same direction and simultaneously. In symport, the movement of these substances is coupled, meaning they are transported together through a carrier protein or a channel protein. Symport may be a form of active transport, requiring energy (usually derived from ATP) to move substances against their concentration gradient. However, there are also symport mechanisms that operate through facilitated diffusion, which is a passive process. A typical example of symport is the cotransport of sodium ions and glucose in the intestinal epithelial cells (SGLT1). In this case, both sodium ions and glucose are transported into the cell against their concentration gradients, and their movement is coupled through a symporter protein. Sodium-potassium-chloride cotransport occurs in the renal thick ascending limb.

Conversely, the movement of two or more solutes in opposing directions across membranes is called countertransport or antiport. One example is the sodium-calcium exchanger, also known as NCX, found in many types of cells, including cardiac muscle fibers and neurons, which allows three sodium ions into the cell in exchange for one calcium ion out of the cell, thus regulating the cellular concentration of calcium ions. Sodium-hydrogen exchanger, also known as NHE, is yet another example and is found in the cell membranes of many cells, including kidneys, gastrointestinal tract, and neurons regulating pH (Figure 16).

Figure 16 | Countertransport |

Table 2: Characteristics of Transport Across the Membrane

Transportation Type	Electrochemical Gradient	Carrier-mediated Transport	Metabolic Energy	Sodium Ion Gradient
Simple Diffusion	Downhill	No	No	No
Facilitated diffusion	Downhill	Yes	No	No
Primary active transport	Uphill	Yes	Yes
Cotransport	Uphill for the solute (not sodium ion) to be transported	Yes	Indirect	Yes, same direction
Countertransport	Uphill for the solute (not sodium ion) to be transported	Yes	Indirect	Yes, opposite direction