4.4 Active Membrane Transport

Active transport mechanisms require the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the substance’s concentration inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Electrochemical Gradient

We have discussed simple concentration gradients—a substance’s differential concentrations across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than the extracellular fluid. Thus in a living cell, the concentration gradient of Na+ tends to drive it into the cell, and its electrical gradient (a positive ion) also drives it inward to the negatively charged interior. However, the situation is more complex for other elements such as potassium. The electrical gradient of K+, a positive ion, also drives it into the cell, but the concentration gradient of K+ drives K+ out of the cell (Figure 1). We call the combined concentration gradient and electrical charge that affects an ion its electrochemical gradient.

Figure 1  Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. Structures labeled A represent proteins. (credit: “Synaptitude”/Wikimedia Commons)

Moving against a gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy comes from ATP generated through the cell’s metabolism. Active transport mechanisms, or pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face of these passive movements. A cell may spend much of its metabolic energy supply maintaining these processes. (A red blood cell uses most of its metabolic energy to maintain the imbalance between exterior and interior sodium and potassium levels that the cell requires.) Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.

Two mechanisms exist for transporting small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.

Carrier proteins for active transport

An important membrane adaptation for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three protein types or transporters (Figure 2). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na+-K+ ATPase, which carries sodium and potassium ions, and H+-K+ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Figure  2  A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. For these carrier proteins to perform active transport, they must also hydrolyze ATP into ADP and phosphate. The energy released in this process will drive these carriers to drive ions and small molecules against their concentration gradients (credit: modification of work by “Lupask”/Wikimedia Commons)

Primary Active Transport

The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still active because it depends on using energy as does primary transport (Figure 3).

Figure  3 Sodium-Potassium Pump The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

One of the most important pumps in animal cells is the sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells. The sodium-potassium pump moves K+ into the cell while moving Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in. The Na+-K+ ATPase exists in two forms, depending on its orientation to the cell’s interior or exterior and its affinity for either sodium or potassium ions.

Several things have happened as a result of this process. At this point, there are more sodium ions outside the cell than inside and more potassium ions inside than out. For every three sodium ions that move out, two potassium ions move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Secondary Active Transport

Secondary active transport uses the kinetic energy of the sodium ions to bring other compounds, against their concentration gradient into the cell. As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will move down its concentration gradient across the membrane. This movement transports other substances that must be attached to the same transport protein in order for the sodium ions to move across the membrane (Figure 4). Many amino acids, as well as glucose, enter a cell this way. This secondary process also stores high-energy hydrogen ions in the mitochondria of plant and animal cells in order to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy then converts ADP into ATP.

Figure  4   An electrochemical gradient (Na+ concentration – green), is generated by primary active transport. The energy stored in the Na+ gradient provides the energy to move other substances against their concentration gradients (Glucose – blue), a process called co-transport or secondary active transport. Credit: Rao, A., Ryan, K., Tag, A. and Fletcher, S. Department of Biology, Texas A&M University.

Figure Descriptions:

Figure 1. The image illustrates a cell membrane separating two regions labeled “Extracellular fluid” on the left and “Cytoplasm” on the right. The membrane is depicted as a double layer, with a central blue channel allowing for ion passage. Various ions are represented as colored shapes with labels: Na+ (sodium ions) as blue hexagons, Cl- (chloride ions) as yellow circles, K+ (potassium ions) as pink squares, and A- (anions) as orange squares. In the extracellular fluid, Na+ and Cl- ions predominate, resulting in a net positive charge. In the cytoplasm, K+ and A- ions dominate, contributing to a net negative charge. An arrow indicates ion movement from the extracellular fluid to the cytoplasm through the channel.

Figure 2. The image illustrates three different types of membrane transport proteins: a uniporter, symporter, and antiporter. Each transport type is represented by a colored rectangle embedded in a gray horizontal membrane. The uniporter on the left is shown in yellow with a single straight arrow labeled “A” passing directly downward through it, indicating one type of substrate moves through in one direction. The symporter in the middle is depicted in red with two curved arrows labeled “A” and “B” entering from above and moving downward through the membrane in the same direction, representing the simultaneous transport of two different substrates. The antiporter on the right is in blue, with two curved arrows labeled “A” and “B” crossing as they pass through the membrane in opposite directions, indicating the exchange of substrates.

Figure 3. The image illustrates a cross-section of a cell’s plasma membrane, depicting the sodium-potassium pump mechanism. The membrane is composed of a double layer of phospholipids, illustrated as brown lines with round, pink heads facing outward. Three large protein structures, depicted in brown color, are integrated into the membrane. Sodium ions, represented as orange hexagons marked “Na+”, and potassium ions, shown as yellow ovals marked “K+”, are seen on either side of the membrane. Arrows indicate the movement of ions: sodium ions are moving out of the cell, while potassium ions are moving into the cell. The sunburst symbol represents ATP conversion to ADP, with associated labels and phosphate. A color gradient bar on the right illustrates the concentration levels of Na+ and K+ with red indicating high concentration and light yellow indicating low concentration. Labels clearly identify different components of the image.

Figure 4. Coming soon