Chapter 9: Cell Membrane and Transport Mechanisms
9.3. Active Transport
Learning Objectives
By the end of this section, you will be able to:
- Understand how electrochemical gradients affect ions.
- Distinguish between primary active transport and secondary active transport.
Active transport mechanisms require the use of 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 concentration of the substance inside the cell must be greater than its concentration in the extracellular fluid, the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight material, such as ions, through the membrane.
In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles. This is referred to as bulk transport or vesicular transport (discussed in section 9.4.)
Electrochemical Gradient
We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because cells contain proteins, most of which are negatively charged, and because ions move into and out of cells, there is 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; at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than does the extracellular fluid. Thus, in a living cell, the concentration gradient and electrical gradient of Na+ promotes diffusion of the ion into the cell, and the electrical gradient of Na+ (a positive ion) tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K+ promotes diffusion of the ion into the cell, but the concentration gradient of K+ promotes diffusion out of the cell (Figure 9.3.1.). The combined gradient that affects an ion is called its electrochemical gradient, and it is especially important to muscle and nerve cells.
Moving Against a Gradient
To move substances against a concentration or an electrochemical gradient, the cell must use energy. This energy is harvested from ATP that is generated through cellular metabolism. Active transport mechanisms, collectively called pumps or carrier proteins, work against electrochemical gradients. With the exception of ions, small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive changes. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. Because active transport mechanisms depend on cellular metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.
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.
Primary active transport
The primary active transport uses ATP directly in the transport of the particles (Figure 9.3.2.).
membrane against their concentration gradients. The energy is provided by the hydrolysis of ATP. Three sodium ions are moved out of the cell for every 2
potassium ions that are brought into the cell. This creates an electrochemical gradient that is crucial for living cells. Credit: Rao, A., Ryan, K. and Fletcher,
S. Department of Biology, Texas A&M University.
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.
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
It 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 9.3.3.). Many amino acids, as well as glucose, enter a cell this way.
License and attributions:
- Concepts of Biology, 2013, Fowler, S. et al. License: CC BY 4.0. Located at https://openstax.org/books/concepts-biology/pages/3-6-active-transport
- Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/5-3-active-transport
