- To sustain life, many substances need to be transported into, out of, and between cells.
- In some cases, this can be accomplished through passive transport, which uses no energy.
- In many cases, however, the cell needs to transport something against its concentration gradient. In these cases, active transport is required.
- 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.
- Active transport uses specific transport proteins, called pumps, which use metabolic energy (ATP) to move ions or molecules against their concentration gradient.
- For example, in both vertebrates and invertebrates, the concentration of sodium ion is about 10 to 20 times higher in the blood than within the cell. The concentration of the potassium ion is the reverse, generally 20 to 40 times higher inside the cell. Such a low sodium concentration inside the cell is maintained by the sodium-potassium pump.
- There are different types of pumps for the different types of ions or molecules such as calcium pump, proton pump, etc.
Features of Active Transport
- During active transport, molecules move from an area of low concentration to an area of high concentration.
- This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient.
- Active transport is called “active” because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport.
- As molecules are moving against their concentration gradients, active transport cannot occur without assistance.
- It requires a transmembrane protein or protein complex called a transporter, which coordinates the entire process, and an energy source like ATP.
- Each type of transport protein, which is designed to transport a specific ion or nutrient into the cell, binds a molecule of its substrate on one side of the membrane, then changes shape and releases the substrate on the other side.
- These proteins are often called “pumps” because they use energy to pump the molecules across the membrane.
Types of Active Transport
Primary active transport
- Primary active transport is also called direct active transport or uniport.
- It involves using energy (usually ATP) to directly pump a solute across a membrane against its electrochemical gradient.
- Substances that are transported across the cell membrane by primary active transport include metal ions, such as Na+, K+, Mg2+, and Ca2+.
- These charged particles require ion pumps or ion channels to cross membranes and distribute through the body.
- Based on the transport mechanism as well as genetic and structural homology, there are considered four classes of ATP-dependent ion pumps:
- P-class pumps
- F-class pumps
- V-class pumps
- ABC superfamily
The P-, F- and V-classes only transport ions, while the ABC superfamily also transports small molecules.
- Most of the enzymes that perform this type of transport are transmembrane ATPases.
- The most studied example of primary active transport is the plasma membrane Na+,K+-ATPase. Other familiar examples of primary active transport are the redox H+-gradient generating system of mitochondria, the light-driven H+-gradient generating system of photosynthetic thylakoid membranes, and the ATP-driven acid (H+) pump found in the epithelial lining of the stomach.
Secondary active transport
- In secondary active transport, also known as coupled transport or cotransport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, it relies upon the electrochemical potential difference created by pumping ions in/out of the cell.
- Secondary active transport moves multiple molecules across the membrane, powering the uphill movement of one molecule. One molecule helps set up the needed gradient to allow for the movement of many chemicals into and out of the cell.
- The energy to produce uphill transport of one solute is derived from the potential energy of a different solute running down its concentration gradient.
- The energy derived from the pumping of protons across a cell membrane is frequently used as the energy source in secondary active transport.
- In humans, sodium (Na+) is a commonly co-transported ion across the plasma membrane, whose electrochemical gradient is then used to power the active transport of a second ion or molecule against its gradient. In bacteria and small yeast cells, a commonly co-transported ion is hydrogen.
- Sodium-calcium exchanger, SGLT2
Carrier Proteins for Active Transport
- An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement.
- There are three types of these proteins or transporters: uniporters, symporters, and antiporters.
- 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 found 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 protein pumps are Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively.
Significance of Active Transport
- Active transport of solutes across biological membranes driven by electrochemical gradients (i.e., secondary active transport) plays a central role in fundamental cellular processes, such as nutrient uptake, excretion of toxic compounds, and signal transduction.
- Active transport is among the most common methods used for the uptake of nutrients such as certain sugars, most amino acids, organic acids, and many inorganic ions by unicellular organisms.
- Secondary active transport is involved in transportation of a diverse range of molecules, such as ions, nutrients, vitamins, and osmolytes in higher organisms.
- Active transport permits the efficient absorption of substances vital for cellular function (and certain drugs that resemble them structurally) and the selective elimination of waste products and foreign chemicals, including many drugs.
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. New York: Garland Science.
- Koolman, J., & Röhm, K.-H. (2005). Color atlas of biochemistry. Stuttgart: Thieme.