4.3 Active transport  (Page 2/18)

Two mechanisms exist for the transport of 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 describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.

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 ( [link] ). 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 proteins are Ca ²⁺ ATPase and H ⁺ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

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 considered active because it depends on the use of energy as does primary transport ( [link] ).

One of the most important pumps in animals 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 interior or exterior of the cell and its affinity for either sodium or potassium ions. The process consists of the following six steps.

1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
2. ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.