Primary Active Transport

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This illustration shows the sodium-potassium pump. Initially, the pumps opening faces the cytoplasm, where three sodium ions bind to it. The antiporter hydrolyzes and A T P to A D P and, as a result, undergoes a conformational change. The sodium ions are released into the extracellular space. Two potassium ions from the extracellular space now bind the antiporter, which changes conformation again, releasing the potassium ions into the cytoplasm.
Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport). (credit: modification of work by Mariana Ruiz Villareal)

OpenStax Biology 2e

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.

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. The process consists of the following six steps.

  1. With the enzyme oriented towards the cell’s interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
  2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.
  3. As a result, the carrier changes shape and reorients itself towards the membrane’s exterior. 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 cell’s interior.
  6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

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.

– What is a type of integral membrane protein that spans the entirety of the cell membrane?

The liver makes and recycles bile acids as an integral part of biliary secretion. Primary bile acids are synthesized from cholesterol in hepatocytes. The activity of a microsomal cholesterol 7 hydroxylase determines the rate of formation of the primary bile acids, and this enzyme is inhibited by bile acids. Thus, the synthesis of the bile acids is under negative feedback control. This degradation of cholesterol is the largest metabolic sink for cholesterol in the body. The primary bile acids are secreted into the bile by a primary active transport mechanism.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

https://www.sciencedirect.com/book/9780123821638/quantitative-human-physiology


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