Got this great question about the sodium pump in Unit 4.4 Pharmacology from one of my tutees: -
"So, we know that the sodium pump actively moves potassium ions into the cell, and sodium ions out of the cell. If every transporter moves ions only one way, how come the sodium pump moves potassium in, and moves sodium out? Does it happen at the same time?"
This fabulous question nicely illustrates how -
As I said to my tutee, Mumtaz, have a look at this video, which explains the movement of potassium and sodium across the plasma membrane; each of the ions is moved in only one direction by the same transporter apparatus.
Sodium is transported out, and potassium is pumped in via a repeating cycle of shape changes in the pump. In each cycle, three sodium (Na+) ions are moved out of the cell, while two potassium (K+) ions enter.
The pump has an inward-facing form with high affinity for sodium (and low affinity for potassium, meaning there's a strong attraction for sodium, but low attraction for potassium); and, an outward-facing form with high affinity for potassium (and low affinity for sodium).
It toggles back and forth between these forms through the addition or removal of a phosphate group, which in turn is controlled by the binding of the ions to be transported across the plasma membrane.
After watching the video, test yourself with the MCQs that follow. Submit your responses to see the answers.
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Hi. I'm Dr Marie McKenzie-Mills, and I help ABPI exam candidates navigate the ins and outs needed to sail through this pharmaceutical industry rite of passage.
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