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.

Please share with your colleagues, if they’re also taking the ABPI exam.

Cheers,

Marie

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