Transport of potassium ions into an animal cell by the sodium-potassium pump requires

Energy

Explanation:

Usually the inside of the cells is hypertonic to the extracellular matrix. This means there are more ions in the cell than outside. Therefore the cell requires energy to pump more ions into the cell against the concentration gradient. The sodium-potassium pump utilizes energy from ATP to pump Na+ and K+ ions 'uphill' into the cell.  

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