The reason cells expend large amount of energy to export sodium ions from cells is because
a) sodium is poisonous for cells
b) sodium gradients are needed to make atp
c) sodium gradients can be used to cotransport items in and out of cells
d) sodium always brings a toxic chloride ion with it.
e) all the above are true

c. sodium gradients can be used to cotransport items in and out of cells.

Explanation:

The sodium-potassium pump results in energy stored through the movement of sodium out of the cell uphill its concentration gradient.  As sodium ions move back into the cell downhill their concentration, the energy released is used to transport glucose up its concentration gradient.

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