most animal cells membranes have proteins that pump Ions out of the cell and potassium ions into the cell

Ans.

The membrane proteins that pump sodium ions out of the cell from inside and potassium ions inside the cells are called as sodium-potassium pumps or sodium-potassium ATPases.

These proteins perform active transport by using energy from ATP hydrolysis and transport two potassium ions inside the cells, along with transport of three sodium ions outside the cells, against the concentration gradient.

Thus, the correct answer to complete the sentence is sodium.