Ignoring all other transmembrane channels or pumps, active Na-K pump proteins establish electrochemical gradients across cell membranes all by themselves. For potassium (K):
a) K+ ions are pumped out of the cell against their concentration gradient.
b) K+ ions are pumped into the cell against their concentration gradient.
c) K+ ions move freely through the membrane.
d) K+ ions are actively transported into the cell along their concentration gradient.