Active Transport
Life cannot exist under equilibrium conditions (page 93).
Nowhere is this more apparent than in the imbalance of oppositely
disposed—the Na concentration is about 150 mM outside the cell and 10–20 mM inside the cell.
The concentration difference for Ca2 is even greater; the typical cytosolic concentration of 107 M is 10,000 times less than that outside the cell.
The ability of a cell to generate such steep concentration gradients across its plasma membrane
cannot occur by either simple or facilitated diffusion.
Rather, these gradients must be generated by active transport.