membrane lipids are also 
highly impermeable to all 
electrically charged molecules, 
no matter how small they are. 
ATP-requiring protein “pumps” 
are employed to transport 
these particles, which are 
called ions. 
One well-studied pump sys- 
tem is the sodium-potassium 
pump. This membrane 
protein consumes more than 
a third of the cell s total ATP 
production in an endless 
cycle of pumping sodium ions 
out of the cell while drawing 
potassium ions in. The differ- 
ence in ion concentration 
inside and outside the cell 
creates a source of potential 
energy that can be used for a 
variety of tasks, including the 
propagation of electrical 
signals in nerve cells. 
47 
Sodium ion outside cell 
Direction of 
potassium 
movement 
i Cell surface membrane 
J 
Direction of 
sodium movement 
Potassium ion inside cell 
