including the propagation of electri- 
cal signals among nerve cells, can 
be performed. 
Another very important function of 
the sodium-potassium pump is to 
keep the total number of ions on both 
sides of the membrane approximately 
equal, thereby preventing the cell 
from swelling or shrinking. Ouabain, 
a crystalline substance derived from 
the seeds of an African shrub, dis- 
rupts the sodium-potassium pump and 
causes cells to swell and burst. 
African hunters use it in large 
amounts to make poisoned darts, 
and doctors sometimes prescribe it in 
small amounts as a heart stimulant. 
This schematic drawing shows how 
the sodium-potassium ion protein 
pump in the cell's membrane forces 
sodium out of the cell, while pulling 
Direction of in potassium. 
Sodium ion outside cell ft potassium movement 
50 
