responses is to "switch off" the G 
protein, causing the message trans- 
mittal to stop. 
The cell appears to employ this 
complex signaling system because it 
increases both the efficiency and 
speed of message transmission. A 
single incoming messenger molecule 
triggers a cascade of reactions that 
eventually results in a large amplifica- 
tion of the original message. Further- 
more, the time elapsed between the 
arrival of a signal at a G protein and 
a cellular response is often only a 
few fractions of a second. For 
example, light-sensitive eye cells 
respond to as little as one photon of 
light in just a few milliseconds 
through a second messenger system. 
In contrast, other cells take as iong 
as 30 seconds to respond to signals 
from the environment. 
Certain diseases impair the function- 
ing of the second messenger system, 
causing profound cellular malfunction. 
A toxin produced by the organism that 
causes cholera, for example, "locks" 
the G proteins of intestinal cells into the 
"on" position so that they are constantly 
stimulating the production of adenylate 
cyclase. This causes the cells to ex- 
crete vast amounts of fluid, accounting 
for the often-fatal diarrhea associated 
with cholera. In addition, because cell 
division is coordinated by a second 
messenger system, many researchers 
are now attempting to discern how a 
breakdown in this system might allow 
a cell to become cancerous. 
