One important feature of any intracellular signaling pathway is its ability to be turned off. Consider the pathway shown in Figure 16−28. Where would off switches be required? Which ones do you suppose would be the most important?

Figure 16−28

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LIGHT one rhodopsin molecule absorbs one photon 500 G protein (transducin) molecules are activated 500 cyclic GMP phosphodiesterase molecules are activated 105 cyclic GMP molecules are hydrolyzed 250 cation channels in the plasma membrane close 106-107 Na+ ions per second are prevented from entering the cell for a period of -1 second membrane potential is altered by 1 mV SIGNAL RELAYED TO BRAIN