Signal transduction pathways involve intracellular events stimulated by an extracellular signal.

• A. They occur only in prokaryotes.
• B. They involve intracellular events stimulated by an extracellular signal.
• C. They do not involve second messengers.
• D. They are limited to changes in membrane potential with no kinase activity.

Answer: (C)

Signal transduction pathways translate an external signal into intracellular actions. When a signal molecule binds a receptor, the message is passed inside the cell often via second messengers such as cyclic AMP, Ca2+, IP3, and DAG. These small molecules amplify the signal and help activate downstream targets, like protein kinases, which then modify other proteins to change cellular behavior. This is why second messengers are a hallmark of many signaling cascades.

The idea that these pathways do not involve second messengers doesn’t fit the pattern of how these signals typically work. In addition to second messengers, signaling often involves kinase cascades and can lead to changes in gene expression or metabolism. It’s also not limited to prokaryotes or to changes in membrane potential; kinase activity and second messengers are common features across many eukaryotic signaling pathways.