Under what condition will a target cell always respond quickly to an extracellular signal?

• A. When the signal requires transcription of new genes.
• B. When the signal involves a complex cascade of intracellular signaling pathways.
• C. When the signal is a small, hydrophobic molecule that can diffuse through the cell membrane.
• D. When the signal activates a receptor that directly opens an ion channel.

Answer: (D)

Verified step by step guidance

1. Understand the nature of the signal: A target cell responds quickly when the signal can directly affect cellular processes without requiring complex or time-consuming steps.
2. Consider the types of signals: Signals that require transcription of new genes or involve complex intracellular pathways typically take longer because they involve multiple steps and processes.
3. Identify the characteristics of the signal: Small, hydrophobic molecules can diffuse through the cell membrane quickly, but they may not always lead to an immediate response if they require further intracellular processing.
4. Focus on ion channels: Signals that activate receptors directly linked to ion channels can cause immediate changes in the cell's membrane potential, leading to a rapid response.
5. Conclude with the mechanism: The quickest response occurs when a signal activates a receptor that directly opens an ion channel, allowing ions to flow across the membrane and rapidly alter cellular activity.