Which statement best describes the all-or-nothing principle in cell signaling?

• A. A cell's response to a stimulus is proportional to the strength of the signal.
• B. A cell either fully responds to a stimulus once a threshold is reached, or does not respond at all.
• C. Cells can partially respond to weak signals and fully respond to strong signals.
• D. The response of a cell to a signal is always gradual and never abrupt.

Answer: (B)

Verified step by step guidance

1. Understand the concept of the all-or-nothing principle in cell signaling. This principle states that a cell's response to a stimulus is binary: it either fully responds once a specific threshold is reached or does not respond at all. There is no partial or gradual response.
2. Analyze the first option: 'A cell's response to a stimulus is proportional to the strength of the signal.' This suggests a gradual response, which contradicts the all-or-nothing principle. Therefore, this option is incorrect.
3. Evaluate the second option: 'A cell either fully responds to a stimulus once a threshold is reached, or does not respond at all.' This aligns perfectly with the definition of the all-or-nothing principle, making it a strong candidate for the correct answer.
4. Consider the third option: 'Cells can partially respond to weak signals and fully respond to strong signals.' This implies a graded response, which is inconsistent with the all-or-nothing principle. Thus, this option is incorrect.
5. Review the fourth option: 'The response of a cell to a signal is always gradual and never abrupt.' This directly contradicts the all-or-nothing principle, as the principle describes an abrupt, binary response. Therefore, this option is also incorrect.