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A signal molecule that cannot cross the plasma membrane must

  • A.be broken down before it can act
  • B.enter through a nuclear pore complex
  • C.be converted into a steroid first
  • D.bind a receptor on the cell surface

Answer: D

The receptor spans the membrane and relays the message inward without the signal itself entering. This is why the external molecule is called the first messenger and the internal relay the second. The message is thus handed across the membrane rather than carried through it.

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