Signaling molecules can be "tagged" by attaching glow-in-the-dark molecules to them.After adding tagged signaling molecules to the fluid around several cells,you observe the cells with a microscope designed to see the glow-in-the-dark molecules.After a few minutes,you notice that the plasma membrane of some of the cells begins to glow.This observation indicates that some cells

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The glow-in-the-dark molecules are attached to the signaling molecules, allowing them to be detected by the microscope. If the plasma membrane of some cells begins to glow, it indicates that the cells have receptors for the tagged signaling molecule and are responding to it. This would not be possible if the cells had no active carrier proteins or were blocking the signaling molecules. The density of the plasma membrane is not relevant to this observation.

Signaling Molecules Can Be "Tagged" by Attaching Glow-In-The-Dark Molecules to Them.After