Answer :
Using the theories of receptor, we got that ligand is the signal molecule that does not enter the cell in the binding of a signal molecule to a surface receptor.
Cell-surface receptors Cell-surface receptors are the membrane-anchored proteins that bind to ligands on outside surface of the cell. In this type of signaling, the ligand actually does not need to cross the plasma membrane.
In other cases, the receptor binds the DNA in either the presence or absence of hormone, but the hormone binding alters the activity of the receptor as the transcriptional regulatory molecule. For example, thyroid hormone receptor acts as the repressor in the absence of hormone.
Hence, in the binding of a signal molecule to a surface receptor, the signal molecule does not enter the cell is ligand.
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