Final answer:
The statement is true, i.e., Option A. The binding of a first messenger like a hormone to its receptor does trigger conformational changes that initiate signal transduction pathways, leading to cellular responses.
Step-by-step explanation:
The statement that the binding of a first messenger to its receptor triggers a conformational change of the membrane-bound receptor, which ultimately leads to the cell's response, is True. So, the correct option among the given ones is Option A. When a first messenger, which is often a hormone, binds to its specific plasma membrane hormone receptor, it causes an allosteric change in the receptor's shape. This change may propagate through to the receptor's intracellular domain or to associated proteins, initiating a signal transduction pathway.
Sometimes, this includes the receptor triggering the activation of a G-protein on the cytoplasmic side of the membrane by converting GDP to GTP in one of the G-protein's subunits. This activation leads to a series of intracellular events, usually involving the generation of a second messenger in the cytoplasm, such as cAMP. The second messenger typically activates a protein kinase, starting a phosphorylation cascade that eventually reaches the nucleus and affects gene expression by activating transcription factors, which interact with DNA to modulate protein synthesis.