Final answer:
A ligand is the correct term for extracellular messenger molecules that bind to transmembrane receptors, thereby initiating signal transduction.
Step-by-step explanation:
The word used to describe extracellular messenger molecules that bind to transmembrane receptors is ligand. A ligand is a molecule that binds specifically to a receptor protein; it delivers a signal by binding to the receptor without necessarily entering the target cell. This interaction triggers a biochemical cascade that can alter cellular functions, including gene expression.
Cell-surface receptors, also known as transmembrane receptors, are integral proteins of the cell membrane that bind ligands. They perform signal transduction, where an extracellular signal is converted to an intracellular one, initiating a cellular response.
For example, when a ligand binds to a receptor, it can lead to changes in the cell such as turning off transcription of a gene as noted by the scientist in the given scenario.