Final answer:
The first step of the Signal Transduction Pathway is the binding of a first messenger to a cell-surface receptor, initiating a signaling pathway that leads to a cellular response.
Step-by-step explanation:
The first step of the Signal Transduction Pathway (STP) involves the binding of a first messenger, typically a hormone, to a cell-surface receptor. This binding triggers a conformational change in the receptor, which then activates the receptor's intracellular components and initiates a series of downstream events, known as a signaling pathway or cascade. Throughout this signaling cascade, various molecules such as second messengers, enzymes, and phosphorylated proteins interact and propagate the signal, leading to cellular responses such as changes in metabolism or gene expression.