When a hormone molecule binds to a receptor on a target cell's surface, it doesn't directly reach inside the cell and cause whatever response the hormone is ultimately meant to trigger. Instead, that single binding event sets off signal transduction: a chemical relay race of internal reactions, each one activating the next, that carries and progressively amplifies the original signal deeper into the cell, until a single hormone molecule binding briefly at the cell's outer surface can end up producing thousands of individual chemical reactions occurring throughout the cell's interior.
Each relay step multiplies the number of molecules actually involved
A typical signal transduction cascade proceeds through several distinct relay steps, and at several of those steps, one activated molecule doesn't just pass the signal to a single next molecule — it activates many copies of the next molecule in the chain, each of which then goes on to activate many copies of the molecule after that, and so on through further steps. This compounding multiplication at each successive relay step is precisely what produces the enormous signal amplification a single receptor-binding event can trigger: a hormone present in only a tiny concentration outside the cell can still reliably produce a large, cell-wide response inside it, because the cascade itself is doing substantial multiplicative amplification work at each successive relay step along the way, not because the original hormone signal was ever large to begin with.
Second messengers carry the signal onward once the receptor's job is done
Many signal transduction pathways rely on molecules called second messengers — small molecules generated inside the cell once the original receptor is activated, which then diffuse onward and continue carrying and amplifying the signal deeper into the cell, well after the original hormone has finished its brief interaction with the surface receptor and, in many cases, has already detached from it. Because a single activated receptor can trigger the production of many second-messenger molecules, and each of those in turn can go on to activate still more downstream targets, second messengers function as a key additional multiplication stage within the broader amplification cascade, extending and further compounding a signal's total reach well beyond what direct binding to the original surface receptor alone could ever have achieved on its own.
What we're still unsure about
The general architecture of signal transduction cascades, including receptor activation, multi-step relay amplification and the role of second messengers, is well-established, extensively confirmed biochemistry. What remains a genuinely active area of ongoing research is mapping out the full, precise details of specific signalling pathways in specific cell types, since a real cell typically runs many different signal transduction pathways simultaneously, and these pathways can interact with, reinforce or inhibit each other in ways that aren't yet fully characterised for every relevant cell type and hormone combination — a genuinely large, still-expanding area of active biochemical and cell-biology research rather than a fully closed catalogue.
This sits inside Signal Transduction, one of seven topics in Biochemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.