The problem: a message stuck outside the door
Most drug molecules — and most of the body's own signalling molecules — are too large or too water-loving to slip through the fatty cell membrane. So when a drug arrives, it usually has to deliver its message from outside the cell. The cell, meanwhile, does its real work inside. Signal transduction is the whole process of getting a message across that wall and turning it into a change in cell behaviour.
The translator is the receptor: a protein with one part facing outward to read the message and another part facing inward to act on it. When an agonist binds the outer part, the receptor changes shape, and that shape change is the message — now expressed in the cell's own internal language.
Relays and amplifiers
Inside the cell, the message rarely travels in a single jump. It passes down a signal cascade: receptor → relay protein → enzyme → small messenger → final action. Each step is a chance to amplify. One activated receptor can switch on many relay molecules; each of those can make hundreds of small messengers. A handful of drug molecules can therefore reshape a whole cell.
The small diffusible molecules that carry the message onward are called second messengers — the drug or hormone being the 'first' messenger. The protein that finally does the cell's bidding (an enzyme, a channel, a pump) is the effector. Hold onto these two words; the rest of the track is mostly about which messengers and which effectors a given receptor uses.
1 agonist molecule
binds 1 receptor
activates ~10 relay proteins (G proteins)
each switches on 1 enzyme
each enzyme makes ~100s of second messengers
messengers activate many effectors
=> tiny outside signal becomes a large inside response