Life is based on the universal principle of the SPECIFIC INTERACTION OF MOLECULES WITH EACH OTHER.   Interacting molecules interact as pairs, one of which is called the LIGAND and the other the RECEPTOR. These ligand/receptor interactions involve associations of atoms that attract one another so as to stick or bind the molecules together through a variety of weak bonds.

The specificity is determined by the SHAPE of the interactive regions on the ligand/receptor pairs that are similar to interactions between a LOCK and its KEY or the pieces of a puzzle.  Imagine a lock/key system that when the key is inserted into the lock it strongly sticks in place so it is difficult to remove. Imagine further, that the interaction between the lock/key cause one or both items to actually change their shape ("morph") and that in doing so some event is triggered to occur, like a bomb going off or a light going on or a door opening etc.

The cartoon above illustrates a number of basic ligand/receptor interactions that I have presented to you during this course.  If you apply this principle to every biological phenomenon described in this course, you will find that understanding them becomes relatively easy.  Further, you will find that your future understanding of biological phenomenon will present little problem for you.
 

Contribution of Dr. R.E. Hurlbert, WSU
via Sigma Co.