Biosensors have some sort of receiving shape, and a transmitting shape. Upon binding with a molecule (either small like a hormone, or large like another protein) the biosensor will change shape to best bind this molecule. When this happens, a protein will do something - change shape, add a modification, bind another protein, change a molecule from A to B, etc. This will happen over and over again - often called a cascade effect. If you lump all the proteins together they make up a signal pathway. For example, an enzyme in our bodies must be modified for us to digest glucose. First, we eat - there are lots of things that happen so that we can start digesting the food (or drink), but eventually it gets broken down far enough that we can absorb small molecules - like glucose - into our blood stream. The levels of glucose rise and at some point, it triggers insulin to start the digestion process. This required glucose to bind to a biosensor, that biosensor to change & give off a signal, which in turn told insulin to start working.

Eventually a signal gets passed to an enzyme that will modify the enzyme that will digest glucose in step 1. Eventually, the products of each of the steps will digest glucose until we get energy - in the form of ATP (adenosine triphosphate).

Cells receive messages in the form of chemicals that get into the cell and activate chemical sensors located inside of the cell. These information transmission molecules can diffuse across the cell membrane, or they can be allowed in through special sites. Different kinds of messages use different molecules for transmission, and some cells are specific to some molecules.