Bio Tool #14 · DE Bio 101 Unit 5 · Campbell Chapter 5
A cell is covered in receptors, and which one a signal can use is decided before anything else happens — by whether the signal can get through the membrane at all. Pick a signal, send it, and run the pathway one stage at a time.
Nothing is submitted. The point is the three pathways side by side: how a signal is received, how far it gets amplified on the way, and what stops working when one step is blocked.
A row fills in once you have run that pathway to the end.