Drug Targets (Receptors, channels, transporters, etc)
Most illustrations combine molecular symbols (usually ligands like agonists and antagonists) with their targets. Most targets are recognized by their 'pocket' that can be filled with an agonist (e.g. symbol for a general receptor).
There are some exceptions though, like the transporter (which merely 'transports' the ligand, thereby not 'activating' a cascade) or the lipoprotein receptor (which binds lipids, hence the 'pocket' is rounded and fits a lipid-symbol).