Answer
All cell signalling starts when a signal (ligand) binds to a receptor somewhere in or on the cell. This leads to a range of further steps which amplify the signal (activation or deactivation of proteins by phosphorylation, production of second messengers, etc.) and lead to changes in what the cell does. These changes can be short term or long term, and gene expression is often the ultimate end of cell signalling.
Work Step by Step
Consider how each of the examples of cell signalling listed in the chapter occur. Note that there is always a signal (ligand) and a receptor of some kind, usually a protein. Then see that there are many steps possible after this point, but that always the cell changes what it is doing. Finally, note that gene expression generally is changed, whether the signalling pathway has many steps or whether the receptor (for example, for a steroid ligand) goes straight to the nucleus to bind to DNA.
