What would a control be for an experiment to see whether fertilizer a plant grow ?

The control would be a  plant grown without any fertilizer.  This is called  a negative control and is important in identifying the influence  of the treatments on the tests. In this case, it would mean that the plant without fertilizer (control) is not expected to grow as large as in the other treatment.

The answer is constant.

Molecules called lipids have long hydrocarbon chains that determine the way they act. they can be fats, oils, or hormones, and even exist in our cell membranes