A signal reaches a cell mostly in the form of a signalling molecule binding to a receptor on the cell surface. This binding activates a chain of events that amplifies the signal and transfers it inside the cell. Protein kinases phosphorylate proteins and cause their activation or deactivation, or in general modify their activity. When a signalling molecule binds to a receptor, a cascade is activated and the second messengers, such as cAMP, are synthesized. cAMP molecules activate the protein kinases, which phosphorylate specific proteins and activate them.
There were two approaches for the planetary paths. The idea about the planets revolving around the Earth, the planetary paths were plotted and were erratic. It was not relevant to the laws of motion and gravity. That fact was neglected by the believers in an Earth-centered Solar System.
Sun-centered Solar System was theorized by Copernicus. The paths assumed to be relatively circular (or elliptical).
