G-protein primary physiological functions is to transmit signals from GPCRs, which serve as GEFs for G-proteins. GPCRs are induced into an active conformational state by interacting with exogenous or endogenous agonists, which alters intracellular G-protein protein binding.
When a ligand (a molecule with a high affinity for the receptor) attaches to a GPCR, the ligand causes a conformational change in the receptor's seven transmembrane region. The C-terminus is activated, which attracts a chemical, which activates the G protein linked with the GPCR.
Receptors are transmembrane proteins that attach to external signalling molecules and then relay the signal to internal signalling pathways via a series of chemical switches. Cellular receptors are signal-receiving proteins found both within and on the outside of cells. A chemical signal occurs when a protein ligand interacts to a protein receptor in normal physiology. A ligand is a chemical messenger that is released by one cell to communicate with another cell.
Thus, G protein, ligand and cell surface receptor works in various ways in cell signalling pathways.