Research has shown that olfactory receptors, used to detect and recognize many different aromatic chemicals by smell, are G-coupled protein receptors. Humans have the ability to detect smells of chemicals in very low concentration, some less than 1 ppm (parts per million) in the air. Describe how G-coupled protein receptors work, and then explain the characteristic of G-coupled protein receptors that allow olfactory (smell) receptors to detect smells in such low concentration.