It's the membrane and this is the extracellular environment. So usually our primary messenger is some type of neurotransmitter or some type of hormone. In this case, the hormone is epinephrine. So basically what happens is we have the epinephrine, we have the transmembrane protein, our protein that basically goes on from the outside to the inside of the cell through the membrane shown in green. And on the inner portion of the membrane, on the cytoplasmic side of this transmembrane, known as beta adrenergic receptor we basically have a complex of other proteins. We have the alpha subunit, the beta subunit and the gamma subunit.