Cause living things can not directly use most of matter which they found in nature. For example carbon, all organism use carbon in complex structure, that is in compound or macromolecule(glucose,fructose,amino acid). we can not directly earn ATP from C which we found in soil/other sources.
Cannabinoid receptors, located throughout the body, are part of the Endocannabinoid system which is involved in a variety of physiological processes including appetite, pain-sensation, mood, and memory.[1]
Cannabinoid receptors are of a class of cell membrane receptors under the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid receptors contain seven transmembrane spanning domains.[5] Cannabinoid receptors are activated by three major groups of ligands: endocannabinoids, produced by the mammillary body; plant cannabinoids (such as Cannabidiol, produced by the cannabis plant); and synthetic cannabinoids (such as HU-210). All of the endocannabinoids and plant cannabinoids are lipophilic, such as fat soluble compounds.