(i) What is termed as first messenger and second messenger in the target cells.
(ii) How do protein hormones acts on target cells?
(i) Hormone is called first messenger and cAMP is called second messenger.
(ii) Protein hormone is water soluble, it binds to the extrinsic receptor present on the cell surface of the plasma membrane. Hormones when binds to them activates, a membrane bound enzyme adenyl cyclase, which catalyses the conversion of ATP to cAMP. The cAMP then activates one more enzyme known as protein kinases. After phosphorylation of protien, alteration of its function occurs which thereby, leads to some metabolic effect.