'A' is a hormone such as FSH that cannot cross lipid bilayer therefore it interacts with the membrane-bound receptors and does not enter the target cell, but generates second messengers. Peptide, polypeptide, protein hormones and catecholamines act through this mechanism.

'B' represents the extracellular receptors present on the cell surface that forms a hormone-receptor complex which brings about conformational changes in the cytoplasmic part of the receptor. This cytoplasmic part can produces second messengers such as Ca +2 , cAMP, IP 3 etc, which activates the existing enzyme system of the cell and accelerates the biochemical reactions in the cell. 'C' represents the second messenger.