Hormone (amines and peptide or protein hormones) are water-soluble and act on the surface of target cells via second messengers; steroid hormones, being lipid-soluble, move through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei. Hormone secretion may occur in many tissues. Endocrine glands are the cardinal example, but specialized cells in various other organs also secrete hormones. Hormone secretion occurs in response to specific biochemical signals from a wide range of regulatory systems. For instance, serum calcium concentration affects parathyroid hormone synthesis; blood sugar (serum glucose concentration) affects insulin synthesis; and because the outputs
Glucose glucose themselves from certain precursors as the need arises. Nerve cells, cells of the renal medulla and erythrocytes depend on glucose for their energy production.<ref name="Löffler/Petrides 195">Peter C. Heinrich: "Löffler/Petrides Biochemie und Pathobiochemie." Springer-Verlag, 2014, , p. 195. (german)</ref> In adult humans, there are about 18 g of glucose, of which about 4 g are present in the blood. Approximately 180 to 220 g of glucose are produced in the liver of an adult in 24 hours. Many of the long-term complications of diabetes (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycation of proteins or