Insulin is synthesized as a preprohormone in the beta cells of the islets of Langerhans. Its signal peptide is removed in the cisternae of the endoplasmic reticulum and then packaged into secretory vesicles in the Golgi. It is folded in its native structure and locked in this conformation by, the formation of 2 disulfide bonds.
In normal individual, insulin is produced by the body in response to the rise in blood glucose level. Apart from it, spurts of insulin are produced throughout the day and night, to look after the body resting needs for insulin and ensure that cells can take up glucose. In other words, function of insulin is to counter the concerted action of the numerous hyperglycemia generating hormones and to sustain low blood glucose levels. There are numerous hyperglycemic hormones untreated disorders associated with insulin, generally leading to severe hyperglycemia and a shortened life span.
In people with type 1 diabetes, the pancreas no longer release the insulin as the beta cells are destroyed and they need insulin shots to use glucose from meals. People with type 2 diabetes can produce insulin but, their body doesn't respond well to it. Some people with type 2 diabetes need diabetes pills or insulin dosages to utilize glucose for energy generation. Insulin cannot be taken as a pill as it will be break down during digestion just like the protein in food. Insulin must be injected into the fat under your skin, to make it get into your blood. By reducing the concentration of glucose in the blood, insulin is thought to prevent or reduce the long-term complications of diabetes, including damage to the blood vessels, eyes, kidneys, and nerves.