Glucose is the primary energy source for the cell. Glucose comes from feeds that are digested and is absorbed into the bloodstream. Insulin is a hormone, and, like many hormones, is a protein. Insulin is secreted by groups of cells within the pancreas called islet cells. So carbohydrates (or sugars) are absorbed from the intestines into the bloodstream after a meal. Insulin is then secreted by the pancreas in response to this detected increase in blood sugar. Most cells of the body have insulin receptors that bind the insulin, which is in the circulation. When a cell has insulin attached to its surface, the cell activates other receptors designed to absorb glucose (sugar) from the bloodstream into the cell. So that's how glucose gets from the blood into the cells to use as energy. What happens if there is a detection in the bloodstream that glucose is too low and can't produce enough glucose for the cells? The scenario could happen during lactation when the energy needs for lactation are really high. Now another hormone comes into play. Glucagon is a protein hormone synthesized as proglucagon and proteolytically processed to yield glucagon within alpha cells of the pancreatic islets. Glucagon stimulates breakdown of glycogen stored in the liver. When blood glucose levels are high, large amounts of glucose are taken up by the liver. Under the influence of insulin, much of this glucose is stored in the form of glycogen. Later, when blood glucose levels begin to fall, glucagon is secreted and acts on hepatocytes to activate the enzymes that depolymerize glycogen and release glucose. Glucagon activates hepatic gluconeogenesis, which is the pathway by which non-hexose substrates such as amino acids are converted to glucose. As such, it provides another source of glucose for blood. This is especially important in animals like cats and sheep that don't absorb much if any glucose from the intestine. In these species, activation of gluconeogenic enzymes is the chief mechanism by which glucagon does its job. Glucagon also appears to have a minor effect of enhancing lipolysis of triglycerides in adipose tissue, which could be viewed as an additional means of conserving blood glucose by providing fatty acid fuel to most cells. Knowing that glucagon's major effect is to increase blood glucose levels, it makes sense that glucagon is secreted in response to hypoglycemia or low blood concentrations of glucose.

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