<P> Hormones released from the pancreas regulate the overall metabolism of glucose . Insulin and glucagon are the primary hormones involved in maintaining a steady level of glucose in the blood, and the release of each is controlled by the amount of nutrients currently available . The amount of insulin released in the blood and sensitivity of the cells to the insulin both determine the amount of glucose that cells break down . Increased levels of glucagon activates the enzymes that catalyze glycogenolysis, and inhibits the enzymes that catalyze glycogenesis . Conversely, glycogenesis is enhanced and glycogenolysis inhibited when there are high levels of insulin in the blood . </P> <P> The level of circulatory glucose (known informally as "blood sugar") is the most important factor determining the amount of glucagon or insulin produced . The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin . Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin . In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells . Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells . </P> <P> Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support (e.g. chitin, cellulose) or for energy storage (e.g. glycogen, starch). However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water - carbohydrate complex . In most organisms, excess carbohydrates are regularly catabolised to form acetyl - CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long - term energy storage . The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates . However, animals, including humans, lack the necessary enzymatic machinery and so do not synthesize glucose from lipids (with a few exceptions, e.g. glycerol). </P> <P> In some animals (such as termites) and some microorganisms (such as protists and bacteria), cellulose can be disassembled during digestion and absorbed as glucose . </P>

Where do living organisms receive carbohydrates proteins and lipids for energy