Carbohydrate metabolism
[1] Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from sunlight internally.
[2] When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells.
[2] Both animals and plants temporarily store the released energy in the form of high-energy molecules, such as adenosine triphosphate (ATP), for use in various cellular processes.
After resorption in the gut, the monosaccharides are transported, through the portal vein, to the liver, where all non-glucose monosacharids (fructose, galactose) are transformed into glucose as well.
[4] Glucose (blood sugar) is distributed to cells in the tissues, where it is broken down via cellular respiration, or stored as glycogen.
[3][4] In cellular (aerobic) respiration, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as endproducts.
[7] In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc.
[8] In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
In humans, substrates for gluconeogenesis may come from any non-carbohydrate sources that can be converted to pyruvate or intermediates of glycolysis (see figure).
Under conditions of prolonged fasting, acetone derived from ketone bodies can also serve as a substrate, providing a pathway from fatty acids to glucose.
[10] The gluconeogenesis pathway is highly endergonic until it is coupled to the hydrolysis of ATP or guanosine triphosphate (GTP), effectively making the process exergonic.
[2][12] The branching of glycogen increases its solubility, and allows for a higher number of glucose molecules to be accessible for breakdown at the same time.
[12] It occurs in the liver, adipose tissue, adrenal cortex, testis, mammary glands, phagocytes, and red blood cells.
[2] Many steps of carbohydrate metabolism allow the cells to access energy and store it more transiently in ATP.
[17] 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.
[15] The level of circulatory glucose (known informally as "blood sugar"), as well as the detection of nutrients in the Duodenum is the most important factor determining the amount of glucagon or insulin produced.
Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin.
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).
