Cellular reactions that convert carbohydrates to the simple sugar GLUCOSE, and subsequently break down glucose to produce energy or raw materials for cell synthesis. Lactose (milk sugar) contains galactose, and sucrose (table sugar) contains fructose (fruit sugar); both must be converted to glucose prior to their being used by cells.
Glucose After Digestion
Following digestion, simple sugars absorbed by the small intestine are carried via the bloodstream to the liver, which converts fructose and galactose
into glucose. After a carbohydrate meal, blood glucose rises rapidly. In response to elevated blood sugar levels, beta cells of the pancreas release the hormone INSULIN, which promotes glucose uptake by most tissues like muscle and fat cells. The brain and the liver do not require insulin to use glucose.
In muscle and in the liver, surplus glucose can be linked up to form long, branched molecules called GLYCOGEN, the major energy reserve in these two tissues. Two hormones, EPINEPHRINE (adrenaline) and GLUCAGON, stimulate glycogen breakdown when energy is needed. The liver’s role is to maintain adequate BLOOD SUGAR levels; when the diet does not supply enough carbohydrate the liver releases glucose from liver glycogen by a process called GLYCOGENOLYSIS. The liver also produces glucose from noncarbohydrate materials like AMINO ACIDS and LACTIC ACID through a branch of carbohydrate metabolism called GLUCONEOGENESIS.
Glucose as a Source of Energy
Once in the cell, glucose can be used in many ways. It can be burned for energy; it can be converted to glycogen for storage; it can produce an agent to supply hydrogen atoms used for biosynthesis, NADPH (reduced nicotinamide adenine dinu-cleotide phosphate), an enzyme helper based on the B vitamin niacin. The carbon atoms of glucose can be used to synthesize lipids. All cells of the body can oxidize glucose to produce ATP, the energetic currency of the cell.
A collection of enzymes work together to carry out the first part of this process, called GLYCOLYSIS, to yield PYRUVIC ACID, a three-carbon acid. Pyruvic acid is shortened to acetic acid and the carbon atom is removed as CARBON DIOXIDE. An activated form of acetic acid called acetyl COENZYME A is used to synthesize FATTY ACIDS and CHOLESTEROL. Alternatively, acetic acid can be oxidized completely to carbon dioxide by mitochondria, the cells’ powerhouses. The oxidation of pyruvate and of acetyl CoA requires the B vitamins NIACIN, RIBOFLAVIN, THIAMIN, and PANTOTHENIC ACID, which form key enzyme helpers (COENZYMES). The complete oxidation of each glucose molecule yields 38 ATP molecules. This is an excellent conservation of energy: it represents an overall efficiency of about 40 percent.
Glucose can also be oxidized by another route, a series of reactions called the pentose phosphate pathway, to produce the NADPH needed in the formation of lipids like cholesterol and in other compounds, and to produce ribose, a simple sugar needed for DNA and RNA synthesis.
Flatt, Jeane-Pierre. “Use and Storage of Carbohydrate and Fat,” American Journal of Clinical Nutrition 61, supp. (1995): 952S–959S.