Monosaccharides are the simplest carbohydrates. These molecules are largely cyclic polyhydroxy aldehydes or polyhydroxy ketones but may have other modifications such as amino groups.

Polysaccharides are linear or branched polymers of monosaccharides. These molecules can be used to store energy, provide structure, or structurally modify other molecules. Both glycogen metabolism and structural polysaccharides are discussed in chapter 8.

Glycolysis is the breakdown of glucose into two copies of the three carbon molecule pyruvate. ATP and NADH are generated in this process. Other monosaccharides (e.g. fructose and galactose) can also be catabolized through glycolysis.

Pyruvate is the three carbon oxo acid that is found at the crossroads of several metabolic pathways. Most pyruvate is decarboxylated to acetyl-CoA and oxidized via the citric acid cycle (chapter 7) but pyruvate can be converted to several other molecules.

Gluconeogenesis is the synthesis of new glucose from other metabolites. Not all metabolites can contribute to gluconeogenesis.

Carbohydrates can be used as a source of stored energy. In animals glucose is polymerized into glycogen, while in plants starch acts as an energy source for the growing plant to use before seedlings can begin photosynthesis. Other organisms can store energy in different molecules, such as inulin a polymer made predominantly of fructose.

Cellulose (a polymer of glucose) is the most common structural carbohydrate, but chitin (found in insect and crab shells) and chondroitin (found in cartilage) are also carbohydrates that play structural roles.