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Polysaccharides have an energy storage function and a structural function. Polysaccharides incorporated by living beings along the food chain are important sources of carbohydrates for the energetic metabolism of organisms of the next trophic levels. 

Starch is the polysaccharide used for energy stored by plants. Glycogen is a macromolecule responsible for the storage of glucose in the liver and muscles. Chitin is a polysaccharide with structural functions that constitute the exoskeleton of the arthropods and the cell wall in fungi.

Hexoses are carbohydrates made of six carbons. Glucose, fructose and galactose are examples of hexoses. Hexoses have an important biological role as energy sources for the metabolism. 

Monosaccharides are simple molecules of carbohydrates that cannot be broken into other carbohydrates. Glucose and fructose are examples of monosaccharides. Disaccharides are carbohydrates made of two monosaccharides and with the loss of one molecule of water (dehydration). The chemical bond between two monosaccharides is known as a glycosidic bond.

Sucrose (table sugar) is a disaccharide made by the union of one molecule of glucose with one molecule of fructose. Maltose is a disaccharide made by two glucose molecules. Lactose (milk sugar) is another disaccharide and it is created by the union of one molecule of galactose with one molecule of glucose.

The molecular formula of glucose is C₆H₁₂O₆.

Structurally glucose is a hexagonal ring formed by one atom of oxygen and five atoms of carbon; a hydroxyl radical and a hydrogen atom bind in each carbon of the ring, except for one of the carbons bound to the oxygen of the ring; this carbon binds to a CH₂OH radical. Spatial sides of hydroxyl bonds are alternated. 

Monosaccharides are simple molecules of carbohydrates that cannot be broken into smaller molecules of other carbohydrates. Oligosaccharides are carbohydrates made by union of a maximum of 10 monosaccharides. Polysaccharides are polymers of monosaccharides made of more than 10 units of such monomers. The most important polysaccharides are cellulose, starch, glycogen and chitin. 

Carbohydrates are also known as sugars (starches, cellulose and other substances are carbohydrates too).

Carbohydrates are polyhydroxylated aldehydes or polyhydroxylated ketones (polyalcohol aldehydes or polyalcohol ketones). 

Polyhydroxylated aldehydes are called aldoses and polyhydroxylated ketones are called ketoses.

Gluconeogenesis