Carbohydrates play two important roles in biological systems by providing energy and structure to living things.

Carbohydrates take many forms, from simple sugars to extensive, branching polymers. Carbohydrates play several important roles in living things, including giving them structure and providing energy. Glucose is the monosaccharide (simple sugar) used for energy in biological systems. This energy is in the form of adenosine triphosphate (ATP), the biological unit of energy, which consists of an adenosine (an adenine group and a ribose sugar) and three phosphate groups. Polymers of carbohydrates store energy for plants in the form of starch and for animals in the form of glycogen. Other polymers of carbohydrates are important to the structural makeup of the cell walls of plants, fungi, and bacteria, and the exoskeletons of insects and arthropods. A carbohydrate also forms the backbone of nucleic acid molecules.

At their simplest, carbohydrates are single sugar molecules, called monosaccharides, with the chemical formula CnH2nOn. They usually contain a ring with four or five carbon atoms and an oxygen atom. Glucose, for example, with the formula C6H12O6 has a six-atom ring core.
A glucose molecule consists of a six-atom ring containing one oxygen atom, and a tail containing an alcohol group (in this case, CH2OH{{-}\rm{CH_2OH}}) connected to carbon 5. Other isomers of glucose contain a five-carbon ring or a tail connected to a different carbon atom. Each carbon atom in the ring is a chiral center (has a mirror image different from itself), where the OH{{-}\rm{OH}} group can be oriented either above or below the plane of the ring.
Glucose has three chiral centers, or atoms around which the orientation of the molecule's other atoms in space may be different, even if the chemical structure is the same. Only a specific configuration of glucose is metabolized by the body, because only that configuration matches with the active site on the enzyme that breaks down glucose in the first stage of glucose metabolism.

Glucose is an example of a monosaccharide, which is a single (simple) sugar molecule. In biological systems, a polysaccharide is a carbohydrate polymer consisting of many monosaccharides that form a long chain. Starch, glycogen, and cellulose are all polymers of glucose. A disaccharide is a carbohydrate made of two monosaccharides. Many familiar carbohydrates are disaccharides. Sucrose (table sugar) and lactose (the sugar in milk) are common examples of disaccharides.

The metabolism of glucose provides energy for the production of ATP. The high-energy molecule is used by all organisms to power the chemical reactions of life. ATP breaks down into adenosine diphosphate (ADP) through a multi-step chemical reaction. The sum of the steps yields the general reaction:
The reaction releases energy. The reaction is often summarized as breaking the bond between the second and third phosphate group of the molecule. This bond is sometimes called a high-energy phosphoanhydride bond because of its low stability.
ATP contains three phosphate groups (PO42{-\!\rm{PO_4}^{2-}}). The phosphoanhydride bond between the second and third phosphate groups provides the energy that drives many cellular processes.
In the body, ADP is recycled. Glucose is broken down through a complex set of cycles and a series of redox reactions. The resulting energy is used to attach another phosphate group to ADP, enabling it to be used for energy again.