Glucose is a type of sugar with the chemical formula C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make ATP. Plants synthesize glucose using carbon dioxide and water, and glucose, in turn, is used for the energy requirements of the plant. Excess glucose is often stored as starch which is catabolized by humans and other animals that feed on plants.

Three-dimensional structures of glucose in both forms are shown above their respective two-dimensional structures. The open-chain form of glucose is a chain of 6 carbons. One O H group is attached to each of the carbons from carbon 2 to 6. Carbon 1 is an aldehyde, so attached to carbon 1 is double-bonded oxygen and single bonded hydrogen. The 6-membered ring form of glucose is made of carbons 1 to 5, with oxygen between carbon 1 and carbon 5.

Figure 1: Depending on the solution, glucose molecules form different isomers.