Hexose is a chemical term for any sugar molecule that contains six carbon atoms. Sugars with 4, 5 or 7 carbons are called tetroses, pentoses and heptoses, respectively. In living organisms, the aldose sugars D-allose, D-altrose, D-glucose, D-mannose, D-Gulose, D-Idose, D-Galactose and D-Talose are important in many biological reactions. Likewise, the ketose sugars D-psicose, D-fructose, D-sorbose and D-tagatose are present in living organisms. When hexoses cyclize into six-membered rings, they are referred to as a furanose. Thus, fructose can cyclize into fructopyranose. When sugars cyclize into a five carbon ring, they are called a furanose. When fructose cyclizes into such a five carbon ring, it is called fructofuranose.