page_title: Glucose
section_title: Open-chain form
hierarchical_section_title: Glucose / Chemical properties / Open-chain form
caption_attribution_description: English: D-Glucose shifting into β-D-glucose Deutsch: Von dem asymmetrischen C-Atom geht das Wasserstoffatom zur Sauerstoffdoppelbindung, verbindet sich und aus der Doppelbindung wird eine Einfachbindung. Dadurch gibt es am obersten Kohlenstoffatom nur 3 Substituenten und deshalb verbindet sich das oberste Kohlenstoffatom mit dem Sauerstoffatom, so entsteht die Ringform. Anmerkung: Die in der Fischer-Projektion rechts stehenden Liganden zeichnet man in der Ringform nach unten und die links stehenden nach oben.
context_page_description: Glucose is a simple sugar with the molecular formula C₆H₁₂O₆. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight, where it is used to make cellulose in cell walls, which is the most abundant carbohydrate. In energy metabolism, glucose is the most important source of energy in all organisms. Glucose for metabolism is partially stored as a polymer, in plants mainly as starch and amylopectin and in animals as glycogen. Glucose circulates in the blood of animals as blood sugar. The naturally occurring form of glucose is d-glucose, while l-glucose is produced synthetically in comparatively small amounts and is of lesser importance. Glucose is a monosaccharide containing six carbon atoms, an aldehyde group and is therefore referred to as an aldohexose. The glucose molecule can exist in an open-chain as well as ring form, the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal.
context_section_description: In its fleeting open-chain form, the glucose molecule has an open (as opposed to cyclic) and unbranched backbone of six carbon atoms, C-1 through C-6; where C-1 is part of an aldehyde group H(C=O)−, and each of the other five carbons bears one hydroxyl group −OH.  The remaining  bonds of the backbone carbons are satisfied by hydrogen atoms −H. Therefore, glucose is both a hexose and an aldose, or an aldohexose. The aldehyde group makes glucose a reducing sugar giving a positive reaction with the Fehling test.
  Each of the four carbons C-2 through C-5 is a stereocenter, meaning that its four bonds connect to four different substituents.  (Carbon C-2, for example, connects to −(C=O)H, −OH, −H, and −(CHOH)₄H.)  In d-glucose, these four parts must be in a specific three-dimensional arrangement. Namely, when the molecule is drawn in the Fischer projection, the hydroxyls on C-2, C-4, and C-5 must be on the right side, while that on C-3 must be on the left side.
  The positions of those four hydroxyls are exactly reversed in the Fischer diagram of l-glucose.  d- and l-glucose are two of the 16 possible aldohexoses; the other 14 are allose, altrose, galactose, gulose, idose, mannose, and talose, each with two enantiomers, “d-” and “l-”.
  It is important to note that the linear form of glucose makes up less than 0.02% of the glucose molecules in a water solution. The rest is one of two cyclic forms of glucose that are formed when the hydroxyl group on carbon 5 (C5) bonds to the aldehyde carbon 1 (C1).
caption: Glucose can exist in both a straight-chain and ring form.