![]() The chain may be branched or unbranched, and it may contain different types of monosaccharides. PolysaccharidesĪ long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide ( poly– = “many”). Common disaccharides include maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane.įigure 5. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 4). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. Disaccharidesĭisaccharides ( di– = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. ![]() When the ring forms, the side chain it closes on is locked into an α or β position. Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. Although glucose, galactose, and fructose all have the same chemical formula (C 6H 12O 6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon all of these monosaccharides have more than one asymmetric carbon (Figure 2).įigure 3. Galactose and fructose are other common monosaccharides - galactose is found in milk sugars and fructose is found in fruit sugars. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants. Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). In humans, glucose is an important source of energy. The chemical formula for glucose is C 6H 12O 6. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. See Figure 1 for an illustration of the monosaccharides.įigure 1. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R′), it is known as a ketose. Most monosaccharide names end with the suffix – ose. In monosaccharides, the number of carbons usually ranges from three to seven. Monosaccharides ( mono– = “one” sacchar– = “sweet”) are simple sugars, the most common of which is glucose. ![]() Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. Carbohydrates also have other important functions in humans, animals, and plants.Ĭarbohydrates can be represented by the stoichiometric formula (CH 2O) n, where n is the number of carbons in the molecule. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates are, in fact, an essential part of our diet grains, fruits, and vegetables are all natural sources of carbohydrates. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. To lose weight, some individuals adhere to “low-carb” diets. Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. ![]()
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