Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix — ose. 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. See Figure 1 for an illustration of the monosaccharides.
Figure 1. Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. 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.
Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively. The chemical formula for glucose is C 6 H 12 O 6.
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 adenosine triphosphate ATP. Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant.
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. Galactose part of lactose, or milk sugar and fructose part of sucrose, or fruit sugar are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula C 6 H 12 O 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.
Figure 2. Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula C6H12O6 but a different arrangement of atoms.
Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms Figure 3. Glucose in a ring form can have two different arrangements of the hydroxyl group -OH around the anomeric carbon carbon 1 that becomes asymmetric in the process of ring formation.
Figure 3. Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. 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.
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. Glycosidic bonds also called glycosidic linkages can be of the alpha or the beta type. Figure 4. Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group.
In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group.
In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose. Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules.
The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose. The chain may be branched or unbranched, and it may contain different types of monosaccharides. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Plants are able to synthesize glucose, and the excess glucose is stored as starch in different plant parts, including roots and seeds.
The starch in the seeds provides food for the embryo as it germinates while the starch that is consumed by humans is broken down by enzymes into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. Glycogen is the storage form of glucose in humans and other vertebrates.
It is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis. Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose and provides structural support to the cell.
Every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.
Carbohydrates serve various functions in different animals. Arthropods have an outer skeleton, the exoskeleton, which protects their internal body parts. This exoskeleton is made of chitin, which is a polysaccharide-containing nitrogen. Chitin is also a major component of fungal cell walls. Carbohydrates are a major class of biological macromolecules that are an essential part of our diet and provide energy to the body.
Biological macromolecules are large molecules that are necessary for life and are built from smaller organic molecules. One major class of biological macromolecules are carbohydrates, which are further divided into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Importantly, carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many basic foods.
The structure of cellulose consists of long polymer chains of glucose units connected by a beta acetal linkage. The graphic on the left shows a very small portion of a cellulose chain.
All of the monomer units are beta-D-glucose, and all the beta acetal links connect C 1 of one glucose to C 4 of the next glucose. Carbon 1 is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal. The Beta position is defined as the ether oxygen being on the same side of the ring as the C 6. In the chair structure this results in a horizontal or up projection. This is the same definition as the -OH in a hemiacetal.
Cellulose: Beta glucose is the monomer unit in cellulose. As a result of the bond angles in the beta acetal linkage, cellulose is mostly a linear chain. Starch: Alpha glucose is the monomer unit in starch.
As a result of the bond angles in the alpha acetal linkage, starch-amylose actually forms a spiral much like a coiled spring. Dietary fiber is the component in food not broken down by digestive enzymes and secretions of the gastrointestinal tract.
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