Arabans are polymers of arabinose. D-xylose — there are small amounts of D-xylose free in fruits, but it occurs mainly in hemicellulose, as xylans and hetero-xylans. Hemicellulose is a polysaccharide of xylose and arabinose a heteroxylan. The ratio of xylose to arabinose seems to affect digestibility as digestibility is reduced as the proportion of xylose increases.
Hemicelluloses constitute a considerable portion of the cell walls of plants so herbivores eat large amounts of them. These sugars are all aldopentoses. D-glucose — an aldohexose with various common names, including grape sugar, dextrose, corn sugar made from cornstarch. Occurs free in plants, fruits, honey, body fluids, including CSF, blood, lymph. It is the major end-product of CHO digestion by non-ruminants and is therefore a primary energy form for non-ruminants.
It is a major component of many oligosaccharides with galactose forms lactose and polysaccharides such as starch and cellulose. In solution D-glucose exists as an equilibrium mixture of the straight chain form with two pyranose ring forms.
Effectively, carbon atom number one reacts with carbon atom number five forming a ring. In fact two forms of the structure exist, called anomers. If the hydrogen atom is above carbon atom one then it is called an alpha anomer but if the hydrogen atom is below the carbon atom it is called a beta anomer. This structural information is very important because it governs how molecules of glucose join together to form larger molecules.
Starch is a polymer of the a- form and is water soluble and digestible by animal enzymes. Cellulose is a polymer of the b- form, it is not soluble and is not digestible by animal enzymes. Changing from a to b via an open chain structure is called mutarotation, and it requires the O-C bond to be broken to allow the C to swivel the H and OH upside down.
Lactose has a beta acetal. The Beta position is defined as the oxygen in the acetal being on the same side of the ring as the C 6.
In the chair structure this results in a horizontal projection. Maltose has an alpha acetal. The Alpha position is defined as the oxygen in the acetal being on the opposite side of the ring as the C 6. In the chair structure this results in a downward projection. The alpha and beta acetal label is not applied to any other carbon - only the anomeric carbon of the left monosaccharide, in this case 1 red. To further identify lactose and maltose, identify the presence of galactose in lactose in the left most structure by the upward -OH on the carbon 4.
Identify glucose in maltose in the left most structure by the horizontal -OH on the carbon 4. They are the building blocks monomers for the synthesis of polymers or complex carbohydrates, as will be discussed further in this section. Monosaccharides are classified based on the number of carbons in the molecule.
General categories are identified using a prefix that indicates the number of carbons and the suffix — ose , which indicates a saccharide; for example, triose three carbons , tetrose four carbons , pentose five carbons , and hexose six carbons Figure 1. The hexose D-glucose is the most abundant monosaccharide in nature.
Other very common and abundant hexose monosaccharides are galactose , used to make the disaccharide milk sugar lactose , and the fruit sugar fructose. Figure 1. Monosaccharides are classified based on the position of the carbonyl group and the number of carbons in the backbone.
Monosaccharides of four or more carbon atoms are typically more stable when they adopt cyclic, or ring, structures. Glucose, for example, forms a six-membered ring Figure 2. Figure 2. Note in these cyclic structural diagrams, the carbon atoms composing the ring are not explicitly shown.
Two monosaccharide molecules may chemically bond to form a disaccharide. The name given to the covalent bond between the two monosaccharides is a glycosidic bond. Glycosidic bonds form between hydroxyl groups of the two saccharide molecules, an example of the dehydration synthesis described in the previous section of this chapter:.
Common disaccharides are the grain sugar maltose , made of two glucose molecules; the milk sugar lactose , made of a galactose and a glucose molecule; and the table sugar sucrose , made of a glucose and a fructose molecule Figure 3. Polysaccharides, also called glycans , are large polymers composed of hundreds of monosaccharide monomers. Unlike mono- and disaccharides, polysaccharides are not sweet and, in general, they are not soluble in water.
Like disaccharides, the monomeric units of polysaccharides are linked together by glycosidic bonds. Polysaccharides are very diverse in their structure. 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; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Figure 7. 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.
As shown in Figure 7, 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. In these animals, certain species of bacteria and protists reside in the rumen part of the digestive system of herbivores and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants.
Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases. Figure 8. Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide. Carbohydrates serve various functions in different animals. Arthropods insects, crustaceans, and others have an outer skeleton, called the exoskeleton, which protects their internal body parts as seen in the bee in Figure 8.
This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya. Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders.
Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed.
Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively.
The long polysaccharide chains may be branched or unbranched.
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