Carbohydrates

Carbohydrates are one of the four main class of macromolecules that comprise all cells and are an essential part of our diet; grains, fruits, and vegetables are all organic sources. If we might be most familiar with the role carbohydrates beat in nutrition, they additionally have a range of other important functions in humans, animals, plants, and also bacteria. In this section, we will discuss and review basic concepts the carbohydrate structure and nomenclature, and a range of features they play in cells.

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Molecular structures

In their most basic form, carbohydrates can be stood for by the stoichiometric formula (CH2O)n, wherein n is the number of carbons in the molecule. For an easy carbohydrates, the ratio of carbon-to-hydrogen-to-oxygen in the molecule is 1:2:1. This formula additionally explains the origin of the ax “carbohydrate”: the contents are carbon (“carbo”) and the contents of water (“hydrate”). An easy carbohydrates space classified right into three subtypes: monosaccharides, disaccharides, and polysaccharides, which will certainly be debated below. While basic carbohydrates loss nicely right into this 1:2:1 ratio, carbohydrate can additionally be structurally much more complex. For example, many carbohydrates save functional teams (remember them from our an easy discussion around chemistry) as well as the obvious hydroxyl. For example, carbohydrates deserve to have phosphates or amino teams substituted in ~ a variety of sites within the molecule. This functional teams can provide extr properties come the molecule and also will transform its in its entirety function. However, even with these species of substitutions, the an easy overall structure of the carbohydrate is retained and easily identified.


Nomenclature

One problem with carbohydrate chemistry is the nomenclature. Here are a couple of quick and straightforward rules:

basic carbohydrates, such together glucose, lactose, or dextrose, end with one "-ose." simple carbohydrates have the right to be classified based upon the variety of carbon atoms in the molecule, similar to triose (three carbons), pentose (five carbons), or hexose (six carbons). An easy carbohydrates can be classified based on the useful group found in the molecule, i.e ketose (contains a ketone) or aldose (contains an aldehyde). Polysaccharides are regularly organized by the variety of sugar molecules in the chain, such together in a monosaccharide, disaccharide, or trisaccharide.

For a short video clip on carbohydrate classification, check out the 10-minute khan Academy video clip by click here.


Monosaccharides

Monosaccharides ("mono-" = one; "sacchar-" = sweet) are an easy sugars; the most common is glucose. In monosaccharides, the variety of carbons usually arrays from three to seven. If the sugar has actually an aldehyde group (the functional group with the framework R-CHO), it is recognized as one aldose; if it has actually a ketone group (the functional group with the framework RC(=O)R"), that is known as a ketose.

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Figure 1. Monosaccharides room classified based on the place of their carbonyl group and the number of carbons in the backbone. Aldoses have actually a carbonyl group (indicated in green) in ~ the finish of the carbon chain and also ketoses have actually a carbonyl group in the center of the carbon chain. Trioses, pentoses, and hexoses have three, five, and also six carbons in their backbones, respectively. Attribution: Marc T. Facciotti (own work)



Fructose versus both glucose and galactose

A 2nd comparison can be made as soon as looking at glucose, galactose, and also fructose (the 2nd carbohydrate that v glucose provides up the disaccharide sucrose and also is a common sugar uncovered in fruit). Every three space hexoses; however, over there is a major structural difference in between glucose and also galactose versus fructose: the carbon that contains the carbonyl (C=O).

In glucose and also galactose, the carbonyl team is top top the C1 carbon, creating an aldehyde group. In fructose, the carbonyl group is on the C2 carbon, creating a ketone group. The former sugars are called aldoses based on the aldehyde team that is formed; the last is designated together a ketose based top top the ketone group. Again, this difference gives fructose different chemical and also structural properties from those that the aldoses, glucose, and also galactose, also though fructose, glucose, and galactose all have the same chemical composition: C6H12O6.

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Figure 2. Glucose, galactose, and fructose are all hexoses. They space structural isomers, meaning they have the same chemical formula (C6H12O6) however a different arrangement of atoms.


Linear versus ring kind of the monosaccharides

Monosaccharides can exist together a linear chain or as ring-shaped molecules. In aqueous solutions, monosaccharides space usually found in ring kind (Figure 3). Glucose in a ring form can have two various arrangements the the hydroxyl team (OH) around the anomeric carbon (C1 the becomes asymmetric in the process of ring formation). If the hydroxyl team is below C1 in the sugar, the is claimed to be in the alpha (α) position, and if that is over C1 in the sugar, the is said to it is in in the beta (β) position.

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Figure 3. Five- and six-carbon monosaccharides exist in equilibrium between linear and ring form. As soon as the ring forms, the next chain it closes on is locked into an α or β position. Fructose and also ribose also kind rings, back they kind five-membered rings as opposed come the six-membered ring the glucose.



Disaccharides

Disaccharides ("di-" = two) kind when two monosaccharides undergo a dehydration reaction (also well-known as a condensation reaction or dehydration synthesis). Throughout this process, the hydroxyl group of one monosaccharide combines v the hydrogen of one more monosaccharide, release a molecule of water and also forming a covalent bond. A covalent shortcut formed in between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is recognized as a glycosidic bond. Glycosidic bond (also called glycosidic linkages) have the right to be of the alpha or the beta type.

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Figure 4. Sucrose is created when a monomer of glucose and a monomer that fructose space joined in a dehydration reaction to kind a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atom in a monosaccharide space numbered indigenous the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed in between the C1 carbon in glucose and also the C2 carbon in fructose.

Common disaccharides incorporate lactose, maltose, and sucrose (Figure 5). Lactose is a disaccharide consist of of the monomers glucose and also galactose. The is uncovered naturally in milk. Maltose, or malt/grain sugar, is a disaccharide formed by a dehydration reaction in between two glucose molecules. The most typical disaccharide is sucrose, or table sugar, which is written of the monomers glucose and also fructose.

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Figure 5. usual disaccharides encompass maltose (grain sugar), lactose (milk sugar), and also sucrose (table sugar).



Polysaccharides

A lengthy chain the monosaccharides connected by glycosidic bonds is known as a polysaccharide ("poly-" = many). The chain may be branched or unbranched, and also it might contain different types of monosaccharides. The molecular weight might be 100,000 Daltons or more, depending on the number of monomers joined. Starch, glycogen, cellulose, and also chitin are primary instances of polysaccharides.

Starch is the stored type of street in plants and also is comprised of a mixture the amylose and also amylopectin; both space polymers of glucose. Plants room able to synthesize glucose. Overabundance glucose, the quantity synthesized that is beyond the plant’s immediate power needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds offers food because that the embryo as it germinates and also can also act together a source of food for humans and animals who might eat the seed. Starch that is consumed by human beings is damaged down through enzymes, such as salivary amylases, into smaller molecules, such together maltose and glucose.

Starch is comprised of glucose monomers that are joined by 1-4 or 1-6 glycosidic bonds; the numbers 1-4 and also 1-6 describe the carbon number of the two residues that have actually joined to type the bond. As illustrated in figure 6, amylose is starch developed by unbranched chain of glucose monomers (only 1-4 linkages), whereas amylopectin is a branched polysaccharide (1-6 linkages in ~ the branch points).

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