83 Carbohydrates
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Carbohydrate Molecules
Carbohydrates are essential macromolecules that are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
LEARNING OBJECTIVES
Describe the structure of mono-, di-, and poly-saccharides
KEY TAKEAWAYS
Key Points
- Monosaccharides are simple sugars made up of three to seven carbons, and they can exist as a linear chain or as ring-shaped molecules.
- Glucose, galactose, and fructose are monosaccharide isomers, which means they all have the same chemical formula but differ structurally and chemically.
- Disaccharides form when two monosaccharides undergo a dehydration reaction (a condensation reaction); they are held together by a covalent bond.
- Sucrose (table sugar) is the most common disaccharide, which is composed of the monomers glucose and fructose.
- A polysaccharide is a long chain of monosaccharides linked by glycosidic bonds; the chain may be branched or unbranched and can contain many types of monosaccharides.
Key Terms
- isomer: Any of two or more compounds with the same molecular formula but with different structure.
- dehydration reaction: A chemical reaction in which two molecules are covalently linked in a reaction that generates [latex]\text{H}_2\text{O}[/latex] as a second product.
- biopolymer: Any macromolecule of a living organism that is formed from the polymerization of smaller entities; a polymer that occurs in a living organism or results from life.
Carbohydrates can be represented by the stoichiometric formula [latex]( \text{CH}_2\text{O})_\text{n}[/latex], where n is the number of carbons in the molecule. Therefore, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. The origin of the term “carbohydrate” is based on its components: carbon (“carbo”) and water (“hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars. In monosaccharides, the number of carbons usually ranges from three to seven. If the sugar has an aldehyde group (the functional group with the structure [latex]\text{R-CHO}[/latex]), it is known as an aldose, and if it has a ketone group (the functional group with the structure [latex]\text{RC(=O)R’}[/latex]), it is known as a ketose. 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). Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms.
Common Monosaccharides
Glucose ([latex]\text{C}_6\text{H}_{12}\text{O}_6[/latex]) is a common monosaccharide and 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,