The structure and function of carbohydrates.

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The structure and function of carbohydrates


Carbohydrates are named for their characteristic content of carbon, hydrogen, and oxygen (CH2O). Carbohydrate molecules are categorized by the number of carbons present in the molecule. Short chains containing from three to seven carbons form the monosaccharides, which are the most basic sugars. Monosaccharides with five or more carbons can form a ring as well as a linear configuration. The rings form through a reaction between two functional groups in the same molecule. Each carbon atom in the chain, except one, carries an -OH group. The remaining carbon carries a -C=O (carbonyl) group. In monosaccharides hydrogen atoms occupy all other available binding sites of carbon.

Glucose, the most common monosaccharide has six carbons per molecule, which is called a hexose. Carbohydrates also have 2:1 hydrogen to oxygen ratio. This aids in the condensation and hydrolysis reactions. A condensation reaction occurs when two monosaccharides join by the removal of water (H2O). During condensation synthesis one monosaccharide losses an OH and the other losses an H. As a result the two monosaccharides bond by forming maltose a disaccharide with a by-product of a free H2O molecule. When three or more monosaccharides or monomer are involved in a condensation synthesis a polymer or polysaccharide is formed with a by-product of water. Starch and Glycogen are two common chain polymers.

Just as the removal of water joins carbohydrates together via condensation synthesis, hydrolysis is the reaction using water to break apart Disaccharides and polysaccharides. A Disaccharide is bound by sharing an oxygen with one hydrogen on each opposing side. When H2O is re-introduced via hydrolysis the two monomers in the disaccharide no longer need to share one H2O, and as a result the opposing sides absorb the OH and H and break apart. This reaction is a common theme in the digestion of complex carbohydrates.

The orientation of other groups attached to the carbon chain of carbohydrates also affects the properties and interactions of monosaccharides. It allows them to be either alpha (a) or beta (ß).

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Three common sugars, glucose, galactose and fructose, share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose. Glucose is the most common monosaccheride and acts as the immediate source of energy for cellular respiration. It has the structure:

                                

The similar structure of both fructose and galactose is shown below:

                                

                          Fructose                               Galactose

Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are ...

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