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NOTES IN BIOLOGY

Carbohydrates

A carbohydrate is an organic compound that

is composed of atoms of carbon, hydrogenand oxygen in a ratio of 1 carbon atom, 2hydrogen atoms, and 1 oxygen atom. Somecarbohydrates are relatively small molecules,the most important to us is glucose which has6 carbon atoms. These simple sugars arecalled monosaccharides.

The primary function of carbohydrates is forshort-term energy storage (sugars are for Energy). A secondary function isintermediate-term energy storage (as in starch for plants and glycogen for animals).

Other carbohydrates are involved as structural components in cells, such as cellulosewhich is found in the cell walls of plants.

Two common Monosaccharides, (single sugars) Glucose and FructoseHooking two monosaccharides togetherforms a more complex sugar, such as theunion of glucose and fructose to givesucrose, or common table sugar.Compounds such as sucrose are calledDisaccharides (two sugars). Bothmonosaccharides and disaccharides aresoluble in water.Larger, more complex carbohydrates areformed by linking shorter units together toform long or very long sugar chains calledPolysaccharides. Because of their size,

these are often times not soluble in water. Many biologically important compounds suchas starches and cellulose are Polysaccharides. Starches are used by plants, andglycogen by animals, to store energy in their numerous carbon-hydrogen bonds, whilecellulose is an important compound that adds strength and stiffness to a plant's cell wall.

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Sugars are most often found in the form of a "RING". The glucose molecule in theimage above and the one in the image below (Glc) are really the same molecule, justarranged differently. The corners of the "stop sign" represent Carbon atoms even

thought they are not labeled with a "C" (itschemistry shorthand). To form these rings,

the Carbonyl (C=0) Carbon of the straight-chain form (above) forms a bond with thenext to last Carbon in the chain, making thering.

The image on the left shows twomonosaccharides, Glucose and Galactose(Gal). Examine their structure and you willnotice there is very little difference. Their

molecular formulas, C6H1206, are even the same. Molecules with the same chemicalformula, but different molecular structures are called Isomers.The sugar subunits can be linked by the reaction, dehydration synthesis, to form largermolecules. The disaccharide, Sucrose, is formed from two monosaccharides, Glucose

and Fructose.

The disaccharide Lactose is a dimer (twosubunits) of Glucose and Galactose, thedisaccharide Maltose is a dimer of Glucose.

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Large polymers of sugars are calledCarbohydrates. Carbohydrates can be 100'sof sugars long and either straight orbranched. The term ComplexCarbohydrate, or sometimes even just

Carbohydrate refers to long chains of sugars.Three common types of complex carbo's wewill examine are: Starch, Cellulose, andGlycogen. All three are composed only ofGlucose. They differ only in the bondingarrangements between the Glucose subunits.Not all complex carbs are composed ofglucose alone, many have highly unusualsugars in their chains.

Starch is a long (100's) polymer of Glucose

molecules, where all the sugars are orientedin the same direction. Starch is one of the primary sources of calories for humans.Cellulose is a long (100's) polymer of Glucose molecules. However the orientation ofthe sugars is a little different. In Cellulose, every other sugar molecule is "upside-down".This small difference in structure makes a big difference in the way we use thismolecule.Glycogen is another Glucose polymer. Glycogen is a stored energy source, found inthe Liver and muscles of Humans. Glycogen is different from both Starch and Cellulosein that the Glucose chain is branched or "forked".