Carbohydrates.

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Carbohydrates are organic compounds that usually contain carbon hydrogen and oxygen in the ratios: 1 Carbon: 2 Hydrogens: 1 Oxygen.

There are four classes of carbohydrates that are of general interest. Monosaccharides, Disaccharides, Oligosaccharides, Polysaccarides

 

 Monosaccharides (simple sugars) have a carbon skeleton of 3 or more carbons depending on the monosaccharide. The most familiar monosaccharide is Glucose (C6 H12 O6). A ball and stick model of glucose is shown here in its ring form, which is the form it takes in water. As a solid, glucose has a straight chain form which is not shown.

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Galactose is another monosaccharide with six carbons. Later we will meet fructose or fruit sugar which also has six cabons. Galacotse is a component of a disaccharide called lactose.

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  Disaccharides. Disaccharides consist of two monosaccharides linked together by a dehydration synthesis.

Sucrose is common disaccharide which functions as a transport sugar in plants. The production of sucrose by means of a dehydration synthesis is shown here. Each sucrose molecule is made by chemically combining a glucose and a fructose molecule.

A hydrogen is removed from the glucose and a hydroxyl(OH) from the fructose leaving an oxygen to link the two molecules together.

Lactose, another disaccharide, is commonly called milk sugar.

This diagram shows the synthesis of sucrose from glucose and fructose via a dehydration synthesis.

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Oligosaccharides: Oligo means a few and oligosaccharides have a few simple sugars linked together but not thousands as do polysacchardes. Oligosacharides are common on cell membranes and surfaces where they often serve as cell markers.

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 Polysaccharides: Polysaccharides are the most abundant organic compounds in the biosphere. The most commonly seen polysaccharide is cellulose and scientist estimate that over one trillion tons of cellulose are synthesized by plants each year. Cellulose forms the cell wall of plants.

Starches can be digested by animals but cellulose cannot. Most animals that injest grass or wood have special micro organisms living in their gut that digest the cellulose and the animals in turn absorp the breakdown product

This diagram compares the way the glucose units bond in starch versus cellulose. What differences do you observe? These differences are largely responsible for the quite different properties of starches and cellulose.

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 Amylopectin: Some starches such as amylopectin are branched and this gives the starch a jelly-like appearence. Pectin is one such starch that is produced by apples and used to thicken jellies.

Even though we typically associate starch as being made by plants, many animals produce a highly branched starch like compound called glycogen. In mammals glycogen is stored in the liver and the muscles where it provides a quick source of energy and serves as a storage substance for excess glucose taken up from the blood.

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pgd. revised 7/15//99

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