Carbohydrates - General Properties

Carbohydrates

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(‘Hydrates of carbon') or saccharides (Greek meaning "sugar") MOST abundant of the biomolecules which also include proteins , fats and nucleic acids Simple organic compounds that are aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group

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The basic carbohydrate unit is called Monosaccharide The general formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater

Sources 

Foods that are high in carbohydrates include breads, pastas, beans, potatoes, bran, rice and cereals.

Classification : 

The most useful Classification Scheme divides the carbohydrates into groups according to the number of individual simple sugar units   

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Monosaccharide Disaccharides Polysaccharides

Number of Carbons Functional Groups

Structural Representation of Carbohydrates 

Open Chain Structure

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Hemi-acetal Structure

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Haworth Structure

Open Chain Structure 

Long Straight chain form of carbohydrates

Hemi-acetal Structure 

The 1st Carbon in glucose condenses with the –OH group of the 5th Carbon to form a ring

Haworth Structure 

Presence of a Pyranose Ring

Physical Properties 

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Stereoisomerism : Same structural formula but differ in spatial configuration With respect to the Penultimate Carbon atom glucose has 2 stereoisomers D-glucose and L-glucose

D-Glucose

L-Glucose

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Optical Activity : Rotation of Plane polarized light and (+)glucose or (–)glucose respectively

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Diastereo Isomers : Configural changes with regard to the C2 , C3 or C4 in glucose. Eg : Mannose , Galactose

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Anomerism : Spatial configuration with respect to the first carbon atom in Aldoses and the second in Ketoses α-glucose has specific rotation of 112 and ß-glucose has a rotation of 19.

Importance – In glucose estimation by GOD/POD , the enzyme acts preferentially on the ß form. Hence fresh glucose solutions give lower values.

Chemical Properties    

Ozazone formation with phenlyhydrazine Benedicts test Oxidation Reduction to Alcohols

Digestion of Carbohydrates 

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Polysaccharides in the mouth are broken through the introduction of amylase, a digestive enzyme in saliva. The high acid content of the stomach inhibits amylase activity, so carbohydrate digestion is suspended in the stomach. Pancreatic amylase is secreted by the pancreas into the duodenum and works with other enzymes to complete the breakdown of carbohydrate into a monosaccharide Is then absorbed into the surrounding capillaries of the villi.

Nutrition  

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Carbohydrates are not essential nutrients The body can obtain all its energy from protein and fats Carbohydrates contain 3.75 Kilocalories An essential diet contains between 40-65% of dietary energy from carbohydrates

Glycemic Index 

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A measure of the effects of carbohydrates on blood glucose levels. Carbohydrates that break down rapidly have a high GI; Those that break down slowly, releasing glucose gradually into the bloodstream, have a low GI A lower glycemic response is often thought to equate to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids.

Classification

GI range

Examples

Low GI

>55 %

Moderate GI

55 – 69 %

High GI

70 and above

most fruit and vegetables, grainy breads, pulses wheat bread, whole wheat products, brown rice, orange corn flakes, baked potato, white bread

Functions of Carbohydrates 

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Carbohydrates spare protein from being used up as an energy source. If there are not enough carbohydrates, then large amounts of fat are used for energy. The body is not able to handle this large amount so quickly, so it accumulates ketone bodies Carbohydrate is necessary for the regulation of nerve tissue and is the source of energy for the brain. Some carbohydrates are high in fibre, which helps prevent constipation Structural components