carbohydrates - general properties

Carbohydrates

(‘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

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 :

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

1) Monosaccharide

2) Disaccharides

3) Polysaccharides

2) Number of Carbons

3) Functional Groups

Structural Representation of Carbohydrates

Open Chain Structure

Hemi-acetal Structure

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

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

L-GlucoseD-Glucose

Optical Activity :

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

Diastereo Isomers :

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

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

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

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

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 % most fruit and vegetables, grainy

breads, pulses

Moderate GI 55 – 69 % wheat bread, whole wheat products,

brown rice, orange

High GI 70 and above

corn flakes, baked potato, white bread

Functions of Carbohydrates 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