3.1..2 Carbohydrates- monosaccharides and disaccharides

3 categories of carbohydrates

Monosaccharides

Disaccharides

Polysaccharides

Monosaccharides

The monomers carbohydrates are made from are monosaccharides and they are the simplest sugars. They are all sweet tasting and water soluble

General formula for carbohydrates

(CH2O)n where n is any number between 3 and 8

What elements do all carbohydrates contain?

C, H and O

What are the 3 main monosaccharides?

Glucose

Fructose

Galactose

They are all hexose sugars and have the general formula C6H12O6

They are isomers- same formula, different structure

Glucose

• Hexose sugar- a monosaccharide with six carbon atoms in each molecule

• alpha and beta glucose (isomers)

Sucrose and lactose

• Disaccharides

• Sucrose is formed from a condensation reaction between a glucose molecule and a fructose molecule

• Lactose is formed from a condensation reaction between a glucose molecule and a galactose molecule

Benedict’s test

• To test for sugars

• Add Benedict’s reagent (blue) to a sample and heat it in a water bath that is been brought to the boil

• If the test is positive it will form a coloured precipitate

Benedict’s test results pt1

• Sample stays blue - no reducing sugar present

• Sample forms green/yellow/orange/brick red precipitate - reducing sugar present

• The results are semi-quantitative as there is a relationship between the conc. of the reducing sugar and the colour of the solution

Benedict’s test results pt2

• The higher the conc. of the reducing sugar, the further the colour change goes

• A more accurate way is to filter the solution and weigh the precipitate, or to remove the precipitate and use a colorimeter to measure the absorbance of the remaining Benedict’s reagent

Non-reducing sugars

• To test for them, first you need to break them down into monosaccharides by:

• Get a new sample of the test solution

• Add dilute hydrochloric acid and carefully heat it in a water bath that’s been brought to the boil

• Neutralise it by adding sodium hydrogen carbonate