Edexcel As level notes - Unit1 – Biomolecules

1) Carbohydrates

• Constituents of carbohydrates are “Carbon, Hydrogen and Oxygen".
• The general formulae of carbohydrates is cx (H2O) y.
• In carbohydrates always Hydrogen and Oxygen are in the ratio 2:1.
• They are made of saccharide units (literally sugar) - a prefix (mono-, di-, poly-) denotes how many units.
• Carbohydrates are classified as Monosaccharides, Disaccharides and polysaccharides.

2) Monosaccharides (Single saccharides)

• They are simplest sugars.
• General formulae of Monosaccharides is (CH2O)n

        

Examples:

• Glucose – Simplest sugar 
• Galactose – Milk sugar
• Fructose – Fruit sugar
• Ribose – present in DNA

3) Disaccharides (two saccharides)

They are double sugars.

Examples:

• Sucrose – formed by condensation of two sugars (Glucose + Fructose)· Plants transport food in the form of sucrose. This is the common table sugar.  Sucrose is a non-reducing sugar. So do not readily answers the benedict test.

• Lactose – formed by condensation of two sugars (Glucose + Galactose)· Lactose is “Milk sugar”.

• Maltose - formed by condensation of two sugars (Glucose + Glucose)

4) Poly saccharides (many saccharides)

• Carbohydrates with three or more sugar units.
• They are made up of either amylose or both amylose and amylopectin.

Examples:

• Starch 
• Glycogen 
• Cellulose 

5) Benedict test – Test for reducing sugars

Sugar + Benedict Solution (blue) ---> green ---> yellow ---> orange ---> red

• The colour intensity increases with increase in percentage of sugars.
• Benedict’s reagent contains blue copper (II) ions (Cu +2) which are reduced to copper (I) ions. These are precipitated as red copper (I) oxide which is insoluble in water.