Biochemical Tests Reducing sugars Non-Reducing sugars Starch Lipid Proteins

Reducing sugars Reducing sugars are identified by the part of their molecule that is the functional group The atoms of the functional group and their arrangement gives the organic compound its characteristic chemical properties

All monosaccharides contain one of two functional groups Some, like glucose, are aldehydes and are also called aldo-sugars or aldoses Others, like fructose are ketones and are called keto-sugars or ketoses

Reducing sugars Monosaccharides, such as glucose, fructose and galactose, and disaccharides, such as maltose and lactose, are known as reducing sugars.Monosaccharides, such as glucose, fructose and galactose, and disaccharides, such as maltose and lactose, are known as reducing sugars. They reduce other compounds, such as Benedict's reagent, producing a precipitate.They reduce other compounds, such as Benedict's reagent, producing a precipitate. The colour of the precipitate depends upon the concentration of the reducing sugar.The colour of the precipitate depends upon the concentration of the reducing sugar.

Reducing Sugars Test Test add 2cm of a solution of the reducing sugar to a test-tube. Add an equal volume of Benedict’s solution. Shake and bring gently to the boil in a water bath Basis of test Benedict’s solution contains copper sulphate. Reducing sugars reduce soluble blue copper sulphate, containing copper(ll) ions to insoluble red- brown copper oxide containing copper(ll) which is seen as a precipitate. Observations The initial blue colouration of the mixture turns green, then yellowish and may finally form a brick-red precipitate

Non-Reducing Sugars Test Add 2cm of sucrose solution to a test-tube. Add 1cm dilute hydrochloric acid. Boil for one minute. Carefully neutralise with sodium hydrogen carbonate (check with ph paper) – care is required because effervescence occurs. Carry out Benedict’s test. Basis of test A disaccharide can be hydrolysed to its monosaccharide constitutents by boiling with dilute hydrochloric acid. Sucrose is hydrolysed to glucose and fructose, both of which are reducing sugars and give the reducing sugar results with the Benedict’s test Observations As Benedict’s test

Starch Test Iodine/potassium iodide test Add 2cm 1% starch solution to a test-tube. Add a few drops of l2/kl solution. Alternatively add the iodine to the solid form of starch Basis of test A polyiodide complex is formed with starch. Observations A blue-black colouration.

Lipid Emulsion test Add 2cm fat or oil to a test-tube containing 2cm of absolute ethanol. Dissolve the lipid by shaking vigorously. Add equal volume of cold water. Basis of test Lipids are immiscible with water. Adding water to a solution of the lipid in alcohol results in an emulsion of tiny lipid droplets in the water which reflect light and give a white, opalescent appearance Observations A cloudy white suspension

Protein Biuret Test Add 2cm protein solution to a test-tube. Add an equal volume of 5% potassium hydroxide solution and mix. Add 2 drops of 1% copper sulphate solution and mix. No heating is required. Basis of test A test for peptide bonds. In the presence of dilute copper sulphate in alkaline solution, nitrogen atoms in the peptide chain form a purple complex with copper(ll) ions. Observations A mauve or purple colour develops slowly