A-Level Biology

3.1.2 Carbohydrates

Carbohydrates contain Carbon, Hydrogen and Oxygen (C6H12O6).                                          (Hydrogen and oxygen is in the ratio 2:1 - like water H2O)

Monosaccharides of Carbohydrates: (Mono = 1)

Alpha glucose and Beta Glucose

Galactose and Fructose

Monosaccharide + Monosaccharide > Disaccharide (Di = 2)

Glucose + Glucose          > Maltose (+water)

Glucose + Galactose      > Lactose (+water)

Glucose + Fructose         >Sucrose (+water)

A condensation reaction occurs (water is produced) and a glycosidic bond is formed.

Hydrolysis breaks down disaccharides using water and an enzyme

Disaccharide > Enzyme

Maltose > maltase

Lactose > lactase

Sucrose > sucrase

Polysaccharides: Starch, Cellulose, and Glycogen

Name of the test: Benedict's test

Test for: Reducing sugar

Reagent: Benedict's solution

Procedures:

1. Add Benedict's solution to sample in the test tube.

2. Shake mixture and heat don't boil (95◦C).

3. Observe colours:

red precipitate - the highest amount of reducing sugar present

orange precipitate

yellow precipitate

green precipitate

blue precipitate - lowest/no amount

Name of the test: Benedict's test

Test for: Non-reducing sugar (SUCROSE)

Reagent: Benedict's solution

Procedures:

1. Boil a sample with dilute hydrochloric acid (HCl) acid for a few minutes to hydrolyse the glycosidic bonds. (CARE NEEDED)

2. Neutralise the solution adding small amounts of solid sodium hydrogen carbonate.

3. Then test as before for reducing sugars using Benedict's reagent

And it should give a positive result.

Name of the test: Iodine test

For: Starch

Reagents: iodine solution

Drops of iodine solution to the sample

dark blue/black = starch present

yellowish-brown = starch absent