1.3 carbohydrates- disaccharides and polysaccharides

List the pairs of disaccharides

Glucose and glucose= Maltese

Glucose and fructose = sucrose

Glucose and galactose= lactose

What happens when monosaccharides join?

What is the bond formed?

A molecule of water is removed and therefore the reaction is called a condensation reaction.

The bond formesnis a glucosidic bond

What happens when water is added to a dissacharide ? (Under suitable conditions)

It breaks the glycosidic bond releasing the monosaccharides. This is called hydrolysis (Addition of water that causes breakdown)

What is hydrolysis?

Addition of a water molecule that causes breakdown

Some dissaccharides are…

Reducing sugars

How do we find reducing sugars?

Use benedicts test

Give an example of a non reducing sugar

Sucrose

How can you tell if something is a non reducing sugar?

They dont change the colour of the bendicts reagent when they are heated with it.

How can you detect a non reducing sugar then?

First must be hydrolysed into its monosaccharide components by hydrolysis.

1) if sample isnt in liquid form,must be ground up in water

2) add 2cm of the food sample being tested to 2cm 3 of benedicts reagent in a test tube and filter

3) place test tube in gently boiling water bath for 5 mins. If benedicts reagent doesn’t change colour(still blue) then reducing sugar is not present.

4) add another 2cm of food sample to 2cm 3 of dilute hydrochloric acid in a tes tube and place the test tube in gently boiling water for 5 mins.

The dilute hydrochloric acid will hydrolyse any dissacharide present into its of monosaccharides

5) slowly add sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. ( benedicts reagent doesn’t work in acidic conditions)

6) test with ph paper to check that solution is alkaline

7) retest the resulting solution by heating it with 2cm 3 ofbeneducts reagent in a gently boiling waterbayh for 5 mins.

8) if non reducing sugar was present in the original sample, benedicts reagent will turn orange-brown. This is due to the reducing sugars thag were produced from the hydrolysis of the non reducing sugar.

Polysaccharides definition

BLURT 5 points

Polymers, formed by combing together many monosaccharide molecules.

The monosaccharides are joined by glycosidic bonds that were formed by condensation reactions

As polysaccharides are very large molecules, they are insoluble. This feature makes them suitable for storage

Although some polysaccharides such as glucose are not used for storage but give structural support to plants.

When they are hydrolysed polysaccharides break down into disaccharides or monosaccharides

Definition of starch

2 points

A polysaccharide that is found in many parts of plants in the form of small granules or grains.

Eg starch grains in chloroplasts

It is formed by the joining of between 200 and 100,000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions

Test for starch result

Iodine in potassium iodide solution turns from yellow to blue black

Test for starch

(3 points)

1) Place 2cm of the sample being tested into a test tube

2) Add 2 drops of iodine solution and shake or stir

3) The presence of starch is indicated by a blue black colouration