Carbohydrates - Disaccharides and Polysaccharides - 1.3 Flashcards
What is a Disaccharide?
A pair of monosaccharides combined.
What does glucose joined to glucose form?
Maltose
What does glucose joined to fructose form?
Sucrose
what does glucose joined to galactose form?
Lactose
What is a condensation reaction?
When monosaccharides join and a molecule of water is removed.
What type of bond is formed from a condensation reaction of monosaccharides?
GLYCOSIDIC BOND
What is hydrolysis?
When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond releasing the constituent monosaccharides.
What are some examples of non-reducing sugars?
Some disaccharides, such as sucrose, are known as non-reducing sugars.
Why are some disaccharides, such as sucrose, known as non-reducing sugars?
They do not change the colour of Benedict’s reagent when they are heated with it.
What must be done first in order to detect a non-reducing sugar?
In order to detect a non-reducing sugar it must first be hydrolysed into its monosaccharide components by hydrolysis.
What is the test for non-reducing sugars?
- First conduct Benedict’s test for reducing sugars, to check if a reducing sugar is present.
- Add 2cm3 of the food sample to 2cm3 of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for five minutes.
- Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid (Use pH paper to check the solution is alkaline).
- Re-test the solution by heating it with Benedict’s reagent in a gently boiling water bath for five minutes.
- If a non-reducing sugar was present in the original sample, the Benedict’s reagent will now turn orange-brown.
What does the hydrochloric acid do in the test for non-reducing sugars?
The dilute hydrochloric acid will hydrolyse any disaccharides present into its constituent parts.
What are polysaccharides?
Polymers formed by combining together many monosaccharide molecules.
Are polysaccharides soluble or insoluble?
Insoluble
What are some common uses for polysaccharides?
- Storage
- Structural support in plants
What is starch?
A polysaccharide that is found in many parts of plants in the form of small granules or grains.
How is starch formed?
By the joining of between 200 and 100,000 alpha-glucose molecules by glycosidic bonds in a series of condensation reactions.
How is starch easily detected?
By its ability to change the colour of the iodine in potassium iodine solution from yellow to blue-black.
In which temperature should the test for starch be carried out?
Room temperature
What is the test for starch?
- Place 2cm3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile).
- Add two drops of iodine solution and shake or stirs.
- The presence of starch is indicated by a blurb;ack colouration.
