1.3 - Carbohydrates (disaccharides And Polysaccharides) Flashcards
What is a disaccharide, and how is it formed?
A disaccharide is formed when two monosaccharides combine in a condensation reaction, which removes a molecule of water and forms a glycosidic bond.
List three examples of disaccharides and their monosaccharide components.
Maltose: Glucose + Glucose
Sucrose: Glucose + Fructose
Lactose: Glucose + Galactose
What is hydrolysis, and what happens to disaccharides during this process?
Hydrolysis is the addition of water that breaks the glycosidic bond in disaccharides, releasing the constituent monosaccharides.
What is the key difference between reducing and non-reducing sugars?
Reducing sugars (e.g., maltose) can change the colour of Benedict’s reagent, whereas non-reducing sugars (e.g., sucrose) cannot unless hydrolysed first.
What is the first step in testing for non-reducing sugars?
- Add 2 cm³ of the food sample to 2 cm³ of Benedict’s reagent in a test tube.
- Heat the mixture in a gently boiling water bath for 5 minutes. If the solution remains blue, a reducing sugar is not present.
How is a non-reducing sugar prepared for testing after the first step?
- Add 2 cm³ of the food sample to 2 cm³ of dilute hydrochloric acid.
- Heat the test tube in a gently boiling water bath for 5 minutes to hydrolyse any disaccharide into monosaccharides
What happens after hydrolysis with hydrochloric acid in the Benedict’s test?
- Slowly add sodium hydrogencarbonate solution to neutralise the acid.
- Check with pH paper to ensure the solution is alkaline.
How do you confirm the presence of a non-reducing sugar?
- Re-test the solution with Benedict’s reagent. Heat for 5 minutes in a gently boiling water bath.
- If a non-reducing sugar was present, the solution will turn orange-brown.
Define a polysaccharide and describe its formation.
A polysaccharide is a polymer formed by combining many monosaccharides through glycosidic bonds in condensation reactions.
Why are polysaccharides suitable for storage?
Polysaccharides are very large molecules and insoluble (won’t affect WP), making them ideal for storage.
What happens to polysaccharides during hydrolysis?
Polysaccharides are broken down into disaccharides or monosaccharides.
What are the two main functions of polysaccharides, with examples?
Storage: Example - Starch (found in plants).
Structural Support: Example - Cellulose (provides support to plant cells).
Describe the composition and location of starch
Starch is a polysaccharide made by joining 200–100,000 α-glucose molecules through glycosidic bonds. It is found in plants as small granules or grains, such as in chloroplasts.
How do you test for the presence of starch?
1) Place 2 cm³ of the sample in a test tube (or 2 drops on a spotting tile).
2) Add two drops of iodine solution.
3) Shake or stir. A blue-black coloration indicates the presence of starch.
