Disaccharides & Polysaccharides (AQA A-Level Biology): Revision Notes
Disaccharides & Polysaccharides
What are disaccharides?
Disaccharides are carbohydrates formed when two monosaccharide molecules combine together. This process creates larger, more complex sugar molecules that serve different biological functions.
The formation of disaccharides represents the first step in building more complex carbohydrate structures, making them essential building blocks in biological systems.
Common examples of disaccharides include:
- Maltose: glucose + glucose
- Sucrose: glucose + fructose
- Lactose: glucose + galactose
Formation of glycosidic bonds
When two monosaccharides join together, they undergo a condensation reaction. During this process, a water molecule (H₂O) is eliminated, and a glycosidic bond forms between the two sugar molecules.
The reverse process can also occur. When water is added to a disaccharide under appropriate conditions, hydrolysis takes place. This breaks the glycosidic bond and releases the individual monosaccharide components.
Understanding these two opposing reactions is crucial - condensation builds up complex carbohydrates by removing water, while hydrolysis breaks them down by adding water back.
Key reactions:
- Condensation: Two monosaccharides → Disaccharide + Water
- Hydrolysis: Disaccharide + Water → Two monosaccharides
Testing for non-reducing sugars
Some disaccharides, like sucrose, are non-reducing sugars because they do not react with Benedict's reagent when heated. To test for these sugars, a two-stage process is required:
Worked Example: Testing for Non-Reducing Sugars
Step 1 - Initial test: Add 2 cm³ of food sample to 2 cm³ of Benedict's reagent and heat in a boiling water bath for 5 minutes. If no colour change occurs (solution stays blue), no reducing sugar is present.
Step 2 - Hydrolysis step: Add 2 cm³ of food sample to 2 cm³ of dilute hydrochloric acid and heat in a boiling water bath for 5 minutes. This breaks down any disaccharides into their monosaccharide components.
Step 3 - Neutralisation: Add sodium hydrogencarbonate solution to neutralise the acid. Check with pH paper that the solution is alkaline.
Step 4 - Second Benedict's test: Re-test with 2 cm³ of Benedict's reagent in a boiling water bath for 5 minutes.
Step 5 - Results: If a non-reducing sugar was present originally, the Benedict's reagent will now turn orange-brown due to the reducing sugars produced from hydrolysis.
Understanding polysaccharides
Polysaccharides are large polymers formed by joining many monosaccharide molecules together through glycosidic bonds created by condensation reactions. These complex carbohydrates have several important properties:
- They are insoluble in water due to their large molecular size
- This insolubility makes them ideal for storage purposes
- When hydrolysed, they break down into disaccharides and eventually monosaccharides
The insolubility of polysaccharides is a key advantage for storage molecules - they don't affect the water potential of cells and can be packed efficiently without taking up excessive space.
Starch is a key example of a polysaccharide found in plants. It consists of between 200 and 100,000 α-glucose molecules joined by glycosidic bonds. Starch appears as small granules in plant cells, particularly in chloroplasts, and serves as an energy storage molecule.
Testing for starch
The iodine test provides a simple method for detecting starch. This test works at room temperature and produces a distinctive colour change.
Worked Example: Iodine Test for Starch
Step 1: Place 2 cm³ of the sample into a test tube (or add two drops to a spotting tile)
Step 2: Add two drops of iodine solution and shake or stir
Step 3 - Results:
- Positive result: The presence of starch is indicated by a blue-black coloration
- Negative result: No starch present if the solution remains yellow-orange
Key Points to Remember:
- Disaccharides form when two monosaccharides join via condensation reactions, creating glycosidic bonds
- Non-reducing sugars like sucrose must be hydrolysed before testing with Benedict's reagent
- Polysaccharides are insoluble storage molecules made from many monosaccharide units
- Starch gives a blue-black colour with iodine solution
- Hydrolysis breaks down complex carbohydrates by adding water, while condensation builds them up by removing water