1.3 Carbohydrates - disaccharides and polysaccharides Flashcards
Formation of maltose
glucose + glucose → maltose
Formation of sucrose
glucose + fructose → sucrose
Formation of lactose
glucose + galactose → lactose
Formation of named disaccharides
glucose + glucose → maltose
glucose + fructose → sucrose
glucose + galactose → lactose
What type of reaction joins two monosaccharides?
Condensation reaction
condensation reaction
the joining of two monomers, producing a polymer and a molecule of water
What type of reaction splits a disaccharide?
Hydrolysis reaction
hydrolysis reaction
the breakdown of a polymer into monomers, using water
What is the bond between two monosaccharides?
Glycosidic bond
Test for non-reducing sugars
- Add 2cm³ of food sample and 2cm³ Benedict’s reagent to a test tube and filter.
- Place test tube into a boiling water bath for 5 minutes. If solution remains blue, no reducing sugar is present.
- Add 2cm³ food sample to 2cm³ hydrochloric acid in a test tube, and place in boiling water bath for 5 minutes. Dilute hydrochloric acid hydrolyses disaccharides into monosaccharides.
- Add sodium hydrogencarbonate solution to test tube to neutralise the hydrochloric acid. Test with pH paper to check solution is alkaline.
- Re-test solution by adding 2cm³ Benedict’s reagent and heating in boiling water bath for 5 minutes.
- If non-reducing sugar was present in first sample, solution will now change colour from blue to orange-brown.
Why is hydrochloric acid added in test for non-reducing sugars?
The acid hydrolyses the glycosidic bond between disaccharides, and forms substituent monosaccharides.
Why is aqueous sodium hydrogencarbonate added when testing for non-reducing sugars?
It neutralises the hydrochloric acid, as Benedict’s reagent cannot work under acidic conditions.
Are polysaccharides soluble?
No, they are too large.
Why are polysaccharides ideal storage molecules?
They are insoluble.
Where is starch found?
Plants, for example starch grains and chloroplasts.
