Monosaccharides and Disaccharides
Monosaccharides are monomers from which larger carbohydrates can be made
The three most common monosaccharides are glucose, fructose and galactose
Disaccharide - 2 monosaccharides joined together, formed by a condensation reaction to make a
glycosidic bond
Disaccharide Monosaccharides Found In
Sucrose Glucose + Fructose Sugar beet
Sugar Cane
Lactose Glucose + Galactose Milk
Maltose Glucose + Glucose Barley
The formation of maltose creates a 1,4 glycosidic bond because it occurs between
the 1C on the left and the 4C on the right
Reducing Sugars:
Can donate electrons (reduce another chemical)
Monosaccharides and some disaccharides
Non-reducing Sugars:
Cannot donate electrons
Polysaccharides and some disaccharides
Testing for Reducing Sugars
When heated with Benedict's Reagent, reducing sugars will reduce blue alkaline copper (II)
sulphate, to red copper (I) oxide
Semi-qualitative test - not just positive or negative, as it gives an approximation of the
concentration of reducing sugar
Method
1. Add 2cm3 of the food sample to test tube (if solid, grind up with water)
2. Add equal volume of Benedict's Reagent
3. Heat mixture in gently boiling water for 5 minutes
Testing for Non-Reducing Sugars
Non-reducing sugars don't change the colour of Benedict's Reagent
You must hydrolyse the sugar into its monosaccharide components (monomers), by
hydrolysis
Method
1. Complete Benedict Reagent test and have a blue, no change result to prove that a reducing
sugar is not present
2. Add 2cm3 of the food sample to 2cm3 of dilute HCl ion test tube and place in gently boiling
water for 5 minutes (the HCl will hydrolyse any disaccharides present
, 3. Neutralise the acid with sodium hydrogen carbonate as Benedict's reagent will not work in
acidic conditions
4. Test solution with pH paper to check it is alkaline
5. Re-test with 2cm3 Benedict's Reagent with the original steps
6. If non-reducing sugar was present - orange/brown
This is due to the reducing sugars that were produced from the hydrolysis of the non-
reducing sugar
Colorimeter
A beam of light is transmitted through the sample that is being tested to a sensor
The percentage of light absorbed which by the sensor will indicate the concentration of
sugar in the sample
Qualitive data
Polysaccharides
Polysaccharides
Formed by condensation polymerisation, by combining monosaccharide molecules together
forming glycosidic bonds
Starch
Made of chains of a-glucose monosaccharides
Branched or Unbranched
Unbranched chains form coils
Glycosidic Bonds - formed by condensation reactions
Found in seeds and storage organs (e.g. potato tubers)
Energy Storage
How is the structure of Starch suited to its function?
Insoluble - does not affect water potential (osmosis)
Large and Insoluble - does not diffuse out of cells
Compact - fits in small spaces
When hydrolysed, they form a-glucose which is easily transported and used in respiration
Branched form - has many ends which can each be acted on simultaneously so glucose
monomers are released rapidly
Cellulose
Made of chains of b-glucose monosaccharides (condensation reaction)
Straight, unchained branches
Each chain runs parallel to one another allowing hydrogen bonds to form cross linkages
Large numbers of hydrogen bonds increase the strength of the material (despite individual
hydrogen bonds adding very little strength)
Molecules group to form microfibrils which arrange in parallel groups to form fibres
Used in plants for structural support
How is the structure of cellulose suited to its function?
B-glucose - creates long, straight unbranched chains
Parallel and hydrogen bonds - build strength
Monosaccharides are monomers from which larger carbohydrates can be made
The three most common monosaccharides are glucose, fructose and galactose
Disaccharide - 2 monosaccharides joined together, formed by a condensation reaction to make a
glycosidic bond
Disaccharide Monosaccharides Found In
Sucrose Glucose + Fructose Sugar beet
Sugar Cane
Lactose Glucose + Galactose Milk
Maltose Glucose + Glucose Barley
The formation of maltose creates a 1,4 glycosidic bond because it occurs between
the 1C on the left and the 4C on the right
Reducing Sugars:
Can donate electrons (reduce another chemical)
Monosaccharides and some disaccharides
Non-reducing Sugars:
Cannot donate electrons
Polysaccharides and some disaccharides
Testing for Reducing Sugars
When heated with Benedict's Reagent, reducing sugars will reduce blue alkaline copper (II)
sulphate, to red copper (I) oxide
Semi-qualitative test - not just positive or negative, as it gives an approximation of the
concentration of reducing sugar
Method
1. Add 2cm3 of the food sample to test tube (if solid, grind up with water)
2. Add equal volume of Benedict's Reagent
3. Heat mixture in gently boiling water for 5 minutes
Testing for Non-Reducing Sugars
Non-reducing sugars don't change the colour of Benedict's Reagent
You must hydrolyse the sugar into its monosaccharide components (monomers), by
hydrolysis
Method
1. Complete Benedict Reagent test and have a blue, no change result to prove that a reducing
sugar is not present
2. Add 2cm3 of the food sample to 2cm3 of dilute HCl ion test tube and place in gently boiling
water for 5 minutes (the HCl will hydrolyse any disaccharides present
, 3. Neutralise the acid with sodium hydrogen carbonate as Benedict's reagent will not work in
acidic conditions
4. Test solution with pH paper to check it is alkaline
5. Re-test with 2cm3 Benedict's Reagent with the original steps
6. If non-reducing sugar was present - orange/brown
This is due to the reducing sugars that were produced from the hydrolysis of the non-
reducing sugar
Colorimeter
A beam of light is transmitted through the sample that is being tested to a sensor
The percentage of light absorbed which by the sensor will indicate the concentration of
sugar in the sample
Qualitive data
Polysaccharides
Polysaccharides
Formed by condensation polymerisation, by combining monosaccharide molecules together
forming glycosidic bonds
Starch
Made of chains of a-glucose monosaccharides
Branched or Unbranched
Unbranched chains form coils
Glycosidic Bonds - formed by condensation reactions
Found in seeds and storage organs (e.g. potato tubers)
Energy Storage
How is the structure of Starch suited to its function?
Insoluble - does not affect water potential (osmosis)
Large and Insoluble - does not diffuse out of cells
Compact - fits in small spaces
When hydrolysed, they form a-glucose which is easily transported and used in respiration
Branched form - has many ends which can each be acted on simultaneously so glucose
monomers are released rapidly
Cellulose
Made of chains of b-glucose monosaccharides (condensation reaction)
Straight, unchained branches
Each chain runs parallel to one another allowing hydrogen bonds to form cross linkages
Large numbers of hydrogen bonds increase the strength of the material (despite individual
hydrogen bonds adding very little strength)
Molecules group to form microfibrils which arrange in parallel groups to form fibres
Used in plants for structural support
How is the structure of cellulose suited to its function?
B-glucose - creates long, straight unbranched chains
Parallel and hydrogen bonds - build strength
