Carbohydrate (reducing/nonreducing Sugars, Disaccharide And Polysaccharides) Flashcards
Describe the reducing sugars test?
Take sample of solution and add equal volume of Benedicta reagent (blue)
Heat in water bath (70-90*C) for 5 minutes
If the solution turns a brick red colour reducing sugars are present
Why do you get a brick red solution when the test is positive?
The reducing sugars have the ability to reduce the Benedicts reagant by donating electrons.
The reducing monosaccharides donate electrons to the Cu2+ ions (which are blue). This causes them to be reduced (gain electron) to form a Cu+ ion (which is a solid red precipitate)
Describe the non reducing sugar test?
First take a clean/new sample from the solution which gave you a negative result in the reducing sugars test.
Add a few drops of acid (HCL) to test tube and heat in boiling water bath for 2 minutes
Remove from water bath and add alkali (sodium hydroxide) solution until fizzing stops (know solution has neutralised) and Benedict’s reagent before heating in water bath (70-90*C) for 5 minutes.
You should get a positive result (brick red precipitate) as the disaccharide is broken down into its constituent reducing monosaccharides
Give an example of a non reducing sugar.
Sucrose - made from alpha glucose and fructose
Why didn’t the non reducing sugar give a positive test for the reducing sugar test?
Non- reducing sugars such as sucrose do not have the ability to donate electron (reduce other substances). However in the non reducing sugar test hydrolysis occurs and breaks the glycosidic bond between the two reducing monosaccharides (alpha glucose and fructose). These monosaccharides have the ability to reduce the Cu2+ ion to form the Cu+ ion (solid red precipitate)
What monosaccharides make up lactose?
Alpha glucose and galactose
What two monosaccharides make up maltose?
Two alpha glucose molecules
What bond forms between two monosaccharides (sugars)?
Glycosidic bond
Define the term isomer?
Organic molecules that have the same chemical formula but a different structural formula
What do we mean when we say the Benedict’s test for reducing sugars is ‘semi-quantitative’?
That the colour of the result can be used to estimate the approximate amount of reducing sugar in a sample
During a nonreducing sugar test why do you add alakali after heating the food sample with acid?
The alkaline helps neutralise the solution so the Benedict’s reagent can work
What is a hydrolysis reaction?
When water is added to a reaction to help break the bonds (glycosidic in this case)
What happens in a condensation reaction?
As the two monosaccharides join together to form a glycosidic bond water is produced/released
What elements make up carbohydrates?
Carbon
Hydrogen
Oxygen
A 3 carbon sugar is called?
Triose
C3H603
What’s an example of a triose sugar?
Glyceraldehyde which is used in the glycolysis stage of respiration
What’s a 4 carbon sugar called?
Tetrose
C4H8O4
What’s a 5 carbon sugar called?
Pentose
C5H10O5
What’s an example of a pentose sugar?
Ribose and deoxyribose in DNA/RNA
What’s the sugar called with 6 carbons?
Hexose
C6H12O6
What’s an example of a hexose sugar?
Glucose (alpha and beta)
- storage molecules
What’s the general chemical formula of a monosaccharide?
Cn H2n On
Define a hydrolysis reaction?
When you add water to a chemical reaction to help break the bonds (glycosidic) and form two smaller molecules
Define a condensation reaction?
When a chemical reaction realises water as a by-product of creating bonds
What are the three different polysaccharides?
Starch
Glycogen
Cellulose
Where is starch found?
It’s found in plants in the form of grains and granules
What is the function of starch?
Energy storage in plants
What monomers join in a condensation reaction to form starch?
Alpha Glucose
What are the two different structures of starch?
Coiled - eg. Amylose
Branched - eg. Amylopectin
Why are some starch molecules coiled?
They form more 1-4 glycosidic bonds
Why are some starch molecules branched?
They form some 1-6 glycosidic bonds
How does starch’s structure make it better adapted for its function?
Coiled - compact so good for storage
Branched - faster hydrolysis of glucose molecules (the branched ends can be simaltaneously worked on by enzymes) for respiration
Large and insoluble- doesn’t diffuse out of cells or affect water potential (osmotically inactive)
Where is glycogen found?
It’s found in animal cells (muscle/liver)
What monomers join to form glycogen?
Alpha glucose
What’s the structure of glycogen?
It has a similar structure to amylopectin but it’s more branched due to the 1-6 glycosidic bonds
What is glycogens main function?
Energy storage in animals
How does glycogens structure make it better adapted for its function?
Branched - faster hydrolysis of glucose molecules releases alpha glucose for respiration
Large and insoluble so doesn’t dissolve out of cells /affect water potential
Coiled - - compact so good for storage
What’s the function of cellulose?
Structural role (provides strength) in plant cell walls
What monomers join to form cellulose?
Beta Glucose
What structure does cellulose have and why?
It forms straight, unbranched chains because every other beta glucose molecule is rotated 180*
The long chains run parallel to one another and a hydrogen bond forms between the OH groups which stick out (cross-linkages) which form microfibrils/macrofibrils/cellulose fibres
How does cellulose’s structure make it better adapted for its function?
The hydrogen bonds (cross-links) between the chains make cellulose strong.
They aid the rigidity and turgidity of the plant to help maximise photosynthesis
How do you test for starch?
- Add iodine in potassium iodide solution
- Shake/stir
- If starch is present the solution will turn from orange to blue/black
