Carbohydrate (reducing/nonreducing Sugars, Disaccharide And Polysaccharides)

Question 1

Describe the reducing sugars test?

Answer 1

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

Question 2

Why do you get a brick red solution when the test is positive?

Answer 2

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)

Question 3

Describe the non reducing sugar test?

Answer 3

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

Question 4

Give an example of a non reducing sugar.

Answer 4

Sucrose - made from alpha glucose and fructose

Question 5

Why didn’t the non reducing sugar give a positive test for the reducing sugar test?

Answer 5

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)

Question 6

What monosaccharides make up lactose?

Answer 6

Alpha glucose and galactose

Question 7

What two monosaccharides make up maltose?

Answer 7

Two alpha glucose molecules

Question 8

What bond forms between two monosaccharides (sugars)?

Answer 8

Glycosidic bond

Question 9

Define the term isomer?

Answer 9

Organic molecules that have the same chemical formula but a different structural formula

Question 10

What do we mean when we say the Benedict’s test for reducing sugars is ‘semi-quantitative’?

Answer 10

That the colour of the result can be used to estimate the approximate amount of reducing sugar in a sample

Question 11

During a nonreducing sugar test why do you add alakali after heating the food sample with acid?

Answer 11

The alkaline helps neutralise the solution so the Benedict’s reagent can work

Question 12

What is a hydrolysis reaction?

Answer 12

When water is added to a reaction to help break the bonds (glycosidic in this case)

Question 13

What happens in a condensation reaction?

Answer 13

As the two monosaccharides join together to form a glycosidic bond water is produced/released

Question 14

What elements make up carbohydrates?

Answer 14

Carbon
Hydrogen
Oxygen

Question 15

A 3 carbon sugar is called?

Answer 15

Triose
C3H603

Question 16

What’s an example of a triose sugar?

Answer 16

Glyceraldehyde which is used in the glycolysis stage of respiration

Question 17

What’s a 4 carbon sugar called?

Answer 17

Tetrose
C4H8O4

Question 18

What’s a 5 carbon sugar called?

Answer 18

Pentose
C5H10O5

Question 19

What’s an example of a pentose sugar?

Answer 19

Ribose and deoxyribose in DNA/RNA

Question 20

What’s the sugar called with 6 carbons?

Answer 20

Hexose
C6H12O6

Question 21

What’s an example of a hexose sugar?

Answer 21

Glucose (alpha and beta)
- storage molecules

Question 22

What’s the general chemical formula of a monosaccharide?

Answer 22

Cn H2n On

Question 23

Define a hydrolysis reaction?

Answer 23

When you add water to a chemical reaction to help break the bonds (glycosidic) and form two smaller molecules

Question 24

Define a condensation reaction?

Answer 24

When a chemical reaction realises water as a by-product of creating bonds

Question 25

What are the three different polysaccharides?

Answer 25

Starch
Glycogen
Cellulose

Question 26

Where is starch found?

Answer 26

It’s found in plants in the form of grains and granules

Question 27

What is the function of starch?

Answer 27

Energy storage in plants

Question 28

What monomers join in a condensation reaction to form starch?

Answer 28

Alpha Glucose

Question 29

What are the two different structures of starch?

Answer 29

Coiled - eg. Amylose
Branched - eg. Amylopectin

Question 30

Why are some starch molecules coiled?

Answer 30

They form more 1-4 glycosidic bonds

Question 31

Why are some starch molecules branched?

Answer 31

They form some 1-6 glycosidic bonds

Question 32

How does starch’s structure make it better adapted for its function?

Answer 32

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)

Question 33

Where is glycogen found?

Answer 33

It’s found in animal cells (muscle/liver)

Question 34

What monomers join to form glycogen?

Answer 34

Alpha glucose

Question 35

What’s the structure of glycogen?

Answer 35

It has a similar structure to amylopectin but it’s more branched due to the 1-6 glycosidic bonds

Question 36

What is glycogens main function?

Answer 36

Energy storage in animals

Question 37

How does glycogens structure make it better adapted for its function?

Answer 37

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

Question 38

What’s the function of cellulose?

Answer 38

Structural role (provides strength) in plant cell walls

Question 39

What monomers join to form cellulose?

Answer 39

Beta Glucose

Question 40

What structure does cellulose have and why?

Answer 40

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

Question 41

How does cellulose’s structure make it better adapted for its function?

Answer 41

The hydrogen bonds (cross-links) between the chains make cellulose strong.
They aid the rigidity and turgidity of the plant to help maximise photosynthesis

Question 42

How do you test for starch?

Answer 42

• Add iodine in potassium iodide solution
• Shake/stir
• If starch is present the solution will turn from orange to blue/black