Carbohydrates (reducing + Non Reducing Sugars)

Question 1

Describe the reducing sugars test

Answer 1

• Add Benedict’s reagent (blue) to your solution and place in a water bath (>80 Celsius)
• leave in water bath for approximately 5 minutes
• After the 5 minutes, if a brick-red precipitate is present then a reducing sugar is present

Question 2

Why do you get a brick-red precipitate when test is positive

Answer 2

• The reducing sugars have the ability to reduce the Benedict’s reagent as they can donate an electron.
• The reducing monosaccharides donate electrons to the Cu2+ ions (which are blue). This causes them to be reduced (gain electron) to form Cu+ ion (which is a solid red precipitate)

Question 3

Describe the test for a non reducing sugar (sucrose)

Answer 3

• Add acid (HCl) to the sample you took from the negative result and heat in a boiling water bath for approximately 2 minutes.
• Remove from water bath and add Alkali (sodium hydroxide) until fizzing/bubbling stops so you know the solution is neutralised.
  -Add some Benedict’s reagent to the solution (turns blue) and place back into the water bath (>80 C)
• After 5 minutes remove from water bath and a solid red precipitate represents a positive test.

Question 4

Why did the non reducing sugar not give a positive result to the reducing sugars test?

Answer 4

• Non reducing sugars, such as sucrose, do not have the ability to donate an electron
• 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 5

What monosaccharides make up lactose

Answer 5

-Galactose
-Alpha glucose

Question 6

What monosaccharides make up maltose

Answer 6

• 2 Alpha glucose

Question 7

What Monosaccharides make up sucrose

Answer 7

• Fructose
• Alpha glucose

Question 8

What bond forms between two monosaccharides (sugars)?

Answer 8

Glycosidic bonds

Question 9

Define a hydrolysis reaction

Answer 9

When you add water to a chemical reaction to help break the bonds (glycosidic) and form 2 smaller molecules.

Question 10

Define a condensation reaction

Answer 10

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

Question 11

What is an isomer?

Answer 11

Molecules that have the same chemical formula but a different arrangement of atoms

Question 12

What are the 3 types of polysaccharides

Answer 12

• starch
• glycogen
• cellulose

Question 13

Where is Starch found

Answer 13

• Found in plant cells in the form of granules

Question 14

What is the function of starch?

Answer 14

Energy storage in plants

Question 15

What monomers join to form starch?

Answer 15

• Alpha glucose

Question 16

What are the 2 different structures of starch?

Answer 16

• coiled e.g amylose
• branched e.g amylopectin

Question 17

Why are some starch molecules coiled?

Answer 17

• they form more 1- 4 glycosidic bonds

Question 18

Why are some starch molecules branched?

Answer 18

• they form some 1 - 6 glycosidic bonds

Question 19

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

Answer 19

• coiled, compact so good for storage
• branched, faster hydrolysis of the alpha glucose molecules for respiration
• Large + insoluble, doesn’t diffuse out of cells or affect water potential

Question 20

Where is glycogen found?

Answer 20

Found in animal cells (muscles / liver)

Question 21

What monomers form glycogen?

Answer 21

Alpha glucose

Question 22

What’s the structure of glycogen?

Answer 22

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

Question 23

What is glycogen’s main function?

Answer 23

• energy storage in animals

Question 24

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

Answer 24

Branched - faster hydrolysed of glucose molecules, helps for more efficient respiration
Coiled - compact so good for storage
Large insoluble - don’t diffuse out of cells / affect water potential

Question 25

What is the function of cellulose?

Answer 25

Structural role ( provides strength) in plant cell walls

Question 26

What monomers join to form cellulose?

Answer 26

Beta glucose

Question 27

What structure does cellulose have and why?

Answer 27

It forms straight up branched chains because every other beta glucose molecule is rotated 180*

The long chains run parallel to one other and and a hydrogen bond forms between the OH groups that stick out (cross linkages) which form microfibrils / macrofibrils / cellulose fibres.

Question 28

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

Answer 28

The hydrogen bonds between the chains make cellulose strong
They aid the rigidity and turgidity of the plant to help maximise photosynthesis

Question 29

How do you test for starch?

Answer 29

• add iodine in a potassium iodide solution
• shake / mix
• if starch is present, solution will change from orange to black/blue

Question 30

What is the General formula for carbohydrates

Answer 30

Cn (H20)n