Lesson 6 - Reducing and non-reducing sugar tests Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

Name 3 common monosaccharides

A

Glucose, galactose and fructose

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Monosaccharides are the monomers from which larger ____________ are made

A

Carbohydrates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What bond does a condensation reaction between 2 monosaccharides form?

A

A glycosidic bond

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Which monosaccharides are these disaccarides formed from?
Maltose
Sucrose
Lactose

A

Maltose - formed from the condensation of 2 glucose molecules
Sucrose - formed from the condensation of a glucose and fructose molecule
Lactose - formed from the condensation of a glucose and galactose molecule

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Which glucose isomers are these polysaccharides formed from the condensation of?
Glycogen & starch
Cellulose

A

Glycogen & starch - alpha glucose
Cellulose - beta glucose

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Why are reducing sugars sometimes called reducing agents?

A

Reducing sugars can donate electrons to reduce another chemical (e.g. in this case Benedict’s reagent)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Which sugars are/aren’t reducing sugars?

A

● ALL monosaccharides are reducing sugars - able to reduce (give electrons to) other molecules
● Some disaccharides are reducing sugars, including maltose & lactose
● Sucrose is not a reducing sugar, we call it a non-reducing sugar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

How can we check for the presence of reducing sugars?

A

● Benedict’s reagent is an alkaline 2+ solution of copper II sulphate (Cu2+ ions)
● When a reducing sugar is heated with
● When using Benedict’s reagent to carry out the reducing sugar test: If there is a colour change from blue to red(orange/yellow/green) there is a reducing sugar present.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What’s oxidised/reduced in Benedict’s test for reducing sugars?

A

The soluble, blue Cu2+ ions are reduced (accept electrons donated by the reducing sugar) and form insoluble, red Cu+ ions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Ionic equation for Benedict’s test for reducing sugars

A

Cu2+ + e- -> Cu+ (insoluble precipitate)
Blue Brick red

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Why can the results for Benedict’s test for reducing sugars be semi-quantitative?

A

You may get some other colours (green, yellow, orange) appear other than brick red, this is due to the concentration of reducing sugar found in the unknown

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Test for non-reducing sugars

A

● add Benedict’s reagent
● heat in water bath
● observe the result - negative (stays blue)
● in a new test tube, add the unknown sugar (that got the -ve result) and boil with HCl in a water bath
● add an alkali (sodium hydrogen carbonate) to neutralise
● add Benedict’s reagent & heat in water bath
● if present, there should be a colour change to brick red (or green, orange, brown) if a non-reducing sugar was present

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Results for glucose in reducing sugars test

A

Glucose showed a colour change from blue to brick red, producing a positive result. It turned red because Glucose is a reducing sugar (as all monosaccharides are reducing sugars), which can act as a reducing agent by donating an electron to the Cu2+ ions in the Benedict’s solution, converting them to Cu+ ions. These Cu+ ions then combine with Oxygen in the solution to form Copper Oxide (CuO), which is a red precipitate.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Results for sucrose in reducing sugars test

A

Sucrose did not show a colour change, it remained blue, showing a negative result. This is because Sucrose is not a reducing sugar, it is a non-reducing sugar.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Results for sucrose in non-reducing sugars test

A

Sucrose, produced a positive result in the non-reducing sugars test (hopefully!), as it turned from blue to brick red. This was because, in the non-reducing sugars test, we heat the sucrose with HCl, which resulted in the hydrolysis of the glycosidic bonds in sucrose, breaking sucrose down into its monomers - glucose and fructose. These are both reducing sugars, so when then heated with Benedict’s solution, produces a positive result (a colour change from blue to brick red). This shows the presence of a non-reducing sugar (sucrose) as before heating with HCl, there was no colour change (in the reducing sugars test).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly