Required practical: biological molecules

Question 1

Reducing sugars meaning

Answer 1

A sugar that acts as a reducing agent
Eg sugar loses electron(s) that other atoms gain and are thus reduced

Question 2

Test for reducing sugar specifically

Answer 2

Add transparent blue Benedict’s solution to a solution of sample
Place in boiling water bath
A change to form a green/yellow/ red precipitate if present

Question 3

What happens depending on how much reducing sugar is present in the sample?

Answer 3

More precipitate (insoluble solid) formed the more sugar
Red colour if a lot present
Yellow if medium
Green if not much

Question 4

Testing glucose with benedicts

Answer 4

Transparent blue ——> red precipitate
Glucose is reducing sugar

Question 5

Testing sucrose with Benedict’s

Answer 5

No colour change
Sucrose is not reducing sugar

Question 6

Second test for sucrose

Answer 6

Using hydrochloric acid to hydrolyse sucrose into glucose and fructose
Then increase pH by adding sodium carbonate as Benedict’s works in alkaline environments (add Benedict’s)
Water bath

Question 7

Results for second sucrose test

Answer 7

Colour change to brick red + precipitate formed showing glucose and fructose are reducing sugars

Question 8

Test for starch

Answer 8

A few drops of iodine in potassium iodide solution to sample
If present colour change from brown to blue/black

Question 9

Test for lipids

Answer 9

Unfiltered sample of oil or ground fats in test tubes
Add 3cm3 of ethanol and mix well
Add water and shake
If lipids present = white cloudy emulsion formed

Question 10

Hazard for lipids test

Answer 10

Ethanol = highly flammable so do away from fire

Question 11

Biuret test

Answer 11

Dissolve a sample of protein in water in a test tube
Add a drop of biuret regent
There should be a colour change from blue to violet

Question 12

What is biuret regent made of?

Answer 12

sodium hydroxide, hydrated copper (II) suphate and sodium tartrate