Biochemical Test for Molecules

Question 1

What is a reducing sugar?

Answer 1

A sugar that acts as a reducing agent because of its aldehyde or ketone group. Examples: all monosaccharides and some disaccharides (e.g., maltose, lactose).

Question 2

How do you test for reducing sugars?

Answer 2

1. Add Benedict’s reagent to the sample.
2. Heat the mixture in a water bath.
3. Observe the color change:
   Blue → No reducing sugar.
   Green/yellow → Low concentration.
   Orange/brick red → High concentration.

Question 3

What do green/yellow/orange/brick red precipitates indicate in a Benedict’s test?

Answer 3

The presence of reducing sugar, with colour intensity increasing with concentration.

Question 4

What does a negative Benedict’s test indicate?

Answer 4

The sample may contain a non-reducing sugar (e.g., sucrose).

Question 5

How do you test for non-reducing sugars?

Answer 5

Hydrolyse the sample by boiling with hydrochloric acid (breaks glycosidic bonds).
Neutralise the solution using sodium hydrogen-carbonate.
Perform the Benedict’s test again.
Positive result (colour change) indicates non-reducing sugar was present.

Question 6

Why does the hydrolysis step in the non-reducing sugar test work?

Answer 6

It breaks sucrose into glucose and fructose, which are reducing sugars that give a positive Benedict’s test.

Question 7

What is starch?

Answer 7

Starch is the storage carbohydrate in plant cells.

Question 8

What reagent is used to test for starch?

Answer 8

Iodine dissolved in potassium iodide solution.

Question 9

How do you perform a starch test?

Answer 9

Add iodine dissolved in potassium iodide solution to the sample.
Observe any colour change.

Question 10

What is a positive result for starch?

Answer 10

The solution changes from orange/brown to blue/black.

Question 11

Which type of cells typically contain starch?

Answer 11

Plant cells, such as those in potatoes.

Question 12

What is the Emulsion test used for?

Answer 12

The Emulsion test is used to detect the presence of lipids in a sample.

Question 13

How do you perform the Emulsion test for lipids?

Answer 13

Add ethanol to the sample and shake thoroughly.
Add an equal volume of water to the mixture.

Question 14

What indicates a positive result for lipids in the Emulsion test?

Answer 14

A milky white suspension forms if lipids are present.

Question 15

What does it mean if the solution remains colorless in the Emulsion test?

Answer 15

No lipids are present in the sample.

Question 16

What is the Biuret test used for?

Answer 16

The Biuret test is used to detect the presence of proteins in a sample.

Question 17

What reagents are used in the Biuret test?

Answer 17

Sodium hydroxide (NaOH) and copper sulfate (CuSO₄) are used, together known as Biuret reagent.

Question 18

How do you perform the Biuret test for proteins?

Answer 18

Add sodium hydroxide to the sample.
Add copper sulphate solution.
Observe the colour change.

Question 19

What indicates a positive result for proteins in the Biuret test?

Answer 19

A colour change from blue to purple indicates the presence of proteins.

Question 20

What does a blue colour in the Biuret test indicate?

Answer 20

A negative result, meaning no proteins are present.

Question 21

What is a biosensor?

Answer 21

A biosensor is an analytical device used to detect the presence of a chemical by using biological molecules to produce a chemical signal, which is then converted into an electrical signal by a transducer.

Question 22

How does a biosensor work?

Answer 22

Biological molecules detect the chemical of interest.
The chemical reaction produces a signal.
The signal is converted into an electrical signal by a transducer.
The electrical signal is processed to determine the concentration of the chemical.

Question 23

What is an example of a biosensor?

Answer 23

A glucose biosensor, used to measure glucose concentration in a solution.

Question 24

How does a glucose biosensor work?

HINT: Good Elephants Taste Everything

Answer 24

G = Glucose oxidase (enzyme) catalyses oxidation
E = Electrons are created (oxidation creates charge at electrodes)
T = Transducer (electrodes convert charge into signal)
E = Electrical signal processed to determine glucose concentration

Question 25

What is the role of glucose oxidase in a glucose biosensor?

Answer 25

Glucose oxidase catalyses the oxidation of glucose, generating a charge that can be detected by the electrodes.