A-level Biology: Carbohydrates

Define Monomer.

Define Monomer.

A smaller unit/molecule from which larger molecules/polymers are made.

Define Polymer.

A larger molecule made up of many smaller/repeating monomer units.

Define Monosaccharide.

A single sugar molecule (e.g. Glucose).

Define Disaccharide.

Two monosaccharides bonded together by a glycosidic bond formed in a condensation reaction.

Define Polysaccharide.

Many sugar units bonded together by glycosidic bonds formed in condensation reactions.

What Is A Glycosidic Bond?

A bond formed between two monosaccharides in a condensation reaction.

What Is A Condensation Reaction And What Does It Result In?

A condensation reaction is where two monomers are joined together and water is expelled. A glycosidic bond is formed.

How Can You Test For Reducing Sugars?

Using Benedict’s reagent.

Give The Steps For The Reducing Sugars Test.

1. Add 2cm^2 of a liquidated food sample in a test tube. If it isn’t liquid, grind the sample until it is.

2. Add an equal volume of Benedict’s reagent into the same test tube.

3. Gently heat the mixture in a boiling water bath for 5 minutes.

What Is The Positive Result For A Reducing Sugar?

The solution/mixture changes from blue to either brick-red.

What Do The Different Results In The Reducing Sugars Test (Green, Yellow, Orange, Red) Indicate?

Green: Very little concentration of R.Ss present.

Yellow: A low concentration of R.Ss present.

Orange: A moderate concentration of R.Ss present.

Brick-Red: A high concentration of R.Ss present.

What Is A Reducing Sugar?

A sugar that can donate electrons to/reduce another chemical.

What Disaccharide Does Glucose + Glucose Form?

Maltose (+water)

What Disaccharide Does Glucose + Fructose Form?

Sucrose (+water)

What Disaccharide Does Glucose + Galactose Form?

Lactose (+ water)

What Is Hydrolysis?

When a glycosidic bond is broken by adding water.

What Is A Non-Reducing Sugar?

Disaccharides such as sucrose that cannot change the colour (reduce) Benedict’s reagent.

How Can Non-Reducing Sugars Be Tested For Using Benedict’s Reagent If They Cannot Reduce It?

The non-reducing sugars must first be broke down into their individual monosaccharides, which can reduce Benedict’s reagent and therefore produce colour change.

Give The Steps For The Non-Reducing Sugars Test.

1. First, conduct the test for reducing sugars. If there’s no colour change, then test for non-reducing sugars.

2. In another test tube, add 2cm^3 of the same sample alongside 2cm^3 of dilute hydrochloric acid. Heat the test tube in a gently boiling water bath for 5 minutes.

3. Remove from the water bath, and then slowly add sodium hydrogen-carbonate to neutralise HCl. Use a pH paper to test if alkaline.

4. Add 2cm^3 of Benedict’s reagent to the test tube to re-test the sample, then heat again for 5 minutes.

5. Positive result: blue to orange-red.

Why Is Sodium Hydrogen-Carbonate Added In The Non-Reducing Sugars Test?

To neutralise the dilute hydrochloric acid.

Why Is Dilute Hydrochloric Acid Added In The Non-Reducing Sugars Test?

To hydrolyse the non-reducing disaccharides into their monosaccharides (reducing sugars).

Why Are Polysaccharides Insoluble?

Because they are very large molecules.

Why Are Polysaccharides Good Storage Molecules?

Because they are insoluble, and so don’t affect the water potential of the cell.

They can also be compact and fit more in small spaces.

Give Examples Of Polysaccharides.

Starch, cellulose, glycogen.

Outline The Test For Starch.

1. Place 2cm^3 of the sample being tested into a test tube.

2. Add two drops of iodine solution and shake or stir the solution.

3. If starch is present, the solution will turn blue-black. If negative, no colour change will take place.