Carbohydrates

Question 1

A biochemical test for reducing sugar produces a negative result with
raffinose solution.

Describe a biochemical test to show that raffinose solution contains a
non-reducing sugar.

Answer 1

Heat with acid and neutralise;

Heat with Benedict’s (solution);

Red precipitate/colour;

Question 2

A precipitate is produced in a positive result for reducing sugar in a
Benedict’s test.

A precipitate is solid matter suspended in solution.

A student carried out the Benedict’s test.

Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution.

Answer 2

Filter and dry (the precipitate)

Find mass/weight

Question 3

Use of a colorimeter in this investigation would improve the repeatability of
the student’s results.

Give one reason why

Answer 3

Quantitative (Colour change is) subjective;

Standardises (the) method;

Question 4

What is a monomer?

Answer 4

(a monomer is a smaller / repeating) unit / molecule from which larger
molecules / polymers are made

Question 5

Lactulose is a disaccharide formed from one molecule of galactose and
one molecule of fructose.

Other than both being disaccharides, give one similarity and one difference
between the structures of lactulose and lactose.

Answer 5

Similarity:
1. Both contain galactose / a glycosidic bond;

Difference:
2. Lactulose contains fructose, whereas lactose contains glucose;

Question 6

Glycogen and cellulose are both carbohydrates.

Describe two differences between the structure of a cellulose molecule
and a glycogen molecule.

Answer 6

Cellulose is made up of β-glucose (monomers) and glycogen is
made up of α-glucose (monomers);

Cellulose molecule has straight chain and glycogen is
branched;

Cellulose molecule has straight chain and glycogen is coiled;

glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds;

Question 7

Starch is a carbohydrate often stored in plant cells.

Describe and explain two features of starch that make it a good storage
molecule.

Answer 7

Insoluble (in water), so doesn’t affect water potential;

Branched / coiled / (α-)helix, so makes molecule compact;
OR
Branched / coiled / (α-)helix so can fit many (molecules) in
small area;

Polymer of (α-)glucose so provides glucose for respiration;

Branched / more ends for fast breakdown / enzyme action;

Large (molecule), so can’t cross the cell membrane

Question 8

What is the test which would be used to show the presence of starch?

Answer 8

Iodine/potassium iodide

Question 9

Describe the structure of glycogen

Answer 9

Polysaccharide of α-glucose;
OR
polymer of α-glucose;

(Joined by) glycosidic bonds
OR
Branched structure;

Question 10

Suggest how glycogen acts as a source of energy

Answer 10

Hydrolysed (to glucose);

Glucose used in respiration

Question 11

Name the monomers from which a maltose molecule is made

Answer 11

Glucose (and glucose)

Question 12

Name the type of chemical bond that joins the two monomers to form
maltose

Answer 12

(a1,4) Glycosidic

Question 13

Starch and cellulose are two important plant polysaccharides

Starch molecules and cellulose molecules have different functions in plant
cells. Each molecule is adapted for its function.

Explain one way in which starch molecules are adapted for their function in
plant cells

Answer 13

Insoluble;

Don’t affect water potential;

Helical;

Compact;

Large molecule;

Cannot leave cell.

Question 14

Explain how cellulose molecules are adapted for their function in plant
cells.

Answer 14

Long and straight chains;

Become linked together by many hydrogen bonds to form
fibrils;

Provide strength (to cell wall).