Carbohydrates - Marks Scheme Answers ( Study Mind)

Question 1

Explain how a sports drink could provide an energy boost when running.

Answer 1

1. (Drink) contains carbohydrates / sugars so High GI / (drink) contains carbohydrates /
   sugars so raises blood glucose concentration quickly;
2. Contains salt so glucose more rapidly absorbed;
3. Increases glucose to muscles for respiration;
4. More / faster respiration so more / faster energy release;

Question 2

In humans, the enzyme maltase breaks down maltose to glucose.

This takes place at normal body temperature.

Explain why maltase:
• only breaks down maltose
• allows this reaction to take place at normal body temperature. (5)

Answer 2

1. Tertiary structure / 3D shape means:
2. Active site complementary to maltose / fits into active site of maltose.
3. Induced fit model (describe)
4. Enzyme is a catalyst by forming enzyme substrate complex.

Question 3

Scientists have investigated the effects of competitive and non-competitive
inhibitors of the enzyme maltase.

Describe competitive and non-competitive inhibition of an enzyme. (5)

Answer 3

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;

(Competitive inhibition),
2. Inhibitor similar shape (idea) to substrate;
3. (Binds) in to active site (of enzyme);
4. (Inhibition) can be overcome by more substrate;

(Non-competitive inhibition),

5. Inhibitor binds to site on enzyme other than active site;
6. Prevents formation of active site / changes (shape of) active site;
7. Cannot be overcome by adding more substrate;

Question 4

Monosaccharides and disaccharides taste sweet.
The lactose-free milk made after hydrolysis with lactase tastes sweeter than the
cow’s milk containing lactose.

Suggest why.

Answer 4

1. (Lactose hydrolysed to) galactose and glucose;
2. (So) more sugar molecules;
3. (So) more / different receptors stimulated / sugars produced are sweeter
   (than lactose).

Question 5

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 5

1. Heat with acid and neutralise;
2. Heat with Benedict’s (solution);
3. Red precipitate/colour;

Question 6

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

Answer 6

1. Filter and dry (the precipitate);
2. Find mass/weight;

Question 7

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 7

Similarity - Both contain galactose / a glycosidic bond;

Difference - Lactulose contains fructose, whereas lactose contains glucose;

Question 8

Glycogen and cellulose are both carbohydrates.

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

Answer 8

1. Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers);
2. Cellulose molecule has straight chain and glycogen is branched;
3. Cellulose molecule has straight chain and glycogen is coiled;
4. glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has
   only 1,4- glycosidic bonds;

Question 9

Starch is a carbohydrate often stored in plant cells.
Describe and explain two features of starch that make it a good storage
molecule. 2m

Answer 9

1. Insoluble (in water), so doesn’t affect water potential;
2. Branched / coiled / (α-)helix, so makes molecule compact;
3. Polymer of (α-)glucose so provides glucose for respiration;
4. Branched / more ends for fast breakdown / enzyme action;
5. Large (molecule), so can’t cross the cell membrane

Question 10

(a) Describe the structure of glycogen.

Answer 10

1. Polysaccharide of α-glucose;
   polymer of α-glucose;
2. (Joined by) glycosidic bonds

Question 11

Suggest how glycogen acts as a source of energy.

Do not include transport across membranes in your answer.

Answer 11

1. Hydrolysed (to glucose);
2. Glucose used in respiration;