1.2-Carbohydrates

Question 1

Explain the difference in the structure of the starch molecule and the cellulose
molecule shown in the diagram above. (2)

Answer 1

1. Starch formed from α-glucose but cellulose formed from β-glucose;
2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.

Question 2

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. (2(

Answer 2

1. Insoluble;
2. Don’t affect water potential;
3. Helical;
   Accept form spirals
4. Compact;
5. Large molecule;
6. Cannot leave cell.

Question 3

Explain how cellulose molecules are adapted for their function in plant cells. (3)

Answer 3

1. Long and straight chains;
2. Become linked together by many hydrogen bonds to form fibrils;
3. Provide strength (to cell wall).

Question 4

Give two ways in which the structure of starch is similar to cellulose. (2)

Answer 4

1. Are polymers / polysaccharides / are made of monomers
2. Contain glucose / carbon, hydrogen and oxygen;
3. Contain glyosidic bonds;
4. Hydrogen bonding (within structure).
5. 1-4 links

Question 5

Give two ways in which the structure of starch is different from cellulose. (2)

Answer 5

(Starch)
1. Contains α / alpha glucose;

2. Helical / coiled / compact / branched / not straight;
3. Glucoses / monomers same way up;
4. No H-bonds between molecules;
5. No (micro / macro) fibres / fibrils.

Question 6

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. (2)

Answer 6

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

Question 7

Maltose is a sugar . Describe how a molecule of maltose is formed (3)

Answer 7

1. two molecules of alpha glucose
2. are joined by glycosidic bond
3. a molecule of water is realsed as condensation takes place

Question 8

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 8

1. Tertiary structure / 3D shape of enzyme (means);
   Accept references to active site
2. Active site complementary to maltose / substrate / maltose fits into active
   site / active site and substrate fit like a lock and key;
   Idea of shapes fitting together
3. Description of induced fit;
4. Enzyme is a catalyst / lowers activation energy / energy required for reaction;
   Accept “provides alternative pathway for the reaction at a
   lower energy level”
5. By forming enzyme-substrate complex;
   Accept idea that binding stresses the bonds so more easily
   broken

Question 9

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 9

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of
   ES complex;
   Max 3 if only one type of inhibition dealt with. Accept maltase
   and maltose as examples of enzyme and substrate (and
   others)
   Only once, for either inhibitor
   (Competitive inhibition),
2. Inhibitor similar shape (idea) to substrate;
3. (Binds) in to active site (of enzyme);
   Accept allows max rate of reaction to be reached / max
   product will eventually be formed
   Accept complementary to active site
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;
   Accept does not allow max rate of reaction to be reached /
   max product will not be formed
7. Cannot be overcome by adding more substrate;

Question 10

Explain how a sports drink could provide an energy boost when running. (3)

Answer 10

(Drink) contains carbohydrates / sugars so High GI / (drink) contains carbohydrates /
sugars so raises blood glucose concentration quickly;
Each alternative requires both aspects for credit
The second alternative requires a reference to speed eg
‘quickly’ or ‘immediately’

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 11

Sugars can be classed as reducing or non reducing .
Describe the test used to identify a non reducing sugar . Include the different results you would expect to see if the test was positive or negative (5)

Answer 11

Take a new sample of test solution , add dilute HCL and heat in water bath thats been brought to the boil

Neutralise with sodium hydrogencarbonate

then add blue blue benedict solution and heat it in a water bath thats been brought to boil

positve test would be brick red prepcipitate

negative woud be blue

Question 12

Explain one way in which the structure of cellulose is linked to its function. (2)

Answer 12

1. H-bonds / micro / macro fibrils / fibres;
   Reject strong hydrogen bonds
2. Strength / rigidity / inelasticity;
   ‘Strong hydrogen bonds’ = 0 but ‘Strong hydrogen bonds give
   strength (to the molecule)’ = 1

Question 13

(a) Starch and protein are biologically important polymers.

(i) Explain what is )meant by a polymer. (1

Answer 13

(a) (i) (Molecule) made up of many identical / similar molecules / monomers /

subunits;
Not necessary to refer to similarity with monomers.

Question 14

In an investigation, the enzyme amylase was mixed in a test tube with a buffer solution
and a suspension of starch. The amylase broke down the starch to maltose. When all
the starch had been broken down, a sample was removed from the test tube and tested
with biuret reagent.
(i) Explain why a buffer solution was added to the amylase-starch mixture. (2)

Answer 14

To keep pH constant;

A change in pH will slow the rate of the reaction / denature
the amylase / optimum for reaction;

Question 15

What colour would you expect the sample to go when tested with biuret reagent? (1)

Answer 15

purple

Question 16

A covering of lignin protects cellulose from enzyme attack (line 14). Use your
knowledge of the way in which enzymes work to explain why cellulose-digesting
enzymes do not digest lignin. (2)

Answer 16

enzymes are specific;
shape of lignin molecules will not fit active site (of enzyme);
OR
shape of active site (of enzyme);
will not fit molecule;

Question 17

Describe the structure of a cellulose molecule and explain how cellulose is adapted for
its function in cells.(6)

Answer 17

1. made from β-glucose;
2. joined by condensation / removing molecule of water / glycosidic bond;
3. 1 : 4 link specified or described;
4. “flipping over” of alternate molecules;
5. hydrogen bonds linking chains / long straight chains;
6. cellulose makes cell walls strong / cellulose fibres are strong;
7. can resist turgor pressure / osmotic pressure / pulling forces;
8. bond difficult to break;
9. resists digestion / action of microorganisms / enzymes;