T1 Biological molecules

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. 3 marks

Answer 1

1. Heat with acid and neutralise;
   Accept boil/water bath for heat
   Accept named alkali for neutralise
   Accept named examples, eg HCl, NaHCO3
2. Heat with Benedict’s (solution);
3. Red precipitate/colour;
   Accept other colours eg orange/ brown/green

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. 2 marks

Answer 2

  1.   Filter and dry (the precipitate);
Accept: correct reference to evaporation after filtration
2. Find mass/weight;

Question 3

What is a monomer? 1 mark

Answer 3

(a monomer is a smaller / repeating) unit / molecule from which larger molecules / polymers
are made;
Reject atoms / elements / ’building blocks’ for units / molecules
Ignore examples

Question 4

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. 2 marks

Answer 4

Similarity
1. Both contain galactose / a glycosidic bond;
Ignore references to hydrolysis and / or condensation
Difference
2. Lactulose contains fructose, whereas lactose contains glucose;
Ignore alpha / beta prefix for glucose
Difference must be stated, not implied

Question 5

Glycogen and cellulose are both carbohydrates.
Describe two differences between the structure of a cellulose molecule and a glycogen molecule. 2 marks

Answer 5

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;
   Ignore ref. to H bonds / microfibrils

Question 6

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

Answer 6

Any two from:
1. Insoluble (in water), so doesn’t affect water potential;
2. Branched / coiled / (α-)helix, so makes molecule compact;
OR
Branched / coiled / (α-)helix so can fit many (molecules) in small area;
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
Require feature and explanation for 1 mark
1. Accept Ψ or WP
1. Accept Insoluble so doesn’t affect osmosis
1. Do not allow ref to ‘doesn’t affect water leaving cells
4. Ignore ‘surface area’
4. Accept ‘branched so glucose readily released’

Question 7

Describe the structure of glycogen 2 marks

Answer 7

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

Question 8

During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.
Suggest how glycogen acts as a source of energy.
Do not include transport across membranes in your answer.
2 marks

Answer 8

1. Hydrolysed (to glucose);
2. Glucose used in respiration;
3. Ignore ‘Broken down’
4. ‘Energy produced’ disqualifies mp2

Question 9

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients. 2 marks

Answer 9

1. Membrane folded so increased / large surface area;
   OR
   Membrane has increased / large surface area for (fast) diffusion / facilitated
   diffusion / active transport / co-transport;
2. Large number of protein channels / carriers (in membrane) for facilitated
   diffusion;
3. Large number of protein carriers (in membrane) for active transport;
4. Large number of protein (channels / carriers in membrane) for co-transport;
5. Accept ‘microvilli to increase surface area’
6. Reject reference to villi.
   Note feature and function required for each marking point and
   reference to large / many / more.
   List rule applies.

Question 10

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 marks

Answer 10

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

Question 11

What term is used to describe the different structures of α-glucose and β-glucose? 1 mark

Answer 11

Isomer

Question 12

Cotton is a plant fibre used to make cloth. Explain how cellulose gives cotton its strength 3 marks

Answer 12

(long) straight / unbranched chains;
(idea of more than 1) chains lie side by side / form (micro)fibrils;
idea of H bonds holding chains together;

Question 13

Describe how a triglyceride molecule is formed. 3 marks

Answer 13

1. One glycerol and three fatty acids;
2. Condensation (reactions) and removal of three molecules of water;
3. Ester bond(s) (formed);
   Accept all marks in suitably labelled diagram OR in a balanced
   equation

Question 14

Describe how you would test a liquid sample for the presence of lipid and how you would recognise a positive result.
2 marks

Answer 14

1. (Mix / shake sample) with ethanol, then water;
   Sequence is important
2. White / milky (emulsion);
   Ignore cloudy
   Reject precipitate

Question 15

Suggest one advantage of the different percentage of cholesterol in red blood cells compared with cells lining the ileum.
1 mark

Answer 15

Red blood cells free in blood / not supported by other cells so cholesterol helps to maintain shape;
Allow converse for cell from ileum – cell supported by others in
endothelium so cholesterol has less effect on maintaining shape.

Question 16

E. coli has no cholesterol in its cell-surface membrane. Despite this, the cell maintains a constant shape. Explain why.
2 marks

Answer 16

1. Cell unable to change shape;
2. (Because) cell has a cell wall;
3. (Wall is) rigid / made of peptidoglycan / murein.

Question 17

Describe the difference between the structure of a triglyceride molecule and the structure of a phospholipid molecule. 1 mark

Answer 17

1. In phospholipid, one fatty acid replaced by a phosphate;
   Ignore references to saturated and unsaturated
   Accept
   Reject P/Phosphorus
   Accept annotated diagrams

Question 18

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst. 3 marks

Answer 18

1. Substrate binds to the active site/enzyme
   OR
   Enzyme-substrate complex forms;
   Accept for ‘binds’, fits
2. Active site changes shape (slightly) so it is complementary to substrate
   OR
   Active site changes shape (slightly) so distorting/breaking/forming bonds
   in the substrate;
3. Reduces activation energy;

Question 19

A competitive inhibitor decreases the rate of an enzyme-controlled reaction.
Explain how. 3 marks

Answer 19

1. Inhibitor similar shape to substrate;
   Reject same shape
   Accept ‘complementary to active site’
2. Fits/binds to active site;
3. Prevents/reduces enzyme-substrate complex forming;

Question 20

When bread becomes stale, the structure of some of the starch is changed. This changed starch is called retrograded starch.
Scientists have suggested retrograded starch is a competitive inhibitor of amylase in the small intestine.
Assuming the scientists are correct, suggest how eating stale bread could help to reduce weight gain.
3 marks

Answer 20

1. Less hydrolysis of starch;
   Accept no hydrolysis
2. (To) maltose;
3. (So) less absorption (of glucose)
   OR
   (So) more egested (starch/carbohydrate);
   Accept description of egestion, eg lost in faeces

Question 21

Explain how the active site of an enzyme causes a high rate of reaction.
3 marks

Answer 21

)  1.   Lowers activation energy;
2.   Induced fit causes active site (of enzyme) to change shape;
3.   (So) enzyme-substrate complex causes bonds to form/break;
Accept: description, of induced fit
Accept: enzyme-substrate complex causes stress/strain on bonds.

Question 22

Formation of an enzyme-substrate complex increases the rate of reaction.
Explain how.
2 marks

Answer 22

1. Reduces activation energy;
   Accept ‘reduces E a’.
2. Due to bending bonds
   OR
   Without enzyme, very few substrates have sufficient energy for reaction;
   Accept ‘Due to stress/pressure/tension on bonds’ OR ‘Due to
   weakening bonds’.
   Ignore references to ‘breaking bonds’.

Question 23

A decrease in temperature decreases the kinetic energy of molecules in a solution. Explain how a decrease in temperature decreases the rate of an enzyme-controlled reaction.
2 marks

Answer 23

molecules moving less / slower; reduces chance of collision
(between enzyme and substrate) / of enzyme-substrate
complexes being formed; (reject converse)

Question 24

Urea breaks hydrogen bonds. Explain how the addition of urea would affect the rate of an enzyme-controlled reaction.

Answer 24

these bonds hold / maintain tertiary / globular structure (of enzyme);
enzyme denatured / tertiary structures destroyed; (shape of) active site
distorted / changes;
substrate no longer fits / enzyme-substrate complex not formed;

Question 25

Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule. 2 marks

Answer 25

1. tRNA is 'clover leaf shape', mRNA is linear; Must be a comparison Reject tRNA is double stranded Accept tRNA is folded for tRNA is ‘clover leaf shaped’ 2. tRNA has hydrogen bonds, mRNA does not; 3. tRNA has an amino acid binding site, mRNA does not; Accept ‘CCA end' for amino acid binding site 4. tRNA has anticodon, mRNA has codon;

Question 26

Adenosine triphosphate (ATP) is a nucleotide derivative. Contrast the structures of ATP and a nucleotide found in DNA to give two difference 2 marks

Answer 26

1. ATP has ribose and DNA nucleotide has deoxyribose; 2. ATP has 3 phosphate (groups) and DNA nucleotide has 1 phosphate (group); 3. ATP – base always adenine and in DNA nucleotide base can be different / varies; Both parts of each MP needed 3. Reject Uracil / U 3. Accept C, T or G for different bases Accept annotated diagram for any of the three marks

Question 27

During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5’ to 3’ direction. Use the figure in part (a) and your knowledge of enzyme action and DNA replication to explain why new nucleotides can only be added in a 5’ to 3’ direction. 4 marks

Answer 27

1. Reference to DNA polymerase; 2. (Which is) specific; 3. Only complementary with / binds to 5’ end (of strand); Reject hydrogen bonds / base pairing 4. Shapes of 5’ end and 3’ end are different / description of how different.

Question 28

The arrows in Figure 2 show the directions in which each new DNA strand is being produced. (d) Use Figure 1, Figure 2 and your knowledge of enzyme action to explain why the arrows point in opposite directions. 4 marks | F1 F2: picture of dna polymerase working on the transcription ## Footnote in opposite directions

Answer 28

1. (Figure 1 shows) DNA has antiparallel strands / described; 2. (Figure 1 shows) shape of the nucleotides is different / nucleotides aligned differently; 3. Enzymes have active sites with specific shape; 4. Only substrates with complementary shape / only the 3’ end can bind with active site of enzyme / active site of DNA polymerase.

Question 29

Describe how an ATP molecule is formed from its component molecules 4 marks

Answer 29

1. and 2. Accept for 2 marks correct names of three components adenine, ribose/pentose, three phosphates;; Accept for 1 mark, correct name of two components Accept for 1 mark, ADP and phosphate/Pi Ignore adenosine Accept suitably labelled diagram 3. Condensation (reaction); Ignore phosphodiester 4. ATP synthase; Reject ATPase

Question 30

State and explain the property of water that can help to buffer changes in temperature 2 marks

Answer 30

1. (water has a relatively) high (specific) heat capacity; Ignore numbers relating to heat capacity 2. Can gain / lose a lot of heat / energy without changing temperature; OR Takes a lot of heat / energy to change temperature; Accept due to H bonding between water molecules

Question 31

Describe the roles of iron ions, sodium ions, and phosphate ions in cells. 5 marks

Answer 31

Iron ions 1.   Haemoglobin binds/associates with oxygen OR Haemoglobin transports/loads oxygen; Ignore reference to 2+ or 3+ in Fe2+ or Fe3+ 4. Sodium ions 2.   Co-transport of glucose/amino acids (into cells); 3.   (Because) sodium moved out by active transport/Na – K pump; 4.   Creates a sodium concentration/diffusion gradient; 5.   Affects osmosis/water potential; Phosphate ions 6.   Affects osmosis/water potential; Accept 5. OR 6. – not both 7.   Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides; 8.   Used in/to produce ATP; Reject ‘energy produced’ 9.   Phosphorylates other compounds (usually) making them more reactive; 10. Hydrophilic/water soluble part of phospholipid bilayer/membrane; Accept for 1 mark, Sodium ions cause water reabsorption in kidneys OR Sodium ions establish resting potential (in neurones) OR Sodium ion diffusion creates action potential

Question 32

The movement of substances across cell membranes is affected by membrane structure. Describe how. 5 marks

Answer 32

  1.   Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances; 1. and 2. Accept correct named examples 1. and 2. Ignore water Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2 Accept water-insoluble Page 6 of 12 www.accesstuition.com 2.   Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipidinsoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer; Accept water-soluble 3.   Carrier proteins allow active transport; 4.   Channel/carrier proteins allow facilitated diffusion/co-transport; Accept aquaporins allow osmosis 5.   Shape/charge of channel / carrier determines which substances move; 6.   Number of channels/carriers determines how much movement; 7.   Membrane surface area determines how much diffusion/movement; 6. and 7. Accept correct reference to faster/slower/rate for ‘how much movement’ Accept microvilli / Golgi (apparatus) / ER / rER Accept surface area to volume for ‘surface area’ 8.   Cholesterol affects fluidity/rigidity/permeability; Accept cholesterol affects vesicle formation/ endocytosis/exocytosis /phagocytosis;

Question 33

Explain five properties that make water important for organisms. 5 marks

Answer 33

1.   A metabolite in condensation/hydrolysis/ photosynthesis/respiration; 2.   A solvent so (metabolic) reactions can occur OR A solvent so allowing transport of substances; 3.   High heat capacity so buffers changes in temperature; For ‘buffer’ accept ‘resist’. 4.   Large latent heat of vaporisation so provides a cooling effect (through evaporation); 5.   Cohesion (between water molecules) so supports columns of water (in plants); For ‘columns of water’ accept ‘transpiration stream’. Do not credit ‘transpiration’ alone but accept description of ‘stream’. For ‘columns of water’ accept ‘cohesion-tension (theory)’. For cohesion accept hydrogen bonding 6.   Cohesion (between water molecules) so produces surface tension supporting (small) organisms; For cohesion accept hydrogen bonding Ignore reference to pH. Allow other suitable properties but must have a valid explanation. For example *   ice floating so maintaining aquatic habitat beneath *   water transparent so allowing light penetration for photosynthesis

Question 34

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample. 5 marks

Answer 34

Lipid 1.   Add ethanol/alcohol then add water and shake/mix OR Add ethanol/alcohol and shake/mix then pour into/add water; Reject heating emulsion test. Accept ‘Add Sudan III and mix’. 2.   White/milky emulsion OR emulsion test turns white/milky; Ignore cloudy. Reject precipitate. Accept (for Sudan III) top (layer) red. Non-reducing sugar 3.   Do Benedict’s test and stays blue/negative; Ignore details of method for Benedict’s test for this mp. 4.   Boil with acid then neutralise with alkali; Accept named examples of acids/alkalis. 5.   Heat with Benedict’s and becomes red/orange (precipitate); Do not credit mp5 if no attempt at mp4. For ‘heat’ ignore ‘warm’/’heat gently’/’put in a water bath’ but accept stated temperatures ≥ 60°C. Heat must be stated again, do not accept using residual heat from mp4. Accept ‘do the Benedict’s test’ if full correct method given elsewhere. Accept ‘sodium carbonate, sodium citrate and copper sulfate solution’ for Benedict’s but must have all three if term ‘Benedict’s’ not used. Amylase 6.   Add biuret (reagent) and becomes purple/violet/mauve/lilac; Accept ‘sodium or potassium hydroxide and copper sulfate solution’ for ‘biuret’. Reject heating biuret test. 7.   Add starch, (leave for a time), test for reducing sugar/absence of starch;

Question 35

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers. Give two named examples of polymers and their associated monomers to illustrate your answer. 5 marks

Answer 35

1.   A condensation reaction joins monomers together and forms a (chemical) bond and releases water; 2.   A hydrolysis reaction breaks a (chemical) bond between monomers and uses water; 3.   A suitable example of polymers and the monomers from which they are made; 3. and 4. Polymers must contain many monomers. 3. and 4: suitable examples include *   amino acid and polypeptide, protein, enzyme, antibody or specific    example *   nucleotide and polynucleotide, DNA or RNA *   Alpha glucose and starch/glycogen *   Beta glucose and cellulose. If neither specific carbohydrate example is given, allow monosaccharide/glucose and polysaccharide. 3. and 4. Reject (once) reference to triglycerides. 4.   A second suitable example of polymers and the monomers from which they are made; 5.   Reference to a correct bond within a named polymer; Reject reference to ester bond.

Question 36

Describe the process of semi-conservative replication of DNA 5 marks

Answer 36

1. DNA helicase unwinds DNA/double helix OR DNA helicase breaks hydrogen bonds; 2. Both strands act as templates; Accept description of ‘template’, eg exposed bases on single (polynucleotide) strands 3. (Free DNA) nucleotides line up in complementary pairs/A-T and G-C; 4. DNA polymerase joins nucleotides (of new strand); Reject forms hydrogen bonds/joins bases 5. Forming phosphodiester bonds; 6. Each new DNA molecule consists of one old/original/template strand and one new strand;

Question 37

Why is HCl important in testing for non-reducing sugars? 3 marks

Answer 37

1. Glycosidic bond is hydrolysed by the acid 2. this frees the reducing sugars to react with Benedict's 3. Non-reducing sugars will not react with Benedict's unless the reducing sugars are released as monosaccharides

Question 38

Phospholipids typically have one unsaturated fatty acid tail. Explain how this would affect membrane fluidity. 3 marks

Answer 38

1. Membrane fluidity would increase 2. Because the unsaturated fatty acid would bend 3. Which prevents close packing of the phospholipids/ pushes phospholipids apart

Question 39

Another name for amide bond

Answer 39

Peptide bond

Question 40

Another name for peptide bond

Answer 40

Amide bond

Question 41

Define activation energy

Answer 41

The minimum energy required to start a chemical reaction