AQA A-Level Biology - Topic 1: Biological Molecules LRQs

A Collection of Common Longer-Response Questions

Explain how a non-competitive inhibitor decreases the rate of an enzyme-controlled reaction. (3)

1.      (Inhibitor) binds (to enzyme) away from active site / to allosteric site

OR

(Inhibitor) does not bind to active site;

2.      Changing (enzyme) tertiary structure

OR

Changing active site (shape);

3.      No / fewer enzyme-substrate complexes (form)

OR

Enzyme-substrate not complementary;

3

Describe the structure of ATP.

          Outline how named enzymes break down and resynthesise ATP. (4)

1.      Ribose, Adenine and 3 phosphates;

2.      ATP to ADP + Pi by ATP hydrolase / ATPase in hydrolysis (reaction);

3.      ADP + Pi to ATP by ATP synthase;

4.      (In) condensation (reaction);

4

Outline the similarities and differences between the structures of DNA and RNA molecules. (6)

Similarities

1.      Polymers of nucleotides;

2.      (Nucleotide has) pentose, (nitrogen-containing organic) base and a phosphate (group);

3.      Cytosine, guanine and adenine (as bases);

4.      Have phosphodiester bonds;

Differences

5.      Deoxyribose v ribose;

6.      Thymine v uracil;

7.      Long v short / DNA is longer;

8.      Double helix / stranded v single stranded;

6 max

Describe how a quaternary protein is formed from its monomers. (5)

1.      Amino acids joined by peptide bond(s);

2.      (By) condensation reaction(s);

3.      Secondary structure is formed by hydrogen bonding;

4.      Tertiary structure formed by interactions (between R groups);

5.      Quaternary structure contains >1 polypeptide

OR

Quaternary structure formed by interactions/bonds between polypeptides;

5

Describe how monomers join to form the primary structure of a protein. (3)

1.      Condensation reaction between amino acids;

2.      (Forming) peptide bonds;

3.      Creating (specific) sequence / order of amino acids;

3

Describe how the structure of glycogen is related to its function. (4)

1.      Helix / coiled / branched so compact;

2.      Polymer of glucose so easily hydrolysed;

3.      Branched so more ends for faster hydrolysis;

4.      Glucose (polymer) so provides respiratory substrate for energy (release);

5.      Insoluble so not (easily) lost (from cell)

OR

Insoluble so does not affect water potential/osmosis;

4 max

Explain five properties that make water important for organisms. (5)

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 (specific) heat capacity so buffers / resists changes in temperature;

4.      Large latent heat of vaporisation so provides a cooling effect (through evaporation);

5.      Cohesion (between water molecules) so supports columns of water / transpiration stream (in plants);

6.      Cohesion / Hydrogen bonding (between water molecules) so produces surface tension supporting (small) organisms;

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

5 max

Describe the structure of DNA. (5)

1.      Polymer of nucleotides;

2.      Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base;

3.      Phosphodiester bonds (between nucleotides);

4.      Double helix / 2 strands held by hydrogen bonds;

5.      (Hydrogen bonds / pairing) between adenine, thymine and cytosine, guanine;

5

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

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;

3

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

Iron Ions

1.   Haemoglobin binds / associates with oxygen

OR

Haemoglobin transports / loads oxygen;

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;

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

5 max

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

Lipid

1.   Add ethanol / alcohol then add water and shake / mix

OR

Add ethanol / alcohol and shake / mix then pour into / add water;

Accept ‘Add Sudan III and mix’

2.   White / milky emulsion

OR

emulsion test turns white / milky;

Accept (for Sudan III) top (layer) red

Non-reducing Sugar

3.   Do Benedict’s test and stays blue / negative;

4.   Boil with acid then neutralise with alkali;

Accept named examples of acids/alkalis

5.   Heat with Benedict’s and becomes red / orange (precipitate);

Protein

6.   Add biuret (reagent) and becomes purple / violet / mauve / lilac;

5 max

Compare and contrast the structure and properties of triglycerides and phospholipids. (5)

1.      Both contain ester bonds (between glycerol and fatty acid);

2.      Both contain glycerol;

3.      Fatty acids on both may be saturated or unsaturated;

4.      Both are insoluble in water;

5.      Both contain C, H and O but phospholipids also contain P;

6.      Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;

7.      Triglycerides are hydrophobic / non-polar and phospholipids have hydrophilic and hydrophobic region;

Accept ‘non-polar’ for hydrophobic and ‘polar’ for hydrophilic

8.      Phospholipids form monolayer (on surface) / micelle / bilayer (in water) but triglycerides don’t;

5 max

Describe how the structures of starch and cellulose molecules are related to their functions. (5)

Starch (max 3)

1.      Helical / spiral shape so compact;

2.      Large (molecule) / insoluble so osmotically inactive / does not affect water potential;

3.      Branched so glucose is (easily) released for respiration;

4.      Large (molecule) so cannot leave cell / cross cell-surface membrane;

Cellulose (max 3)

5.      Long, straight / unbranched chains of β glucose;

6.      Joined by hydrogen bonding;

7.      To form (micro / macro)fibrils;

8.      Provides rigidity / strength;

5 max

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

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) / complementary to active site;

Accept allows max rate of reaction to be reached / max product will eventually be formed

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;

5 max

Explain how the structure of DNA is related to its functions. (6)

1.      Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;

2.      Long / large molecule so can store lots of information;

3.      Helix / coiled so compact;

Accept: can store in a small amount of space for ‘compact’

4.      Base sequence allows information to be stored / base sequence codes for amino acids / protein / base sequence allows transcription;

5.      Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;

6.      (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;

6

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

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;

(allow maximum of 4 marks for structural features)

6 max

Explain why ATP is useful in many biological processes. (4)

1.      Releases energy in small / manageable amounts;

2.      (Broken down) in a one step / single bond broken immediate energy compound / makes energy available rapidly;

2. Accept easily broken down

3.      Phosphorylates / adds phosphate makes (phosphorylated substances) more reactive / lowers activation energy;

4.      Reformed / made again;

4