AQA AS Level Biology Paper 2, November 2021: Question 9
10 marks · Medium difficulty · Short Answer
Explain five properties of water that make it important for organisms and describe the process of semi-conservative replication of DNA.
Practise this questionQuestion
Question text
09.1 Explain five properties that make water important for organisms.
[5 marks]
09.2 Describe the process of semi-conservative replication of DNA.
[5 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
09.1 1. A metabolite in condensation/hydrolysis/
photosynthesis/respiration; 3. For ‘buffer’ accept
‘resist’.
2. A solvent so (metabolic) reactions can occur
4. Reject latent heat of
OR evaporation
A solvent so allowing transport of substances; 5. For ‘columns of
water’ accept
3. High (specific) heat capacity so buffers changes ‘transpiration
in temperature; stream’. Do not
credit ‘transpiration’
4. Large latent heat of vaporisation so provides a alone but accept
cooling effect (through evaporation); description of
‘stream’.
5. Cohesion (between water molecules) so 5. For ‘columns of
supports columns of water (in plants); water’ accept
‘cohesion-tension
6. Cohesion (between water molecules) so 5 max (theory)’.
produces surface tension supporting (small) 5. and 6. For cohesion
organisms; 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
– LOGYpenetration for––JUNE 2021
photosynthesis
09.2 1. DNA helicase unwinds DNA/double helix
OR
DNA helicase breaks hydrogen bonds;
2. Both strands act as templates; 2. Accept description
of ‘template’, eg
3. (Free DNA) nucleotides line up in exposed bases on
complementary pairs/A-T and G-C; 5 max single
(polynucleotide)
4. DNA polymerase joins nucleotides (of new strands
strand); 17
4. Reject forms
5. Forming phosphodiester bonds; hydrogen
bonds/joins bases
6. Each new DNA molecule consists of one
old/original/template strand and one new strand;
TOTAL 10
How to answer it
Water Properties & DNA Replication Study Guide
What this question tests
This assessment targets core biological molecules and cellular processes from AS Level Biology. Part 1 evaluates your ability to link specific, physical, and chemical properties of water directly to biological functions in organisms. Part 2 tests your recall and precise sequencing of semi-conservative DNA replication, focusing heavily on enzyme actions, base-pairing rules, and the antiparallel nature of strands.
Explain five properties of water that make it important for organisms. [5 marks]
✅ Correct Answer Framework
- Metabolite: Acts as a reactant in condensation, hydrolysis, photosynthesis, and respiration.
- Solvent: Dissolves metabolic reactions and allows transport of substances.
- Specific Heat Capacity: High value buffers changes in environmental/body temperature.
- Latent Heat: Large latent heat of vaporisation provides a cooling effect through evaporation.
- Cohesion: Molecules stick together, supporting water columns in plant xylem and producing surface tension for small organisms.
💡 Key Knowledge & Marking Rules
- You must state the property AND give its biological consequence (the "so" link).
- Hydrogen bonding underpins many of these properties (cohesion, high specific heat capacity).
- Alternative acceptable marking points include ice floating (insulating aquatic habitats below) and transparency (allowing light penetration for photosynthesis).
🧠 Exam Technique
- Do not just list keywords like "cohesion" or "solvent". Examiners award the mark only when the functional explanation (the "so...") is included.
- Ensure you list 5 distinct properties; redundancy within a single property won't count as a second mark.
❌ Common Errors
- Writing "latent heat of evaporation" instead of latent heat of vaporisation (though notes show evaporation is credited for the cooling effect description).
- Confusing specific heat capacity with latent heat.
- Mentioning "transpiration" alone without linking it to the transpiration stream or water columns.
Describe the process of semi-conservative replication of DNA. [5 marks]
✅ Correct Answer Framework
- Step 1: DNA helicase unwinds the double helix / breaks hydrogen bonds between bases.
- Step 2: Both exposed strands act as templates.
- Step 3: Free DNA nucleotides align next to their complementary exposed bases (A-T, G-C).
- Step 4: DNA polymerase joins the nucleotides of the new strand together.
- Step 5: Phosphodiester bonds form in the sugar-phosphate backbone (condensation reaction).
- Step 6: Each new DNA molecule contains one original/template strand and one newly synthesized strand.
💡 Key Knowledge
- Enzyme specificity: DNA helicase breaks bonds; DNA polymerase forms bonds. Mixing these up loses marks instantly.
- Complementary base pairing rules must be explicitly stated or implied clearly (A with T, C with G).
🧠 Exam Technique
- Write chronologically. Start with unwinding, move to template action and nucleotide alignment, then enzymatic joining, and finish with the final composition of the replicated molecules.
- Use precise biological terminology ( DNA polymerase , phosphodiester bonds , hydrogen bonds ).
❌ Common Errors
- Stating that DNA polymerase "forms hydrogen bonds" between bases (it joins nucleotides via phosphodiester bonds; hydrogen bonds form spontaneously due to base attraction).
- Vague descriptions like "DNA is copied" or "enzymes split the DNA" without naming DNA helicase.
- Forgetting to mention that *both* strands act as templates.
Topics
Biology · 3.1 Biological molecules
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.