AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 9

11 marks · Standard Demand difficulty · Short Answer

Identify oxygen gas from photosynthesis, name naturally occurring polymers, identify functional groups in glycine and products of condensation polymerisation, determine early atmosphere gases, and describe the structure of DNA.

Practise this question

Question

Question text

09 This question is about algae.

A student:

• placed algae in water containing dissolved carbon dioxide

• shone bright light on the algae.

Gas bubbles were collected as the algae photosynthesised.

09.1 Describe a test that would identify the gas collected.

Give the result of the test.

[2 marks]

Test

Result

09.2 Glucose is produced when algae photosynthesise.

Name two naturally occurring polymers produced from glucose.

[2 marks]

and

Figure 6 shows the displayed structural formula of an amino acid called glycine.

Figure 6

09.3 How many functional groups are there in the molecule in Figure 6?

[1 mark]

Tick ( ) one box.

12 3 4

09.4 Glycine reacts by condensation polymerisation to produce a polypeptide and one

other substance.

Name the other substance produced.

[1 mark]

09.5 Scientists think that algae may have used gases in Earth’s early atmosphere.

Algae need an element to produce the molecule in Figure 6 which is not present in

water or carbon dioxide.

Which two gases from Earth’s early atmosphere could have provided this element?

[2 marks]

27and

09.6 The development and function of algae are controlled by a naturally

occurring polymer.

Figure 7 represents the shape and structure of this polymer.

Figure 7

Describe the shape and structure of this polymer.

[3 marks]

Mark scheme

Show the mark scheme

Question 9

AO /

Question Answers Extra information Mark

Spec. Ref.

09.1 test: (use a) glowing splint do not accept burning splint 1 AO1

4.8.2.2

result: relights dependent on correct test in 1 4.9.1.3

MP1

ignore with a pop

starch 1 AO1

09.2 4.7.3.4

cellulose 1

allow glycogen

AO1

09.3 2 1 4.7.3.3

09.4 water allow H2O 1 AO1

4.7.3.3

AO3

09.5 ammonia 1 4.7.3.3

4.9.1.2

nitrogen 1

if no other mark awarded, allow

1 mark for NO / NO2 / N2O / NOx

or equivalent named

compounds

Question 9 (continued)

AO /

Spec. Ref.

09.6 two polymer chains allow two polymer strands 1 AO1

4.7.3.4

four (different) monomers / allow four (different) bases 1

nucleotides allow cytosine, guanine, adenine

and thymine

allow C G A T

20 (double) helix allow spiral 1

if no other mark awarded, allow

1 mark for DNA

Total 11

How to answer it

Algae, Photosynthesis, Natural Polymers & DNA

📘 What This Question Tests

This question covers organic chemistry and Earth's early atmosphere. Specifically, you need to know:

The chemical test for oxygen produced in photosynthesis (Topic 8: Chemical Analysis).
Naturally occurring polymers derived from glucose (Topic 7: Organic Chemistry).
The structure and functional groups of amino acids and condensation polymerisation.
Gases in Earth's early atmosphere that provided essential biological elements (Topic 9: Chemistry of the Atmosphere).
The chemical structure and shape of DNA as a natural polymer.

Part 09.1

Gas Identification Test

Identifying oxygen gas produced by photosynthesis (2 marks)

✅ Correct Answer

Test: (Use a) glowing splint [1 mark]

Result: Relights [1 mark]

Mark scheme note: The result mark is strictly dependent on naming a "glowing splint" for the test.

❌ Common Errors & Traps

"Burning splint": Confusing the test for oxygen with hydrogen. A burning splint is NOT accepted.
Writing "squeaky pop" — this is for hydrogen (H₂), not oxygen (O₂).
Writing "it burns brighter" without saying it relights.

🧠 Exam Technique

Algae photosynthesise using CO₂ and H₂O to release oxygen (O₂). Always learn gas tests as fixed pairs: "glowing splint" goes with "relights".

Part 09.2

Polymers from Glucose

Naming two naturally occurring glucose polymers (2 marks)

✅ Correct Answer

Any two from:

Starch [1 mark]
Cellulose [1 mark]
Glycogen [1 mark]

💡 Key Knowledge

Glucose (C₆H₁₂O₆) is a monomer sugar. Plants and algae join glucose units into natural polymers:

Starch: Energy storage carbohydrate in plants.
Cellulose: Structural polymer forming strong plant cell walls.
Glycogen: Carbohydrate storage polymer in animals and fungi.

Parts 09.3 & 09.4

Amino Acids & Condensation Polymerisation

Functional groups and polymerisation byproducts (2 marks total)

✅ Part 09.3 Answer (1 mark)

Tick the box for: 2

Glycine contains exactly two different functional groups: an amine group (-NH₂) and a carboxylic acid group (-COOH).

✅ Part 09.4 Answer (1 mark)

Water (or H₂O ) [1 mark]

Condensation polymerisation eliminates a small molecule (water) as the peptide link forms.

💡 Anatomy of an Amino Acid

Figure 6 shows glycine:

Left side: Basic amine group ( -NH₂ ).
Right side: Acidic carboxylic acid group ( -COOH ).

Because amino acids have two distinct functional groups, they can react together end-to-end to form polypeptide chains.

🧠 Condensation vs. Addition Polymers

Addition polymerisation: Monomer contains C=C double bonds; only one product (the polymer) is formed.
Condensation polymerisation: Monomers join while eliminating a small molecule — in GCSE, this small molecule is always water.

Part 09.5

Earth's Early Atmosphere & Biological Elements

Identifying gases supplying nitrogen to early organisms (2 marks)

📐 Step-by-Step Deduction

Check what is in water and CO₂: Hydrogen (H), Oxygen (O), and Carbon (C).
Check the formula of glycine: Glycine has C, H, O, and Nitrogen (N).
Identify missing element: The missing element is Nitrogen.
Recall early atmosphere gases containing nitrogen: Volcanoes released ammonia (NH₃) and nitrogen (N₂).

✅ Correct Answer

Any two of:

Ammonia [1 mark]
Nitrogen [1 mark]

If no other mark awarded, allow 1 mark for oxides of nitrogen (NO / NO₂ / N₂O / NOₓ).

❌ Common Misconceptions

Students often write "oxygen" or "carbon dioxide" because they forget to look at the missing element in the amino acid structure. Water and carbon dioxide already provide C, H, and O — the organism specifically needed nitrogen to make amino acids!

Part 09.6

DNA Structure & Shape

Describing the naturally occurring polymer shown in Figure 7 (3 marks)

✅ Correct Answer (3 marks)

Award 1 mark for each of the following points (max 3):

Two polymer chains (allow two polymer strands) [1 mark]
Made from four (different) monomers / nucleotides (allow four bases / cytosine, guanine, adenine, thymine / C, G, A, T) [1 mark]
In the shape of a (double) helix (allow spiral) [1 mark]

Consolation mark: If 0 marks scored from the points above, 1 mark is awarded simply for identifying the polymer as DNA.

🧠 How to Get 3/3 on DNA Description

Make sure your answer covers three distinct aspects: number of strands, monomer type, and overall shape.

Model sentence: "The polymer is made of two polymer chains wound in a double helix, formed from four different nucleotide monomers."

💡 Key Knowledge: Natural Polymers Overview

1. Polynucleotides (DNA):

Monomer = nucleotides (4 types: A, T, C, G). Forms double helix.

2. Polypeptides / Proteins:

Monomer = amino acids (e.g. glycine). Formed via condensation polymerisation.

3. Polysaccharides:

Monomer = glucose. Produces starch, cellulose, and glycogen.

Topics

Chemistry · C7: Organic Chemistry · C8: Chemical Analysis · C9: Chemistry of the Atmosphere

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.