AQA AS Level Biology Paper 2, June 2019: Question 9
10 marks · Medium difficulty · Extended Answer
Describe the roles of iron ions, sodium ions, and phosphate ions in cells, and explain how membrane structure affects the movement of substances across cell membranes.
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Question text
09.1 Describe the roles of iron ions, sodium ions, and phosphate ions in cells.
[5 marks]
[Extra space]
09.2 The movement of substances across cell membranes is affected by membrane
structure. Describe how.
[5 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
Iron ions 5 max Must have MP1 for 5 max
1. Haemoglobin binds/associates with 3 max for sodium and 3 max for
oxygen phosphate
OR + +
Haemoglobin transports/loads 1. Ignore reference to 2 or 3 in
Fe2+ or Fe3+
oxygen;
Accept 5. OR 6. – not both
Sodium ions
Accept for 1 mark,
2. Co-transport of glucose/amino Sodium ions cause water
acids (into cells); reabsorption in kidneys
3. (Because) sodium moved out by OR
active transport/Na – K pump; Sodium ions establish resting
4. Creates a sodium potential (in neurones)
09.1
concentration/diffusion gradient; OR
5. Affects osmosis/water potential;
Sodium ion diffusion creates
Phosphate ions action potential
8. Reject ‘energy produced’
6. Affects osmosis/water potential;
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;
5 max
1. and 2. Accept correct named
1. Phospholipid (bilayer) allows
examples
movement/diffusion of non-
polar/lipid-soluble substances; 1 and 2 Ignore water
2. Phospholipid (bilayer) prevents 1. Accept phospholipid (bilayer)
movement/diffusion of polar/ allows movement/diffusion of
charged/lipid-insoluble substances O /CO
OR
(Membrane) proteins allow 1. Accept water-insoluble
polar/charged substances to cross 2. Accept water-soluble
the membrane/bilayer;
3. Carrier proteins allow active 4. Accept aquaporins allow
transport; osmosis
09.2
4. Channel/carrier proteins allow 6 and 7 accept correct reference
facilitated diffusion/co-transport; to faster/slower/rate for ‘how
5. Shape/charge of channel / carrier much movement’
determines which substances
move; 7. Accept microvilli / Golgi
6. Number of channels/carriers (apparatus) / ER / rER
determines how much movement; 7. Accept surface area to
7. Membrane surface area volume for ‘surface area’
determines how much
diffusion/movement; 8. Accept cholesterol affects
vesicle formation/
8. Cholesterol affects
endocytosis/exocytosis/phagocy
fluidity/rigidity/permeability;
tosis;
TOTAL 10
How to answer it
Roles of Ions and Cell Membrane Structure
What this question tests
This assessment tests your core biological knowledge of inorganic ion functions (specifically iron, sodium, and phosphate) and how the structural components of cell membranes (phospholipids, proteins, and cholesterol) govern the movement of substances across membranes via passive and active transport mechanisms.
Describe the roles of iron ions, sodium ions, and phosphate ions in cells.
✅ Correct Mark Scheme Points (5 Max)
- Iron ions: Haemoglobin binds, associates with, or transports/loads oxygen.
- Sodium ions: Involved in the co-transport of glucose/amino acids; moved out by active transport (Na-K pump) creating a concentration gradient; affects water potential/osmosis.
- Phosphate ions: Affects water potential/osmosis; joins nucleotides with phosphodiester bonds in DNA/RNA; component of ATP; phosphorylates compounds to make them more reactive; forms the hydrophilic/water-soluble part of phospholipids.
💡 Key Knowledge
- You must cover multiple ion types to max out marks (typically capped at 3 marks max if only discussing sodium and phosphate).
- Phosphate has dual roles: structural (DNA backbones, phospholipids) and metabolic (ATP, phosphorylation).
🧠 Exam Technique
- Use precise biological terminology like co-transport , phosphodiester bond , and water potential .
- Ensure you link the ion directly to its cellular function rather than just naming it.
❌ Common Errors
- Saying sodium or phosphate "produces energy" (always state they produce/are part of ATP ).
- Failing to mention iron, resulting in an automatic mark cap since answers must span across the specified ion categories.
The movement of substances across cell membranes is affected by membrane structure. Describe how.
✅ Correct Mark Scheme Points (5 Max)
- Phospholipid bilayer allows movement/diffusion of non-polar / lipid-soluble substances.
- Phospholipid bilayer prevents movement/diffusion of polar / charged / lipid-insoluble substances.
- Membrane proteins (channel/carrier) allow polar/charged substances to cross or facilitate active transport / co-transport / facilitated diffusion.
- Shape/charge of proteins determines which specific substances move.
- Number of channel/carrier proteins determines the amount of movement.
- Membrane surface area determines how much diffusion/movement occurs.
- Cholesterol affects membrane fluidity, rigidity, and permeability.
💡 Key Knowledge
- The hydrophobic core of the phospholipid bilayer acts as a barrier to water-soluble molecules.
- Proteins provide selective pathways for specific ions and polar molecules.
🧠 Exam Technique
- Explicitly contrast what can pass through the phospholipid bilayer versus what cannot, and how proteins compensate for this.
- Mention quantitative structural factors like surface area and number of proteins to easily pick up descriptive marks.
❌ Common Errors
- Confusing active transport and facilitated diffusion regarding energy requirements and protein involvement.
- Vague descriptions of cholesterol—must link it directly to fluidity , rigidity , or permeability .
Topics
Biology · 3.1 Biological molecules · 3.2 Cells
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.