A Level H2 Biology Practice Paper 3

TuitionGoWhere Practice Paper - Biology H2 A-Level

Answer Key and Marking Scheme

Version: 3 of 5

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Section A: Structured Questions (20 marks)

1. (a) [2 marks]

• P: Phospholipid (1)
• Q: Protein / integral protein / channel protein / carrier protein (1)

1. (b) [2 marks]

• Phospholipids have a hydrophilic (phosphate) head and hydrophobic (fatty acid) tails (1).
• The hydrophobic core of the membrane restricts the passage of polar molecules, ions, and large molecules, allowing only small, non-polar molecules to pass through freely (1).

1. (c) [1 mark]

• Any one from: transport of molecules/ions across the membrane / acts as a channel or carrier / cell recognition / enzymatic activity / receptor for signalling molecules.

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2. (a) [2 marks]

• As temperature increases, absorbance increases (1).
• The increase is gradual from 10 °C to 40 °C, but becomes steeper above 40 °C / the relationship is non-linear / exponential increase above 40 °C (1).

2. (b) [3 marks]

• At temperatures above 40 °C, the proteins in the membrane begin to denature (1).
• The tertiary structure of membrane proteins is disrupted, creating gaps/holes in the membrane (1).
• This increases membrane permeability, allowing more betalain pigment to leak out, increasing absorbance (1).

2. (c) [1 mark]

• Some pigment leaks out even at low temperatures due to natural diffusion / the membrane is partially permeable / some cells may have been damaged during cutting (1).

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3. (a) [2 marks]

• Independent variable: pH (1)
• Dependent variable: Rate of reaction / enzyme activity / rate of starch hydrolysis (1)

3. (b) [3 marks]

• At pH 2.0, the enzyme amylase is denatured / the active site shape is altered (1).
• The ionic and hydrogen bonds that maintain the tertiary structure of the enzyme are disrupted by excess H⁺ ions (1).
• The substrate (starch) can no longer bind to the active site, so enzyme-substrate complexes cannot form, and the rate of reaction is very low (1).

3. (c) [2 marks]

• 37 °C is the optimum temperature for amylase / human body temperature (1).
• This ensures that temperature is not a limiting factor / the enzyme works at its maximum rate, so any change in rate is due to pH alone (1).

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Section B: Data Interpretation and Analysis (20 marks)

4. (a) [1 mark]

• Peptide bond (1)

4. (b) [1 mark]

• Condensation (reaction) (1)

4. (c) [3 marks]

• Each amino acid has a different R-group / side chain (1).
• R-groups vary in size, shape, charge, and polarity/hydrophobicity (1).
• These differences affect how the polypeptide chain folds (tertiary structure) and how the protein interacts with other molecules, giving proteins their diverse properties (1).

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5. (a) [4 marks] Graph marking:

• Correct axes labelled: x-axis = Sucrose concentration (mol dm⁻³), y-axis = Percentage change in mass (%) (1)
• Appropriate scales chosen (1)
• All points plotted correctly (1)
• Smooth line of best fit drawn (1)

5. (b) [3 marks]

• The water potential of the potato cells is equal to the sucrose concentration where there is no net change in mass / where the line crosses the x-axis (1).
• From the graph, this occurs at approximately 0.28 mol dm⁻³ (accept 0.27–0.29) (1).
• At this point, the water potential of the solution equals the water potential of the cells, so there is no net movement of water (1).

5. (c) [3 marks]

• The 0.0 mol dm⁻³ solution (distilled water) has a higher water potential (Ψ = 0) than the potato cells (negative Ψ) (1).
• Water moves into the potato cells by osmosis, down the water potential gradient (1).
• The entry of water increases the mass of the potato cylinders (1).

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6. (a) [2 marks]

• Hydrophilic region correctly labelled on the phosphate head (1)
• Hydrophobic region correctly labelled on the fatty acid tails (1)

6. (b) [3 marks]

• In an aqueous environment, the hydrophilic phosphate heads orientate towards the water (both inside and outside the cell) (1).
• The hydrophobic fatty acid tails orientate away from water, facing each other in the interior of the membrane (1).
• This arrangement forms a stable bilayer, with the hydrophobic tails shielded from water, which is the basic structure of all cell membranes (1).

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Section C: Extended Response (20 marks)

7. (a) [6 marks] Primary structure:

• The primary structure of collagen consists of a repeating sequence of amino acids, mainly glycine, proline, and hydroxyproline / Gly-X-Y repeating triplet, where X is often proline and Y is often hydroxyproline (1).

Secondary structure:

• Each polypeptide chain forms a left-handed helix / extended helix (not an α-helix) (1).

Tertiary structure:

• The polypeptide chain is wound into a left-handed helix; glycine (small R-group) allows tight packing of the three chains (1).

Quaternary structure:

• Three polypeptide chains wind around each other to form a triple helix / tropocollagen molecule (1).
• Hydrogen bonds form between the chains, stabilising the structure (1).
• Covalent cross-links form between lysine residues of adjacent tropocollagen molecules, forming collagen fibrils, which assemble into collagen fibres (1).

7. (b) [4 marks]

• Collagen has a triple helix structure, which provides high tensile strength / resistance to stretching (1).
• The covalent cross-links between tropocollagen molecules and between fibrils further increase strength and stability (1).
• The fibrous, insoluble nature of collagen makes it suitable for structural support (1).
• In tendons, collagen fibres are arranged in parallel bundles, allowing them to withstand the pulling forces when muscles contract (1).

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8. (a) [4 marks]

• The active site of the enzyme is not a rigid, perfectly complementary shape to the substrate (1).
• When the substrate binds to the active site, the enzyme undergoes a conformational change (1).
• The active site moulds itself around the substrate, forming a precise fit (1).
• This induced fit puts strain on bonds in the substrate / lowers activation energy, facilitating the reaction / formation of the transition state (1).

8. (b) [6 marks] Comparison:

• Lock-and-key model: the active site has a rigid, fixed shape that is exactly complementary to the substrate (1).
• Induced-fit model: the active site is flexible and changes shape upon substrate binding (1).

Why induced-fit is better:

• The induced-fit model explains how enzymes can catalyse reactions involving more than one substrate / how the enzyme can stabilise the transition state (1).
• The conformational change puts strain/stress on substrate bonds, lowering the activation energy more effectively than a rigid active site (1).
• The induced-fit model accounts for the broad specificity of some enzymes (e.g., they can act on a range of similar substrates) (1).
• Experimental evidence (e.g., X-ray crystallography) shows that enzyme structures change upon substrate binding, supporting the induced-fit model over the lock-and-key model (1).

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END OF ANSWER KEY