A Level H2 Biology Practice Paper 3

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TuitionGoWhere Practice Paper - Biology H2 A-Level

TuitionGoWhere Practice Paper (AI)

Subject: Biology H2 Level: A-Level Paper: Practice Paper (Cells & Biomolecules) Version: 3 of 5 Duration: 1 hour 30 minutes Total Marks: 60

Name: _________________________ Class: _________________________ Date: _________________________

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Instructions to Candidates

1. This paper consists of three sections: Section A, Section B, and Section C.
2. Answer all questions.
3. Write your answers in the spaces provided.
4. The number of marks is given in brackets [ ] at the end of each question or part question.
5. You are advised to spend no more than 30 minutes on Section A, 30 minutes on Section B, and 30 minutes on Section C.
6. You may use a scientific calculator.

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Section A: Structured Questions (20 marks)

Answer all questions in this section.

1. Figure 1.1 shows the fluid mosaic model of a cell surface membrane.

(a) Identify the molecules labelled P and Q in Figure 1.1. [2]

P: _________________________ Q: _________________________

(b) Explain how the structure of molecule P contributes to the selective permeability of the membrane. [2]

(c) State one function of molecule Q in the membrane. [1]

2. A student investigated the effect of temperature on the permeability of beetroot cell membranes. Beetroot discs were placed in water at different temperatures for 15 minutes. The absorbance of the surrounding solution was measured using a colorimeter.

The results are shown in Table 2.1.

Table 2.1

Temperature (°C)	Absorbance (arbitrary units)
10	0.05
20	0.08
30	0.15
40	0.32
50	0.68
60	1.12
70	1.45

(a) Describe the relationship between temperature and absorbance shown in Table 2.1. [2]

(b) Explain why the absorbance increases at temperatures above 40 °C. [3]

(c) Suggest why the absorbance at 10 °C is not zero. [1]

3. Amylase is an enzyme that hydrolyses starch into maltose. A student set up an experiment to investigate the effect of pH on amylase activity. The experiment was carried out at 37 °C.

(a) State the independent variable and the dependent variable in this investigation. [2]

Independent variable: _________________________ Dependent variable: _________________________

(b) The student found that the rate of reaction was highest at pH 7.0 and very low at pH 2.0. Explain why the rate of reaction is low at pH 2.0. [3]

(c) Explain why the experiment was carried out at 37 °C. [2]

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Section B: Data Interpretation and Analysis (20 marks)

Answer all questions in this section.

4. Figure 4.1 shows the structure of a dipeptide formed from two amino acids.

(a) Name the type of bond labelled R in Figure 4.1. [1]

(b) Name the chemical reaction that forms bond R. [1]

(c) With reference to the general structure of an amino acid, explain why different R-groups give proteins their diverse properties. [3]

5. A biologist investigated the water potential of potato tuber cells. Cylinders of potato tissue were placed in a series of sucrose solutions of different concentrations. After 30 minutes, the change in mass of each cylinder was measured.

The results are shown in Table 5.1.

Table 5.1

Sucrose concentration (mol dm⁻³)	Percentage change in mass (%)
0.0	+18.5
0.1	+12.0
0.2	+5.5
0.3	-1.0
0.4	-8.5
0.5	-15.0
0.6	-22.0

(a) Plot a graph of percentage change in mass against sucrose concentration on the grid below. Draw a line of best fit. [4]

[Grid space provided]

(b) Use your graph to estimate the water potential of the potato tuber cells. Explain your answer. [3]

(c) Explain why the potato cylinders gained mass in the 0.0 mol dm⁻³ sucrose solution. [3]

6. Figure 6.1 shows the molecular structure of a phospholipid.

(a) On Figure 6.1, label the hydrophilic region and the hydrophobic region. [2]

(b) Explain how the properties of phospholipids cause them to form a bilayer in an aqueous environment. [3]

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Section C: Extended Response (20 marks)

Answer all questions in this section.

7. Collagen is a fibrous protein found in connective tissues such as tendons, ligaments, and skin.

(a) Describe the primary, secondary, tertiary, and quaternary structure of collagen. [6]

(b) Relate the structure of collagen to its function as a structural protein in tendons. [4]

8. The induced-fit model is currently accepted as the mechanism of enzyme action.

(a) Describe the induced-fit model of enzyme action. [4]

(b) Compare the induced-fit model with the lock-and-key model. Explain why the induced-fit model is considered a better explanation for enzyme specificity and catalysis. [6]

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END OF PAPER

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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