A Level H2 Biology Practice Paper 3

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TuitionGoWhere Exam Practice (AI)

Subject: Biology H2
Level: A-Level
Paper: Practice Paper 2 (Structured Questions)
Version: 3 of 5
Duration: 2 hours
Total Marks: 75

Name: __________________________ Class: __________ Date: __________

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Instructions to Candidates

1. Answer all questions.
2. Write your answers in the spaces provided.
3. Use a black or dark blue ink pen.
4. Diagrams should be drawn with a sharp pencil.
5. The number of marks allocated to each question is given in brackets [ ].

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Section A: Cells and Biomolecules

Question 1 Figure 1.1 shows the structure of a protein involved in the transport of ions across a cell membrane. (Imagine Figure 1.1: A transmembrane protein with a specific binding site and a conformational change indicated by arrows)

(a) With reference to Figure 1.1, explain how the binding of a specific ion triggers the transport of that ion across the membrane. [3]



(b) Suggest why a mutation that replaces a hydrophilic amino acid with a hydrophobic amino acid in the extracellular domain of this protein would likely inhibit its function. [2]



(c) Describe the role of the hydrophobic interactions in maintaining the tertiary structure of this transmembrane protein. [3]


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Question 2 A researcher is studying the diagnosis of a genetic blood disorder using gel electrophoresis. (a) Describe and explain how gel electrophoresis is used to distinguish between individuals who are homozygous for a normal haemoglobin allele (HbA) and those who are heterozygous for a mutation (HbS). [4]





(b) Explain why the bands for the mutant allele (HbS) migrate at a different rate compared to the normal allele (HbA) in the electric field. [2]



(c) If an individual shows only one band at the same position as the HbS allele, what is their genotype and predicted phenotype? [2]


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Question 3 Figure 3.1 shows the results of an experiment where a suspension of mitochondria was placed in a buffer containing ADP and inorganic phosphate (Pi). The oxygen concentration was monitored over time. (Imagine Figure 3.1: A graph showing a steep decline in $O_2$ concentration, which levels off as ADP is depleted)

(a) Explain the relationship between the rate of oxygen consumption and the availability of ADP in the mitochondria. [3]



(b) A chemical inhibitor, sodium azide, which blocks the final electron acceptor in the electron transport chain, is added to the suspension. Predict and explain the effect on the oxygen concentration graph. [3]




(c) Describe how the proton gradient across the inner mitochondrial membrane is used to synthesize ATP. [4]




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Question 4 Prokaryotes often utilize operons to regulate gene expression. (a) Contrast the mechanism of an inducible operon (e.g., lac operon) with that of a repressible operon (e.g., trp operon). [4]





(b) Suggest and explain why it is metabolically advantageous for a bacterium to possess a repressible operon for the synthesis of an essential amino acid. [3]




(c) Describe the effect on the lac operon if a mutation occurred that prevented the repressor protein from binding to the operator region. [3]



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Question 5 Certain neurodegenerative diseases are characterized by the accumulation of misfolded proteins. (a) With reference to the molecular structure of proteins, suggest why misfolded proteins tend to aggregate within the cytoplasm of a cell. [3]



(b) Explain how the exposure of hydrophobic regions in a misfolded protein leads to the formation of insoluble aggregates. [3]




(c) Discuss how these protein aggregates might interfere with normal cellular functions. [4]




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(Note: For the purpose of this practice paper, the remaining marks are distributed across other core ideas as per the syllabus blueprint, but the focus here is the requested Cells & Biomolecules topic.)

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[End of Section A]

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Answer Key - Biology H2 Practice Paper 2 (Version 3)

Section A: Cells and Biomolecules

Question 1 (a) [3 marks]

• Binding of the specific ion to the binding site [1]
• Causes a conformational change/change in shape of the protein [1]
• This opens the channel/transports the ion to the opposite side of the membrane [1]

(b) [2 marks]

• The extracellular domain must be hydrophilic to interact with the aqueous environment/ion [1]
• A hydrophobic amino acid would repel the ion or cause the protein to misfold, preventing ion binding [1]

(c) [3 marks]

• Hydrophobic R-groups cluster together to avoid water [1]
• This occurs primarily in the transmembrane region (alpha-helices) [1]
• Stabilizes the protein within the phospholipid bilayer/maintains the 3D fold [1]

Question 2 (a) [4 marks]

• An electric field/potential difference is applied across the gel [1]
• Proteins migrate based on charge and molecular mass/size [1]
• Homozygotes (HbA/HbA) show a single band at the HbA position [1]
• Heterozygotes (HbA/HbS) show two distinct bands (one at HbA, one at HbS) because they possess two different alleles [1]

(b) [2 marks]

• The mutation changes the amino acid sequence (e.g., Glu to Val) [1]
• This alters the overall charge or shape of the protein, changing its migration speed through the gel matrix [1]

(c) [2 marks]

• Genotype: Homozygous for the mutant allele (HbS/HbS) [1]
• Phenotype: Sickle cell anaemia [1]

Question 3 (a) [3 marks]

• Oxygen consumption is coupled to ATP synthesis [1]
• High ADP levels stimulate the ETC to pump protons and consume $O_2$ to regenerate ATP [1]
• As ADP is converted to ATP, the rate of $O_2$ consumption decreases/levels off [1]

(b) [3 marks]

• Oxygen concentration would remain constant/stop decreasing [1]
• Sodium azide prevents electrons from being transferred to oxygen (final acceptor) [1]
• This halts the ETC and the associated proton pumping/oxygen uptake [1]

(c) [4 marks]

• ETC pumps $H^+$ ions from matrix to intermembrane space [1]
• Creates a proton gradient/electrochemical gradient [1]
• $H^+$ ions flow back into the matrix through ATP synthase [1]
• This provides energy for the phosphorylation of ADP to ATP (chemiosmosis) [1]

Question 4 (a) [4 marks]

• Inducible: Normally "off"; turned "on" by the presence of a substrate/inducer (e.g., lactose) [1]
• Repressible: Normally "on"; turned "off" by the presence of a corepressor/end-product (e.g., tryptophan) [1]
• Inducible repressor is inactivated by the inducer [1]
• Repressible repressor is activated by the corepressor [1]

(b) [3 marks]

• Prevents wasteful overproduction of amino acids [1]
• When the amino acid is abundant, the operon is switched off [1]
• Conserves cellular energy and resources [1]

(c) [3 marks]

• The repressor cannot bind to the operator [1]
• RNA polymerase has unimpeded access to the promoter [1]
• Genes are expressed constitutively (always "on") regardless of lactose presence [1]

Question 5 (a) [3 marks]

• Misfolding exposes hydrophobic R-groups [1]
• These regions are normally buried in the protein core [1]
• Hydrophobic regions seek to avoid the aqueous cytoplasm [1]

(b) [3 marks]

• Exposed hydrophobic regions of one misfolded protein interact with those of another [1]
• This is driven by hydrophobic interactions/attraction [1]
• Leads to the formation of large, insoluble clumps/aggregates [1]

(c) [4 marks]

• Aggregates can physically obstruct intracellular transport [1]
• May sequester other essential proteins, preventing them from functioning [1]
• Can trigger apoptosis/programmed cell death [1]
• Leads to loss of neuron/cell function in the affected tissue [1]