A Level H2 Biology Practice Paper 3

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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: __________

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 [ ].

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.)

[End of Section A]