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A Level H2 Biology Practice Paper 4

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Questions

A-Level Biology H2 Quiz - Cells Biomolecules

Name: ____________________ Class: ____________________ Date: ____________________ Score: ________ / 65

Duration: 1 hour 15 minutes
Total Marks: 65
Instructions: Answer all questions. Write your answers in the spaces provided. Use a black or blue pen.

Section A: Short Answer Questions (1-8)

Focus: Fundamental Knowledge and Recall

State the primary difference between the structure of a prokaryotic cell and a eukaryotic cell regarding genetic material. [1]
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Define the term 'amphipathic' in the context of phospholipids in the plasma membrane. [1]
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Name the bond that links two amino acids together in a polypeptide chain. [1]
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State the role of the nucleolus within the nucleus. [1]
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Distinguish between a competitive inhibitor and a non-competitive inhibitor in terms of where they bind to an enzyme. [2]
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Explain why the interior of the mitochondrial matrix must be maintained at a different pH than the intermembrane space. [2]
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Describe the structure of a triglyceride molecule. [2]
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State the function of the Golgi apparatus in the secretion of proteins. [2]
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Section B: Structured Response Questions (9-15)

Focus: Application and Mechanism

(a) Describe the process of facilitated diffusion. [2]
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(b) Explain why facilitated diffusion is faster than simple diffusion for polar molecules. [2]
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A protein is synthesized on a ribosome and is destined for secretion outside the cell. (a) Trace the path of this protein from the ribosome to the extracellular space. [3]
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(b) Explain the role of signal peptides in this process. [2]
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(a) Explain how the primary structure of a protein determines its tertiary structure. [3]
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(b) Describe the effect of a mutation that replaces a hydrophobic amino acid with a hydrophilic one in the core of a globular protein. [2]
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With reference to the fluid mosaic model, explain how the presence of cholesterol affects the fluidity of the plasma membrane at high temperatures. [3]
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(a) Describe the mechanism of the sodium-potassium pump ( $\text{Na}^+/\text{K}^+$ ATPase). [4]
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(b) Explain why this process is termed 'active transport'. [2]
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(a) Explain the relationship between the $V_{max}$ and the $K_m$ of an enzyme. [3]
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(b) If a non-competitive inhibitor is added, explain the change in $V_{max}$ and $K_m$ . [3]
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(a) Describe the structure of DNA, highlighting the complementary nature of the bases. [3]
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(b) Explain why the 5' to 3' directionality of DNA is biologically significant during replication. [2]
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Section C: Analysis and Synthesis (16-20)

Focus: Higher-Order Reasoning and Data Interpretation

A researcher treats a cell with a drug that prevents the formation of hydrogen bonds. Predict and explain the effect this would have on the secondary structure of proteins. [4]
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Compare and contrast the roles of the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER) in a liver cell. [4]
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Explain how the structure of the inner mitochondrial membrane (cristae) is an adaptation for the efficient production of ATP. [4]
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Discuss how the properties of water (e.g., polarity, hydrogen bonding) are essential for the stability of the DNA double helix. [4]
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A mutation causes a protein to misfold, exposing hydrophobic regions that are normally buried. Explain why this leads to the formation of insoluble aggregates in the cytoplasm. [5]
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Answers

A-Level Biology H2 Quiz - Cells Biomolecules (Answer Key)

Section A

Prokaryotes: Genetic material is circular, naked (no histones), and located in the nucleoid region. Eukaryotes: Genetic material is linear, associated with histones, and enclosed within a nuclear envelope. [1]
A molecule containing both a hydrophilic (polar) head and a hydrophobic (non-polar) tail. [1]
Peptide bond. [1]
Synthesis of ribosomal RNA (rRNA) and assembly of ribosome subunits. [1]
Competitive: Binds to the active site. Non-competitive: Binds to an allosteric site (site other than the active site). [2]
To maintain a proton gradient ( $\text{H}^+$ concentration gradient) across the inner membrane, which provides the proton motive force required for ATP synthesis via ATP synthase. [2]
One glycerol molecule esterified to three fatty acid chains. [2]
Modifies proteins (e.g., glycosylation), sorts them, and packages them into vesicles for transport to specific destinations. [2]

Section B

(a) Movement of molecules across a membrane via specific transmembrane proteins (channels or carriers) down a concentration gradient. [2] (b) Polar molecules are hydrophilic and cannot pass through the hydrophobic lipid bilayer; proteins provide a hydrophilic pathway, reducing the activation energy for transport. [2]
(a) Ribosome $\rightarrow$ Rough ER $\rightarrow$ Transport vesicle $\rightarrow$ Golgi apparatus $\rightarrow$ Secretory vesicle $\rightarrow$ Plasma membrane (exocytosis). [3] (b) Signal peptides are amino acid sequences that direct the ribosome-protein complex to the RER membrane for co-translational translocation. [2]
(a) The sequence of amino acids (primary) determines the specific R-group interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) that fold the protein into its 3D shape. [3] (b) The protein may fail to fold correctly as the hydrophilic residue will seek the aqueous environment, potentially destabilizing the core and causing denaturation or misfolding. [2]
At high temperatures, cholesterol restricts the movement of phospholipids, preventing the membrane from becoming too fluid or disintegrating. [3]
(a) 3 $\text{Na}^+$ ions bind inside $\rightarrow$ ATP hydrolyses to ADP + Pi $\rightarrow$ conformational change pumps $\text{Na}^+$ out $\rightarrow$ 2 $\text{K}^+$ ions bind outside $\rightarrow$ conformational change pumps $\text{K}^+$ in. [4] (b) It moves ions against their concentration gradients, requiring an input of metabolic energy (ATP). [2]
(a) $V_{max}$ is the maximum rate of reaction when the enzyme is saturated with substrate. $K_m$ is the substrate concentration at which the rate is half of $V_{max}$ (indicates affinity). [3] (b) $V_{max}$ decreases because the inhibitor reduces the number of functional enzyme molecules. $K_m$ remains unchanged because the affinity of the remaining active sites for the substrate is unaffected. [3]
(a) Double helix of two antiparallel polynucleotide strands. Bases pair specifically: Adenine with Thymine (2 H-bonds), Cytosine with Guanine (3 H-bonds). [3] (b) DNA polymerase can only add nucleotides to the 3' end; therefore, one strand is synthesized continuously (leading) and the other discontinuously (lagging). [2]

Section C

Secondary structures ( $\alpha$ -helices and $\beta$ -pleated sheets) are stabilized by hydrogen bonds between the amino and carboxyl groups of the polypeptide backbone. Without H-bonds, these structures would collapse, resulting in a random coil and loss of protein function. [4]
RER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion. SER: No ribosomes; synthesizes lipids/steroids, detoxifies chemicals (e.g., drugs in liver), and stores calcium ions. [4]
The inner membrane is folded into cristae, which significantly increases the surface area. This allows for a higher number of electron transport chain complexes and ATP synthase molecules to be embedded, maximizing the rate of ATP production. [4]
Water's polarity allows it to form hydrogen bonds with the polar phosphate backbone. The hydrophobic effect drives the non-polar nitrogenous bases to the interior, while the hydrophilic backbone interacts with the aqueous environment, stabilizing the helix. [4]
Normally, hydrophobic R-groups are buried in the core to avoid water. Misfolding exposes these hydrophobic regions to the aqueous cytoplasm. To minimize contact with water, these exposed regions interact with hydrophobic regions of other misfolded proteins via hydrophobic interactions, leading to the aggregation of insoluble protein clumps. [5]