A Level H2 Biology Practice Paper 5

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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.

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Section A: Molecular Foundations (Questions 1-7)

1. Describe the structural difference between a saturated and an unsaturated fatty acid and explain how this affects the fluidity of a cell membrane. [3]
   
   
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2. Explain why the primary structure of a protein determines its tertiary structure. [3]
   
   
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3. A protein is found to have a high proportion of cysteine residues. Suggest the likely type of bonding that stabilizes its tertiary structure and explain its significance. [3]
   
   
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4. Distinguish between the roles of the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER) in terms of the molecules they synthesize. [3]
   
   
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5. Explain how the structure of a phospholipid molecule allows it to form a bilayer in an aqueous environment. [3]
   
   
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6. Describe the process of hydrolysis in the formation of a glycosidic bond between two $\alpha$-glucose molecules. [3]
   
   
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7. Compare the structure of glycogen and cellulose, explaining how their different linkages lead to different biological functions. [4]
   
   
   
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Section B: Organelles and Transport (Questions 8-14)

8. Explain the importance of the highly folded inner membrane (cristae) of the mitochondrion in relation to ATP production. [3]
   
   
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9. Describe the mechanism of facilitated diffusion and explain why it is limited by a maximum rate of transport. [3]
   
   
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10. With reference to the fluid mosaic model, explain how cholesterol regulates membrane fluidity at very high temperatures. [3]
    
    
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11. Explain the role of the Golgi apparatus in the modification and secretion of proteins. [4]
    
    
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12. Contrast the mechanisms of endocytosis and exocytosis, providing one example of a molecule transported by each. [4]
    
    
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13. Describe how the sodium-potassium pump maintains a resting potential across a cell membrane. [4]
    
    
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14. Explain why a cell would require more mitochondria in tissues with high metabolic activity, such as cardiac muscle. [3]
    
    
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Section C: Enzyme Kinetics and Regulation (Questions 15-20)

15. Define the term 'induced fit' and explain how it increases the rate of an enzyme-catalyzed reaction. [3]
    
    
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16. Describe the effect of a non-competitive inhibitor on the $V_{max}$ and $K_m$ of an enzyme. Explain the molecular basis for these changes. [4]
    
    
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17. Explain how a change in pH can lead to the denaturation of an enzyme. [3]
    
    
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18. Discuss the advantage of allosteric regulation in a metabolic pathway involving multiple sequential enzymes. [4]
    
    
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19. A competitive inhibitor is added to an enzyme reaction. Describe how the effect of this inhibitor can be overcome. [3]
    
    
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20. Explain the relationship between the activation energy of a reaction and the rate of reaction, and describe how enzymes influence this. [4]
    
    
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A-Level Biology H2 Quiz - Cells Biomolecules (Answer Key)

Section A: Molecular Foundations

1. Saturated: No double bonds between carbons; straight chains. Unsaturated: One or more $\text{C}=\text{C}$ double bonds; creates kinks. Fluidity: Kinks prevent tight packing of phospholipids, increasing membrane fluidity. [3]
2. Primary structure is the specific sequence of amino acids. This sequence determines the R-group interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) that fold the protein into its specific 3D tertiary shape. [3]
3. Bonding: Disulfide bridges (covalent bonds between sulfur atoms of two cysteine residues). Significance: Provides strong, permanent stabilization of the tertiary structure, making the protein more resistant to denaturation. [3]
4. RER: Synthesizes proteins (via attached ribosomes), specifically those for secretion or membrane insertion. SER: Synthesizes lipids (phospholipids, steroids) and is involved in detoxification. [3]
5. Phospholipids are amphipathic. The hydrophilic phosphate heads face the aqueous environment (extracellular/cytosol), while the hydrophobic fatty acid tails face inward, away from water, creating a stable bilayer. [3]
6. Hydrolysis is the addition of water to break a bond. However, the question asks for the formation of a bond (which is condensation). Correction for marking: If the student describes condensation (removal of water to form a glycosidic bond), award marks. If they describe hydrolysis of a bond, award 1 mark for the concept of water addition. [3]
7. Glycogen: $\alpha$-glucose, 1,4 and 1,6 linkages; branched; used for rapid glucose mobilization (storage). Cellulose: $\beta$-glucose, 1,4 linkages only; unbranched straight chains; forms microfibrils via H-bonding; provides structural strength (cell walls). [4]

Section B: Organelles and Transport

8. Cristae increase the surface area of the inner membrane. This allows for more copies of the Electron Transport Chain (ETC) and ATP synthase complexes, maximizing the rate of ATP production via oxidative phosphorylation. [3]
9. Movement of molecules down a concentration gradient via a channel or carrier protein. Limited by the number of available transport proteins (saturation point), where all binding sites are occupied. [3]
10. At high temperatures, phospholipids move more rapidly. Cholesterol interacts with the hydrophobic tails, restricting their movement and reducing membrane fluidity/permeability to prevent the membrane from becoming too leaky. [3]
11. Proteins from RER arrive in vesicles. Golgi modifies them (e.g., glycosylation/adding sugar chains), sorts them, and packages them into secretory vesicles for transport to the plasma membrane or lysosomes. [4]
12. Endocytosis: Invagination of membrane to take in materials (e.g., LDL particles). Exocytosis: Fusion of vesicle with membrane to release materials (e.g., insulin). [4]
13. The pump uses ATP to actively transport 3 $\text{Na}^+$ ions out and 2 $\text{K}^+$ ions in. This creates a concentration gradient and a net negative charge inside the cell relative to the outside. [4]
14. Cardiac muscle requires a continuous, high supply of ATP for contraction. More mitochondria increase the capacity for aerobic respiration to meet this high energy demand. [3]

Section C: Enzyme Kinetics and Regulation

15. The active site is not a rigid lock; it changes shape slightly as the substrate binds to fit more tightly. This puts strain on the substrate bonds, lowering activation energy and increasing reaction rate. [3]
16. $V_{max}$: Decreases (inhibitor reduces the number of functional enzyme molecules). $K_m$: Remains unchanged (the affinity of the remaining active sites for the substrate is unchanged). Basis: Inhibitor binds to an allosteric site, changing the active site shape. [4]
17. pH changes alter the ionization of R-groups in the active site. This disrupts ionic and hydrogen bonds, changing the 3D conformation of the active site so the substrate can no longer bind. [3]
18. Allows for feedback inhibition. The end-product of a pathway can bind to the allosteric site of the first enzyme in the sequence, shutting down the pathway when product levels are sufficient, preventing waste of resources. [4]
19. Increase the substrate concentration. Since the inhibitor and substrate compete for the same active site, a higher substrate-to-inhibitor ratio increases the probability of substrate binding. [3]
20. Relationship: Higher activation energy = slower reaction rate. Enzymes: Lower the activation energy by providing an alternative pathway (e.g., stabilizing the transition state), allowing more molecules to react per unit time. [4]