A Level H1 Biology Practice Paper 4

Answer Key - A-Level Biology H1 Quiz: Cells Biomolecules

Section A: Cell Structure and Organelles

1. Prokaryotic: Circular DNA, not enclosed in a nucleus. Eukaryotic: Linear DNA, enclosed within a nuclear envelope. (1)
2. Rough Endoplasmic Reticulum (RER). (1)
3. Cell Type: Muscle cell / Heart cell / Liver cell. (1) Reason: High demand for ATP to power contraction/metabolic activities. (1)
4. Modifies proteins (e.g., adding carbohydrate chains to form glycoproteins) (1) and packages them into secretory vesicles for transport to the cell surface. (1)
5. Increases the surface area of the inner membrane (1), allowing for more electron transport chain complexes and ATP synthase enzymes to be embedded, thus increasing ATP production. (1)
6. Composition: Wall is primarily cellulose; membrane is a phospholipid bilayer with proteins. (1) Permeability: Wall is fully permeable to most small molecules; membrane is selectively/partially permeable. (2)
7. Function: Breakdown of waste/foreign materials via hydrolytic enzymes. (1) pH: The enzymes (acid hydrolases) only function at low pH (1), which prevents them from digesting the rest of the cell if the lysosome leaks. (1)

Section B: Biological Molecules

8. Bond: Glycosidic bond. (1) Reaction: Condensation reaction. (1)
9. Structure: Amylose is unbranched/helical; amylopectin is branched. (1) Role: Amylose is more compact for storage (1); amylopectin's branching allows for faster hydrolysis of glucose ends for rapid energy release. (1)
10. One glycerol molecule (1) ester-bonded to three fatty acid chains. (1)
11. Hydrophilic heads face the aqueous environments (inside and outside) (1), while hydrophobic tails face inward, away from water (1). This creates a stable, self-sealing bilayer that acts as a barrier. (1)
12. Primary: Linear sequence of amino acids. (1) Tertiary: The overall 3D folding/shape of a single polypeptide chain. (1)
13. A different amino acid may have different properties (e.g., polar vs non-polar) (1), altering the R-group interactions (hydrogen/ionic bonds) (1), which changes the 3D shape of the active site/binding site, rendering the protein non-functional. (1)
14. Structure: Pentose sugar, phosphate group, and a nitrogenous base. (1) Identity: The specific sequence of bases (codons) (1) determines the sequence of amino acids (1) via translation, which determines the final 3D fold and function of the protein. (1)

Section C: Membrane Transport and Application

15. Definition: The tendency of water to move from a region of higher water potential to lower water potential. (1) Unit: Kilopascals (kPa). (1)
16. Process: Movement of polar/charged molecules via specific channel or carrier proteins. (1) Difference: Simple diffusion occurs directly through the phospholipid bilayer (1), whereas facilitated diffusion requires a protein mediator. (1)
17. Mechanism: The pump uses ATP to move 3 Na+ ions out of the cell and 2 K+ ions into the cell against their concentration gradients. (2) ATP: Required because the movement is against a gradient (active transport), necessitating energy to change the protein's conformation. (2)
18. Prediction: The cell becomes flaccid or plasmolysed. (1) Explanation: Water leaves the cell by osmosis (1) from a region of higher water potential (inside) to lower water potential (outside). (1)
19. Role: Sodium ions move down their gradient into the cell via a co-transporter, providing the energy (secondary active transport) (2) to pull glucose molecules into the cell against their concentration gradient. (2)
20. Fluidity: Phospholipids and proteins can move laterally, allowing the membrane to be flexible and self-repair. (2) Mosaic: The variety of proteins (channels, carriers, receptors) embedded in the bilayer (2) allows the cell to be selectively permeable, controlling which ions/molecules enter or exit based on the specific proteins present. (2)