A Level H1 Biology Practice Paper 1

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

Section A: Cell Structure and Organelles

1. Prokaryotic: Circular DNA, not enclosed in a nucleus (naked). Eukaryotic: Linear DNA, enclosed within a nuclear envelope. [1]
2. Function: Secretory cell (e.g., pancreatic cell). [1] RER synthesizes proteins for secretion, and Golgi modifies/packages them into vesicles. [1]
3. Site of ribosomal RNA (rRNA) synthesis [1] and assembly of ribosomal subunits. [1]
4. They contain their own circular DNA [1] and their own 70S ribosomes, allowing them to synthesize some of their own proteins. [1]
5. Plant: Composed primarily of cellulose. [1] Bacterium: Composed of peptidoglycan (murein). [1]
6. Function: Contain hydrolytic enzymes to break down waste/foreign materials. [1] Mechanism: Proton pumps in the membrane actively transport $\text{H}^+$ ions into the lumen [1] to maintain low pH for enzyme activity. [1]
7. Cristae increase the surface area of the inner membrane [1]. This allows for more electron transport chain complexes and ATP synthase proteins [1], maximizing the rate of ATP production via oxidative phosphorylation. [1]

Section B: Biological Molecules

8. Glycosidic bond [1] formed via the removal of a water molecule between two hydroxyl groups. [1]
9. Triglycerides are more reduced/contain more $\text{C-H}$ bonds per unit mass [1], thus releasing more energy upon oxidation [1]. They are hydrophobic and do not attract water, making them more compact for storage than glycogen. [1]
10. Primary: The specific sequence of amino acids in a polypeptide chain. [1] Tertiary: The overall 3D folding of the polypeptide due to interactions between R-groups. [1]
11. High energy is required to break hydrogen bonds between water molecules [1]. When water evaporates from leaves (transpiration) or skin (sweating), it absorbs significant heat [1], cooling the organism/plant. [1]
12. Ensures a constant width of the helix [1]. Allows for semi-conservative replication as each strand serves as a template [1]. Ensures the genetic code is copied accurately. [1]
13. DNA: Double-stranded [1], contains deoxyribose sugar [1], uses Thymine. RNA: Single-stranded [1], contains ribose sugar [1], uses Uracil. (Any 3)
14. Hydrogen bonds form between the $\text{C=O}$ group of one amino acid and the $\text{N-H}$ group of another [1] four residues along the chain [1]. This stabilizes the coiled helical shape. [1]

Section C: Membrane Transport and Application

15. Arrangement: Phospholipids form a bilayer with hydrophilic heads facing aqueous environments and hydrophobic tails facing inward [2]. Barrier: Small non-polar molecules pass through; polar/charged molecules are blocked by the hydrophobic core [2], requiring specific transport proteins.
16. Facilitated Diffusion: Passive (no ATP), moves down concentration gradient via channel/carrier proteins [2]. Active Transport: Requires ATP, moves substances against concentration gradient via carrier proteins/pumps [2].
17. Water moves out of the cell by osmosis [1]. The cell becomes plasmolysed (plant) or shrunken/crenated (animal) [1] as the cytoplasm loses volume. [1]
18. Mechanism: 3 $\text{Na}^+$ ions are pumped out and 2 $\text{K}^+$ ions are pumped in using ATP hydrolysis [2]. Significance: Maintains the resting membrane potential (electrochemical gradient) [1], which is necessary for the depolarization required to trigger an action potential. [1]
19. The bilayer is fluid, allowing phospholipids to move laterally [1]. Proteins are embedded or attached [1] and can drift within the bilayer, allowing for dynamic interactions and signaling. [1]
20. $\text{CO}_2$: Moves into the leaf via stomata and then into mesophyll cells by simple diffusion [2] down a concentration gradient. Glucose: Produced in chloroplasts and exported to the cytoplasm/phloem via facilitated diffusion or active transport [2] depending on the concentration gradient. Significance: Regulates the supply of raw materials and the removal of products to maintain photosynthetic efficiency. [1]