A Level H1 Biology Practice Paper 3

Answer Key - Biology H1 Practice Paper 2 (Version 3)

Section A: Structured Questions

Question 1 (a) Phospholipids form a bilayer [1]; hydrophilic heads face the aqueous environments (extracellular/cytoplasm) and hydrophobic tails face inward, away from water [1]. (b) Polar molecules move through the channel protein (A) [1] via facilitated diffusion, moving down a concentration gradient [1]. (c) Structure B (carrier protein) facilitates passive transport/facilitated diffusion down a gradient [1]; Structure C (pump) performs active transport [1], moving substances against a concentration gradient using ATP hydrolysis [1].

Question 2 (a) Period Y [1]. (b) Radioactive thymidine is a nucleotide analogue [1]; it is incorporated into DNA during the S phase (DNA replication), which corresponds to period Y [1]. (c) Period X (G1) has a baseline DNA amount [1]; during period Y (S phase), DNA replication occurs [1], resulting in the doubling of the DNA amount by the end of the period [1].

Question 3 (a) Pyruvate can enter the mitochondrial matrix and be converted to acetyl-CoA to enter the Krebs cycle [1], where decarboxylation reactions release $\text{CO}_2$ [1]. Glucose cannot enter the mitochondria directly [1]; it requires glycolysis in the cytoplasm to be converted to pyruvate, and isolated mitochondria lack glycolytic enzymes [1]. (b) Mitochondrial matrix [1]. (c) It provides a surface for the electron transport chain [1] and allows for the establishment of a proton gradient for ATP synthesis via chemiosmosis [1].

Question 4 (a) Rough Endoplasmic Reticulum [1]. It provides a surface for ribosomes to synthesize proteins that are translocated into the lumen for folding/transport [1]. (b) Proteins are packaged into vesicles [1], transported to the Golgi apparatus for modification (e.g., glycosylation) [1] and sorting/packaging into secretory vesicles [1]. (c) Secretory cells produce large quantities of proteins/enzymes [1]; more RER allows for higher rates of protein synthesis to meet this demand [1].

Question 5 (a) "Fluid": phospholipids and proteins can move laterally within the layer [1]; "Mosaic": proteins are embedded in the bilayer in a random/varied pattern [1]. This allows the membrane to be flexible [1] and selectively permeable [1]. (b) Cholesterol interacts with the phospholipid tails [1], reducing their movement and preventing the membrane from becoming too fluid or disintegrating at high temperatures [1].

Section B: Data Interpretation and Application

Question 6 (a) Enzyme P has an optimal pH of 2.0 [1], which matches the highly acidic environment of the human stomach, ensuring maximum catalytic activity [1]. (b) Activity would decrease/stop [1]. 45°C is likely above the optimal temperature (25°C) for the soil bacterium enzyme [1], leading to the breaking of hydrogen bonds and denaturation of the enzyme's active site [1]. (c) Change in primary structure (amino acid sequence) [1] leads to incorrect folding of the tertiary structure [1], altering the shape of the active site so the substrate can no longer bind (loss of complementarity) [1].

Question 7 (a) Water moves from the region of higher water potential (lower sucrose) to lower water potential (higher sucrose) [1] via osmosis [1]. (b) Sucrose molecules are solute particles [1]; they bind to water molecules via hydrogen bonding, reducing the number of "free" water molecules available to move [1]. (c) Sucrose would move from the area of higher concentration to lower concentration [1] via simple diffusion [1].

Question 8 (a) Prokaryotes lack a nucleus (DNA is circular/naked) [1]; they lack membrane-bound organelles like mitochondria/chloroplasts [1]; they are generally much smaller [1]. (b) Aerobic respiration occurs across the plasma membrane [1]; the membrane contains the electron transport chain and ATP synthase [1], functioning similarly to the inner mitochondrial membrane [1].

Section C: Extended Response

Question 9 (a) Significance of membrane transport in photosynthesis:

• $\text{CO}_2$ uptake: Diffuses across the stomatal pore and then across the plasma membrane/chloroplast membrane [1] down a concentration gradient to reach the stroma [1].
• Water uptake: Absorbed by root hairs via osmosis [1]; essential for photolysis in the light-dependent reaction [1].
• Ion transport: Active transport of $\text{Mg}^{2+}$ and $\text{N}$ ions [1] required for the synthesis of chlorophyll and enzymes like Rubisco [1].
• Product export: Triose phosphates/glucose must be transported out of the chloroplast [1] to be used for energy or stored as starch [1].
• Regulation: Control of stomatal opening via $\text{K}^+$ ion transport across guard cell membranes [1] regulates $\text{CO}_2$ entry and water loss [1].

(b) Phospholipid structure:

• Amphipathic nature: contains a hydrophilic phosphate head and two hydrophobic fatty acid tails [1].
• In water, tails associate with each other to avoid water (hydrophobic interaction) [1].
• Heads face the aqueous environment [1].
• This spontaneously forms a bilayer that is stable and acts as a barrier to polar substances [1].

(c) SA:Vol Ratio and Active Transport:

• As a cell grows, volume increases faster than surface area (SA:Vol ratio decreases) [1].
• Diffusion is only efficient over very short distances [1].
• In large cells, the center of the cell is too far from the membrane for passive diffusion to supply nutrients or remove waste quickly enough [1].
• To compensate, cells use active transport to "pump" substances into the cell faster than diffusion would allow [1].
• This allows the cell to maintain high internal concentrations of ions/nutrients despite a low SA:Vol ratio [1].
• Without active transport, metabolic rates would be limited by the slow rate of diffusion [1].
• This explains why highly active cells often have folded membranes (e.g., microvilli) to increase SA:Vol ratio [1].
• Thus, a lower SA:Vol ratio increases the physiological necessity for energy-dependent transport mechanisms [1].