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A Level H1 Biology Practice Paper 3
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A-Level Biology H1 Quiz - Cells Biomolecules — Answer Key
Section A: Short Answer
1. (a) Plant cell / Eukaryotic plant cell [1]
(b) Presence of a nucleus / membrane-bound nucleus / double-membrane organelles (e.g., chloroplasts, mitochondria); [1] Accept: true nucleus / nuclear envelope. Do not accept: cell wall alone (some prokaryotes have cell walls).
2. Phospholipids form a bilayer (1); with hydrophilic (phosphate) heads facing outward toward the aqueous environment (on both sides of the membrane) and hydrophobic (fatty acid) tails facing inward, away from water / forming a hydrophobic core (1). Award full marks for a clear description of bilayer arrangement with correct orientation of heads and tails. [2]
3. (a) Na⁺ is most likely to enter by simple diffusion (1). Because Na⁺ has a higher concentration in pond water (40 units) than in the cytoplasm (15 units) / Na⁺ concentration gradient is from outside to inside the cell, allowing passive movement down its concentration gradient (1). [2]
(b) The plant cell uses active transport to move K⁺ ions into the cell against the concentration gradient (1); this requires energy in the form of ATP, provided by respiration / specific carrier proteins (K⁺ pumps) in the membrane use ATP to transport K⁺ into the cell (1). [2]
4. Any two from:
- Channel proteins — form hydrophilic pores for facilitated diffusion of specific ions/molecules (1).
- Carrier proteins — bind specific molecules and change shape to transport them across the membrane (1).
- Receptor proteins — bind signalling molecules (e.g., hormones) to trigger a cellular response (1).
- Enzymes — catalyse reactions at the membrane surface (1).
- Cell adhesion molecules / glycoproteins — involved in cell-cell recognition and attachment (1).
Award 1 mark each, maximum 2 marks. [2]
Section B: Structured Response
5. (a) Mitochondrion (accept: mitochondria). [1]
(b) Liver cells have high metabolic activity / high demand for ATP (1); mitochondria are the site of aerobic respiration / oxidative phosphorylation / ATP synthesis, so large numbers are required to meet the energy needs of the liver (1). Accept reference to liver functions e.g., detoxification, protein synthesis, bile production requiring ATP. [2]
(c) Magnification = image length ÷ actual length (1) = 52.5 µm ÷ 2.5 µm = ×21 (1) Award 1 mark for correct formula/substitution and 1 mark for correct answer with appropriate units/expression. [2]
6. (a) Rate of uptake increases as pH rises from (e.g., pH 5.5) to the optimum pH of approximately 7.2 (1); rate then decreases as pH continues to rise above the optimum / above pH 7.2 (1). Accept: description of a bell-shaped relationship / reference to an optimum pH. [2]
(b) Facilitated diffusion of substance X depends on the specific transport protein maintaining its correct three-dimensional shape / conformation (1); at pH below 6.5, the change in hydrogen ion concentration causes the protein to denature / lose its specific tertiary structure (1); this alters the shape of the binding site / transport channel so that substance X can no longer bind or pass through (1). [3]
7. (a) S phase. [1]
(b) Phase: S phase (1). Explanation: Radioactive thymine is a nucleotide / a component of DNA / is incorporated into new DNA strands during DNA replication, and DNA replication occurs during the S phase of the cell cycle (1). Do not accept G1 or G2. [2]
8. A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms (1) [accept: bent / V-shaped structure]; the oxygen atom is slightly negative (δ⁻) and the hydrogen atoms are slightly positive (δ⁺), making water a polar molecule / a dipole (1). Because water is polar, it can form hydrogen bonds with / surround and separate polar solutes such as ions and polar molecules (e.g., sugars, amino acids), allowing them to dissolve / acting as a solvent (1). Award 1 mark for structure, 1 mark for explanation linking polarity to solvent function. Maximum 2 marks. [2]
Section C: Data-Based and Extended Response
9. (a) Pyruvate enters the mitochondrion and is converted to acetyl-CoA (1); acetyl-CoA then enters the Krebs cycle / citric acid cycle, where decarboxylation reactions occur, releasing carbon dioxide (1). Accept: pyruvate is the substrate for the link reaction and Krebs cycle which produce CO₂. [2]
(b) Very little CO₂ is produced because glucose cannot be metabolised directly by isolated mitochondria — the enzymes for glycolysis are located in the cytoplasm, not in mitochondria (1); glycolysis must first break down glucose to pyruvate in the cytoplasm before pyruvate can enter the mitochondrion (1); since only isolated mitochondria were used in the experiment, glycolysis does not occur and very little pyruvate is available for the Krebs cycle (1). Credit also reference to the trace amount (5 units) possibly due to small amounts of glucose breakdown during isolation or minor contamination. [3]
10. Marking scheme: 6 marks
Award marks for the following points:
| Mark | Content point |
|---|---|
| 1 | CO₂ uptake: CO₂ diffuses from the atmosphere into the leaf through open stomata; CO₂ then diffuses across the cell surface membrane and chloroplast membranes into the stroma, where it is fixed in the Calvin cycle. |
| 2 | Diffusion depends on a concentration gradient; CO₂ concentration is maintained lower inside chloroplasts because it is continuously used in carbon fixation, favouring inward diffusion. |
| 3 | Water uptake: Water enters root hair cells by osmosis down a water potential gradient across the partially permeable cell membrane. Water is required as an electron donor in the light-dependent reactions (photolysis). |
| 4 | Ion/mineral transport: Mineral ions such as Mg²⁺ (for chlorophyll synthesis), K⁺, and nitrate ions are taken up into root cells by active transport across membranes, using ATP from respiration; these ions are essential for photosynthetic enzyme activity and pigment production. |
| 5 | Product export: Triose phosphate / glucose produced in photosynthesis is transported out of the chloroplast across the chloroplast membrane into the cytoplasm and then into phloem for translocation; this prevents end-product inhibition of photosynthesis. |
| 6 | Conclusion / Integration: The regulated movement of substances across membranes (both uptake of raw materials and removal of products) is essential for maintaining the rate and efficiency of photosynthesis; without effective membrane transport, photosynthesis would slow or cease. |
Marks should be awarded holistically for a coherent discussion that links membrane transport mechanisms specifically to photosynthetic processes. A simple list of transport types without reference to photosynthesis should be limited to a maximum of 2 marks.
Accept alternative correct points, such as oxygen diffusion out of chloroplasts, within the marking framework. [6]
END OF ANSWER KEY