A Level H1 Biology Practice Paper 4

TuitionGoWhere Exam Practice (AI) - Biology H1 A-Level

Answer Key and Marking Scheme Paper: Practice Paper (Version 4 of 5) Topic: Cells & Biomolecules

Section A: Structured Questions

1. (a) Phospholipid Arrangement:

• Phospholipids form a bilayer [1].
• Hydrophilic (polar) heads face outward towards the aqueous environment, and hydrophobic (non-polar) tails face inward, away from water [1].

(b) Transport Mechanisms:

• Substance X is non-polar/lipid-soluble, so it can dissolve in and diffuse through the hydrophobic fatty acid tail region (B) of the membrane [1].
• Substance Y is polar/large, so it is repelled by the hydrophobic interior of the membrane [1].
• Therefore, Substance Y requires a transport protein (like C) to cross, whereas X does not [1].

2. (a) Temperature Increase (10-40°C):

• As temperature increases, kinetic energy of enzyme and substrate molecules increases [1].
• This leads to more frequent collisions between enzyme and substrate [1].
• More enzyme-substrate complexes are formed per unit time, increasing the rate of reaction [1].

(b) Temperature at 60°C:

• The high temperature breaks the hydrogen bonds/ionic bonds holding the tertiary structure of the enzyme [1].
• The active site changes shape (denaturation), so the substrate can no longer bind [1].

3. (a) Organelle M:

• Name: Mitochondrion [1].
• Function: Site of aerobic respiration / production of ATP [1].

(b) Protein Synthesis and Secretion:

• RER: Ribosomes on the RER synthesize proteins which enter the lumen of the RER for folding/modification [1].
• Transport vesicles carry proteins from RER to the Golgi apparatus [1].
• Golgi: Modifies (e.g., glycosylation), sorts, and packages proteins into secretory vesicles for transport out of the cell [1].

4. (a) Glucose vs. Pyruvate:

• Glucose cannot enter the Krebs cycle directly; it must first be broken down into pyruvate via glycolysis [1].
• Glycolysis occurs in the cytoplasm, not in the mitochondria [1].
• Isolated mitochondria lack the enzymes for glycolysis, so glucose cannot be metabolized to produce CO₂. Pyruvate can enter the mitochondrial matrix and undergo the Link Reaction/Krebs cycle, producing CO₂ [1].

(b) Stage:

• Krebs Cycle (Citric Acid Cycle) / Link Reaction [1]. (Accept either, as both produce CO2 in mitochondria, but Krebs is the major source).

5. (a) Solvent Properties:

• Water is polar [1].
• It forms hydrogen bonds with charged/polar solutes, allowing them to dissolve [1].

(b) Specific Heat Capacity:

• Hydrogen bonds between water molecules require a large amount of energy to break [1].
• This allows water to absorb much heat with only a small rise in temperature, stabilizing thermal conditions [1].

Section B: Data and Diagram Interpretation

6. (a) Phase S:

• Synthesis phase / S phase [1].

(b) Process in S Phase:

• DNA replication occurs [1].
• Each chromosome is replicated to form two sister chromatids, doubling the DNA content [1].

(c) Halving of DNA (Meiosis I to II):

• During Meiosis I, homologous chromosomes separate into different cells [1].
• This reduces the DNA content by half in each daughter cell compared to the cell at the end of Meiosis I [1].

7. (a) Reaction Type:

• Condensation / Esterification [1].

(b) Suitability for Storage:

• Triglycerides are insoluble in water, so they do not affect the water potential of cells/osmotic balance [1].
• They have a high energy-to-mass ratio (more C-H bonds than carbohydrates), providing more energy per gram [1].
• They are compact/hydrophobic, allowing large amounts of energy to be stored in a small volume [1].
• (Any 2 points, 2 marks each for explanation) -> Total 4 marks.

8. (a) Mass Gain at 0.0 M:

• The sucrose solution has a higher water potential (is hypotonic) than the potato cells [1].
• Water enters the potato cells by osmosis [1].
• This causes the cells to gain mass [1].

(b) Estimation of Water Potential:

• The point where there is no change in mass (0% change) represents isotonic conditions [1].
• From the table, this occurs between 0.2 and 0.4 mol dm⁻³ (closer to 0.4). Estimate approx 0.3 - 0.35 mol dm⁻³ [1].

9. (a) Dipeptide Structure:

• Correct linkage of two amino acids via a peptide bond (C-N) [1].
• Removal of water molecule (H from amine, OH from carboxyl) shown or implied in structure [1].

(b) Sequence and Structure:

• The sequence of amino acids (primary structure) determines the interactions between R-groups [1].
• These interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) cause the polypeptide to fold [1].
• This folding results in the specific tertiary (3D) shape of the protein [1].

10. (a) Optimum pH:

• pH 7 [1].

(b) Shape at pH 2:

• Low pH (high H⁺ concentration) disrupts ionic and hydrogen bonds in the enzyme's tertiary structure [1].
• The active site changes shape (denaturation) [1].
• Substrate can no longer bind effectively, reducing/Stopping activity [1].

Section C: Extended Response

11. Significance of Membrane Transport in Photosynthesis:

• CO₂ Uptake:

  • CO₂ is a raw material for the Calvin Cycle (light-independent reaction).
  • It enters leaf mesophyll cells via diffusion through stomata and cell membranes down a concentration gradient.
  • Without efficient CO₂ entry, the rate of photosynthesis is limited.

• Water Uptake:

  • Water is a raw material for the light-dependent reaction (photolysis).
  • Water enters root hair cells via osmosis and is transported to leaves.
  • Water movement into guard cells (via active transport of ions followed by osmosis) regulates stomatal opening, controlling CO₂ entry.

• Ion Transport:

  • Minerals like Magnesium (for chlorophyll) and Nitrates (for enzymes/proteins) are absorbed by root cells via active transport against concentration gradients.
  • These are essential for the synthesis of photosynthetic pigments and enzymes.

• Product Export:

  • Glucose produced in photosynthesis is converted to sucrose for transport.
  • Sucrose is loaded into phloem sieve tubes via active transport (co-transport with H⁺).
  • This removes products from the leaf, preventing feedback inhibition and maintaining the concentration gradient for continued photosynthesis.

• Thylakoid Membrane Function:

  • The thylakoid membrane contains the electron transport chain and ATP synthase.
  • It maintains a proton gradient (H⁺) by pumping protons into the thylakoid lumen during the light-dependent reaction.
  • This gradient drives chemiosmosis to produce ATP, which is essential for the Calvin Cycle.

(Marking Guide: 6 marks available. Award 1 mark for each distinct, well-explained point linking membrane transport to photosynthesis. Max 6 marks.)