AQA AS Level Biology Paper 2, June 2025: Question 9

✅ Marking Points (5 Marks Available)

1. Two nuclear divisions result in 4 haploid daughter cells / gametes / nuclei.
2. Separation of homologous chromosomes occurs during the first meiotic division (Meiosis I).
3. Separation of sister chromatids occurs during the second meiotic division (Meiosis II).
4. Independent segregation (or random assortment) of homologous chromosomes reshuffles maternal and paternal chromosomes.
5. Crossing over results in the exchange of alleles / sections of chromatids between homologous chromosomes.

💡 Key Knowledge Breakdown

• Meiosis I (Reduction Division): Homologous pairs line up along the equator (metaphase I) and are pulled to opposite poles by spindle fibres (anaphase I). Diploid (2n) becomes haploid (n).
• Meiosis II: Centromeres divide; sister chromatids are pulled apart into individual chromosomes.
• Independent Assortment: The orientation of maternal and paternal homologous pairs at the equator during metaphase I is completely random.
• Crossing Over (Chiasmata): Non-sister chromatids of homologous pairs twist around each other, break, and swap equivalent alleles, creating recombinant chromatids.

🧠 Exam Technique: Two-Pronged Prompts

• Notice the question has two distinct halves:
  (1) "how it results in haploid cells"
  (2) "how it increases genetic variation".
• Allocate your sentences evenly: explain the two divisions (homologous pairs in division 1, chromatids in division 2) to answer the first half, then name and describe independent segregation and crossing over for the second half.
• Always state what is separating in which division: Division 1 = homologous pairs; Division 2 = sister chromatids.

❌ Common Student Pitfalls

• Chromosomes vs. Chromatids: Confusing the order of separation. Saying "chromatids separate in the first division" forfeits the mark.
• Vague Variation Descriptions: Writing that "genes swap" during crossing over. Genes do not change locus; alleles (or parts of chromatids) are exchanged.
• Omitting Homologous Pairs: Mentioning independent segregation without identifying that it applies to homologous chromosomes.
• Ignoring Division Count: Forgetting to clearly state that meiosis consists of two divisions producing four cells.

✅ Marking Points (Any 5 of 6)

Formation at the Arteriole End:

1. High hydrostatic pressure (blood pressure) at the arterial end of the capillary bed.
2. Water, ions, and small soluble molecules (glucose, amino acids) are forced out of the capillary into intercellular spaces.
3. Plasma proteins remain inside the capillary blood (too large to pass through capillary pores).

Return at the Venule End & Lymph:

1. Retained plasma proteins lower the water potential (create a water potential gradient / more negative Ψ) inside the capillary.
2. Water moves back into the capillary blood by osmosis down the water potential gradient.
3. Excess (unabsorbed) tissue fluid drains into the lymphatic system (lymph capillaries/vessels).

💡 Pressure Balance Concept

• At Arteriole End: Hydrostatic pressure generated by ventricular systole (~4.3 kPa) is greater than the inward osmotic pull (~2.8 kPa), creating an overall net outward filtration pressure.
• Ultrafiltration: Only small molecules cross fenestrations/endothelial gaps. Red blood cells, platelets, and large plasma proteins (e.g. albumin, fibrinogen) stay inside.
• At Venule End: Friction reduces hydrostatic pressure (~1.6 kPa). Hydrostatic pressure is now lower than the inward osmotic pull, so net movement of water is inward via osmosis.
• Lymphatics: Roughly 10–15% of fluid cannot re-enter capillaries directly; it enters blunt-ended lymph capillaries and returns to the blood near the subclavian veins.

🧠 Exam Technique: "Describe and Explain"

• Always state both the physical force and the resulting movement:
  • Force: High hydrostatic pressure → Movement: forces water/solutes out.
  • Force: Lower water potential inside → Movement: water returns by osmosis.
• Use the term osmosis explicitly. Saying "water diffuses back" will lose the return mark.
• Use standard symbol Ψ or the full words water potential . Abbreviations like "wp" or "WP" are rejected by examiners.

❌ Common Student Pitfalls

• "Water moves by diffusion": Water specifically moves down a water potential gradient across a partially permeable membrane via osmosis.
• Forgetting the Lymph: Over 40% of candidates miss out on the final mark by failing to mention that remaining excess fluid drains into the lymph system.
• Saying Proteins Move Out: Stating that proteins leave the blood is an automatic contradiction. Proteins are retained because they are too large to cross the capillary endothelium.
• Vague Pressure Terms: Mentioning just "pressure" without specifying hydrostatic pressure or water potential.