Biology interleaved practice
Instructions: Answer all questions. This set deliberately mixes topics from the digestive, circulatory, and respiratory systems in random order — read each carefully and choose the correct system/concept before answering. Show reasoning where asked. Total: 50 marks.
1. A patient's blood test shows very low hemoglobin. Explain the specific molecular consequence for gas transport, and state which gas hemoglobin primarily binds and where. (5 marks)
2. Trace, in correct sequence, the path of a bolus of bread from the mouth to the point where the remaining waste is compacted. Name every major structure it passes through. (6 marks)
3. A student writes: "Chewing food and stomach acid breaking proteins are both mechanical digestion." Identify and correct the error, defining both types of digestion. (4 marks)
4. Trace the pathway of a single oxygen molecule of air from the nostrils to the alveoli, naming each structure in order. (5 marks)
5. Explain how the structure of the small intestine wall maximises absorption. Name two structural features and one absorption mechanism used for glucose. (6 marks)
6. Starting from the right atrium, trace the pulmonary circulation until oxygenated blood returns to the heart. Name the chambers and vessels. (5 marks)
7. Match each enzyme to its substrate and product: amylase, pepsin, lipase. State where each acts. (6 marks)
8. The SA node fails but the heart continues beating at a slower rate. Explain why, referring to the conduction system. (4 marks)
9. A person with blood group O Rh⁻ needs a transfusion but only group AB Rh⁺ blood is available. Explain why this is dangerous, and state what makes O Rh⁻ the "universal donor." (5 marks)
10. Compare an artery and a capillary with respect to wall thickness, function, and lumen. Then explain one role of the pancreas that assists digestion in the region the capillaries here serve. (4 marks)
Answer keyMark scheme & solutions
1. Tests 4.2.2 (hemoglobin function) + 4.2.1. Hemoglobin is the iron-containing protein in RBCs that binds oxygen (primarily) in the lungs (high pO₂) and releases it in tissues (low pO₂). Low hemoglobin → reduced oxygen-carrying capacity → tissues receive less O₂ (fatigue, breathlessness). Also carries some CO₂ back to lungs. Why: The word "hemoglobin" cues the molecular transport topic, not RBC counts generally.
2. Tests 4.1.2 (path of food) + 4.1.10. Mouth → pharynx → oesophagus → stomach → small intestine (duodenum → jejunum → ileum) → large intestine (colon), where waste is compacted → rectum. Why: "Bolus...to compaction" signals the alimentary canal path ending at the large intestine (water reabsorption/compaction), not respiratory pathway.
3. Tests 4.1.3. Error: stomach acid breaking proteins is chemical digestion, not mechanical. Mechanical digestion = physical breakdown without changing chemical composition (chewing, churning). Chemical digestion = enzymes/acids break chemical bonds into smaller molecules. Chewing is mechanical; acid/enzyme protein breakdown is chemical. Why: Contrasts the two — must classify each action correctly.
4. Tests 4.3.1 (air pathway). Nostrils/nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Why: "Air from nostrils to alveoli" is respiratory, deliberately placed near Q2's food path to force discrimination between the two tracts (both pass through the pharynx!).
5. Tests 4.1.6 + 4.1.9. Features: villi (finger-like folds increasing surface area) and microvilli (brush border on villi cells, further increasing area); thin single-cell-thick wall and rich blood/lymph supply. Glucose absorption uses active transport (secondary, via Na⁺-glucose cotransport) then facilitated diffusion into blood. Why: "Maximises absorption" cues structure→function; glucose specifically requires active transport, not simple diffusion.
6. Tests 4.2.4 (pulmonary circulation) + 4.2.3. Right atrium → right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium. Why: "Right atrium → back to heart" and "pulmonary" restrict to the pulmonary loop only — students must not include systemic vessels.
7. Tests 4.1.8 (enzymes).
| Enzyme | Substrate | Product | Site |
|---|---|---|---|
| Amylase | Starch | Maltose/sugars | Mouth (saliva) & small intestine |
| Pepsin | Proteins | Peptides | Stomach |
| Lipase | Fats/lipids | Fatty acids + glycerol | Small intestine |
Why: Requires enzyme–substrate matching and site recall together.
8. Tests 4.2.6 (conduction system). The SA node is the primary pacemaker. If it fails, the AV node takes over as a secondary pacemaker but fires more slowly (~40–60 bpm vs 60–100), so the heart keeps beating at a reduced rate. Why: The failure scenario forces reasoning about backup pacemaker hierarchy.
9. Tests 4.2.10 (ABO/Rh). AB Rh⁺ blood carries A, B, and Rh(D) antigens. The recipient (O Rh⁻) has anti-A and anti-B antibodies (and can form anti-Rh), which would attack the donor cells → agglutination/hemolysis. O Rh⁻ is the universal donor because its RBCs carry no A, B, or Rh antigens, so no recipient antibodies attack them. Why: Antigen–antibody direction is the trap; must distinguish donor vs recipient compatibility.
10. Tests 4.2.7 (vessels) + 4.1.7 (pancreas). Artery: thick muscular/elastic wall, carries blood away from heart under high pressure, narrow lumen relative to wall. Capillary: single-cell (endothelium) wall, site of exchange, tiny lumen. The pancreas secretes digestive enzymes (amylase, lipase, proteases) and bicarbonate into the duodenum, plus bile (from liver, stored in gall bladder) emulsifies fats — aiding absorption in the small intestine served by these capillaries. Why: Two-part interleave: vessel comparison plus pancreatic role in the absorptive region.
[
{
"claim": "AV node backup rate (~50 bpm) is lower than SA node rate (~75 bpm)",
"code": "sa_rate = 75\nav_rate = 50\nresult = av_rate < sa_rate"
},
{
"claim": "Alimentary path in Q2 has 6 distinct named stages mouth->rectum",
"code": "path = ['mouth','pharynx','oesophagus','stomach','small_intestine','large_intestine']\nresult = len(path) == 6 and path[0]=='mouth' and path[-1]=='large_intestine'"
},
{
"claim": "O Rh- donor has zero antigens (A,B,Rh all absent)",
"code": "antigens = {'A':0,'B':0,'Rh':0}\nresult = sum(antigens.values()) == 0"
}
]