Biology Year 11 · Module 2

Checkpoint Quiz 3

Covers Lessons 11–15: mammalian digestion, absorption and elimination, transport systems, the cardiovascular system, and gas exchange between internal and external environments.

20 questions15 MC + 5 short answer

Lessons coveredL11 · L12 · L13 · L14 · L15

Suggested time25–30 minutes

Section A — Multiple Choice

15 questions · 1 mark each

1. In the mammalian digestive system, where does chemical digestion of proteins begin?

Mouth — salivary amylase begins protein digestion alongside starch

Stomach — pepsin (activated from pepsinogen by HCl) hydrolyses proteins to polypeptides

Small intestine — pancreatic proteases first contact ingested protein here

Large intestine — bacteria produce proteases that digest remaining protein

2. The primary role of bile in digestion is to:

Chemically digest fats by cleaving ester bonds between fatty acids and glycerol

Neutralise stomach acid to create an alkaline environment for enzymes

Emulsify fats — break large fat droplets into smaller ones, increasing SA for lipase

Activate pepsinogen to pepsin in the stomach by lowering pH

3. Which correctly describes villi and microvilli in the small intestine?

Villi absorb amino acids and glucose; microvilli absorb fatty acids only

Microvilli are large folds of the intestinal wall; villi are microscopic cell projections

Villi increase SA at organ level; microvilli reduce diffusion distance by thinning the epithelium

Villi are finger-like projections of intestinal wall containing capillaries and lacteals; microvilli are projections on individual epithelial cells — both increase absorptive SA at different scales

4. Absorbed fatty acids and glycerol enter lacteals rather than blood capillaries because:

They are reassembled into large chylomicron complexes in epithelial cells — too large for blood capillaries, so they enter wider lacteals and travel via lymph

Fatty acids would dissolve in blood plasma, disrupting clotting mechanisms

Blood capillaries lack transport proteins for fatty acids; lacteals have specific fatty acid carriers

Fatty acids are absorbed passively through capillary walls and do not enter lacteals

5. Which correctly distinguishes open from closed circulatory systems?

Open systems have a heart; closed systems do not

Open systems contain haemoglobin; closed systems contain colourless haemolymph

In open systems, haemolymph leaves vessels and directly bathes tissues; in closed systems, blood remains within vessels at all times

Open systems are found only in vertebrates; closed systems only in invertebrates

6. Blood leaving the right ventricle flows to the:

Aorta → systemic circulation → body tissues → vena cava

Pulmonary artery → lungs → pulmonary vein → left atrium

Pulmonary vein → lungs → pulmonary artery → left atrium

Left atrium → bicuspid valve → left ventricle → aorta

7. Which structural feature makes capillaries uniquely suited for exchange of materials?

Thick muscular walls that push substances into surrounding tissue

Pocket valves ensuring unidirectional flow through the capillary bed

Elastic fibre walls that slow blood flow, increasing exchange time

Walls only one endothelial cell thick (~0.5 μm), minimising diffusion distance; network density ensures no cell is more than ~100 μm from a capillary

8. Which combination of O₂ and CO₂ levels is physiologically correct for the three locations: pulmonary artery, pulmonary vein, vena cava?

Pulmonary artery: low O₂ high CO₂ · Pulmonary vein: high O₂ low CO₂ · Vena cava: low O₂ high CO₂

Pulmonary artery: high O₂ low CO₂ · Pulmonary vein: low O₂ high CO₂ · Vena cava: high O₂ low CO₂

Pulmonary artery: low O₂ low CO₂ · Pulmonary vein: high O₂ high CO₂ · Vena cava: low O₂ low CO₂

All three vessels have identical O₂ and CO₂ concentrations — blood composition is regulated constant throughout the circuit

9. Which change would DECREASE the rate of gas exchange across the alveolar membrane, according to Fick's law?

Increasing the partial pressure gradient of O₂ between alveolar air and blood

Increasing alveolar SA by recruiting additional alveoli during exercise

Fluid accumulation between alveolar epithelium and capillary wall (pulmonary oedema), increasing membrane thickness

Increasing ventilation rate to maintain higher O₂ in alveolar air

10. Why do large multicellular animals require a specialised circulatory system rather than relying on diffusion alone?

Large animals require more complex molecules that diffusion cannot transport

Diffusion rate decreases with distance — interior cells are too far from the external environment for diffusion to supply O₂ and nutrients fast enough to meet metabolic demand

Large animals have lower SA:V ratios so diffusion is faster — but they still need circulation for other reasons

Diffusion only works through aqueous solutions; large terrestrial animals lack sufficient internal water

11. Which correctly states the partial pressures driving O₂ from alveolar air into pulmonary blood?

pO₂ alveolar air ≈ 40 mmHg; pulmonary blood ≈ 100 mmHg — O₂ moves from blood into air

pO₂ alveolar air ≈ 100 mmHg; pulmonary blood ≈ 100 mmHg — O₂ moves by active transport

pO₂ atmospheric air ≈ 160 mmHg; pulmonary blood ≈ 40 mmHg — O₂ moves by bulk flow

pO₂ alveolar air ≈ 100 mmHg; deoxygenated pulmonary blood ≈ 40 mmHg — O₂ diffuses down partial pressure gradient from air into blood

12. How is the majority of CO₂ transported in the blood?

~70% as bicarbonate ions (HCO₃⁻) in plasma — CO₂ enters RBCs, reacts with water → carbonic acid → H⁺ + HCO₃⁻

Entirely bound to haemoglobin — CO₂ binds to the haem group displacing O₂

Dissolved directly as CO₂ gas in plasma — ~100% transported in dissolved form

As carbonic acid molecules attached to plasma proteins

13. The key advantage of double circulation (pulmonary + systemic) over single circulation is:

Blood is completely separated from respiratory surfaces, preventing O₂ loss into lung tissue

Deoxygenated and oxygenated blood mix in the heart, providing tissues with a mixture for optimal delivery

Blood returning from the lungs at low pressure is re-pressurised by the left ventricle before the systemic circuit, allowing high-pressure delivery to all body tissues

Double circulation allows the heart to pump twice as much blood per minute, doubling metabolic rate

14. Which blood component is primarily responsible for defending the body against pathogens?

Platelets — they engulf and destroy bacteria through phagocytosis

White blood cells (leucocytes) — phagocytes engulf pathogens; lymphocytes produce antibodies

Red blood cells — haemoglobin binds to pathogen surface proteins, neutralising them

Plasma proteins — fibrinogen and albumin form an antibody network in the bloodstream

15. A patient with emphysema has destruction of alveolar walls, reducing alveoli count but enlarging remaining spaces. Which aspect of Fick's law explains impaired gas exchange?

Membrane thickness increases as remaining walls thicken to compensate

The O₂ concentration gradient is reduced because fewer alveoli produce less O₂

Blood flow to the lungs is reduced because fewer capillaries can surround enlarged spaces

Total surface area for gas exchange is dramatically reduced — fewer alveoli means smaller total exchange surface, directly reducing diffusion rate according to Fick's law

Section B — Short Answer

5 questions · variable marks

16. Trace the chemical digestion of a ham and cheese sandwich from mouth to small intestine. For each region where chemical digestion occurs, name one enzyme, its substrate, its product, and the organ/gland that produces it. 6 MARKS

One mark per complete enzyme set (name + substrate + product + source) — three regions required: mouth, stomach, small intestine.

17. Compare the structure and function of arteries and veins. Refer to: wall structure, relative lumen size, pressure, direction of blood flow, and valves. 5 MARKS

Must use comparative language (whereas/in contrast) — describing each separately scores Band 3. Need at least 4 of 5 features compared explicitly.

18. A patient develops pulmonary fibrosis — scar tissue accumulates between the alveolar epithelium and pulmonary capillaries, thickening the gas exchange membrane. Using Fick's law, predict and explain the effects on both O₂ uptake and CO₂ removal. 4 MARKS

Two marks O₂ (Fick prediction + mechanism), two marks CO₂ (Fick prediction + explanation of why CO₂ is less affected).

19. Explain the role of the large intestine in the formation and elimination of faeces. In your answer, describe what enters the large intestine, what is absorbed there, and what constitutes faeces. 3 MARKS

20. A student places a red dye solution (representing oxygenated blood) on one side of a thin permeable membrane and a clear solution (representing tissue fluid) on the other. After 10 minutes both sides are the same colour. The student claims "this demonstrates that gas exchange in alveoli works by the same mechanism." Evaluate this claim — what does the model correctly demonstrate, and state two significant limitations. 4 MARKS

1 mark correct demonstration + 1 mark per explained limitation (×2) + 1 mark evaluative conclusion.

Answers — Section A

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1. B — Protein digestion begins in the stomach with pepsin. Salivary amylase digests starch only. Pancreatic proteases continue digestion in the small intestine but are not the first site of protein digestion.

2. C — Bile emulsifies (physically breaks up) fat — it is not an enzyme and does not cleave covalent bonds. Sodium bicarbonate in pancreatic juice (not bile) neutralises stomach acid.

3. D — Villi are macroscopic projections of the intestinal wall itself (containing capillaries and lacteals). Microvilli are submicroscopic projections of individual epithelial cells on the surface of villi. Both increase SA at different structural scales.

4. A — Fatty acids and glycerol are reassembled into chylomicrons inside epithelial cells — too large to enter narrow blood capillaries. They enter the wider lacteals, travel in lymph through the thoracic duct, and enter blood at the left subclavian vein.

5. C — Open: haemolymph leaves vessels and bathes tissues directly in a haemocoel. Closed: blood stays inside vessels at all times, enabling higher pressure and more directed delivery.

6. B — Right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium. Pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood — named by direction, not O₂ content.

7. D — The one-cell-thick wall and high network density are the structural basis for exchange. Thick walls, valves, and elastic fibres belong to arteries and veins, not capillaries.

8. A — Pulmonary artery: deoxygenated blood going to lungs (low O₂, high CO₂). After lung gas exchange: pulmonary vein (high O₂, low CO₂). After systemic circuit: vena cava (low O₂, high CO₂).

9. C — Fick's law: Rate ∝ (SA × gradient) / thickness. Increasing thickness (denominator) decreases rate. Pulmonary oedema adds fluid between alveolus and capillary, increasing effective diffusion distance.

10. B — The diffusion distance problem: rate of diffusion decreases with distance squared. Interior cells of large organisms are too far from external surfaces for diffusion alone to supply metabolic needs at adequate rates.

11. D — Alveolar air pO₂ ≈ 100 mmHg; deoxygenated pulmonary blood pO₂ ≈ 40 mmHg. A gradient of ~60 mmHg drives O₂ diffusion from alveolar air into blood. (Atmospheric pO₂ ≈ 160 mmHg but alveolar air is diluted with water vapour and residual gas.)

12. A — ~70% as HCO₃⁻ (bicarbonate) in plasma. ~20–25% as carbaminohaemoglobin (CO₂ bound to globin chains, not haem). ~5–10% dissolved as CO₂ in plasma. CO₂ does NOT displace O₂ from the haem group — it binds to globin chains instead.

13. C — After passing through pulmonary capillaries, blood pressure has fallen. The left ventricle re-pressurises it to ~120 mmHg for high-pressure systemic delivery. Fish single circulation: blood pressure falls significantly after the gills and oxygen delivery to tissues is less effective.

14. B — WBCs (leucocytes) perform immune functions: phagocytes engulf pathogens; lymphocytes produce antibodies. Platelets are for clotting. RBCs transport O₂. Plasma proteins include transport proteins and clotting factors, not antibodies.

15. D — Emphysema destroys alveolar walls → fewer alveoli → reduced total SA → rate of diffusion falls proportionally (Fick's law: rate ∝ SA). Fewer but larger air spaces have less total SA than many small alveoli.

← Lesson 15: Gas Exchange — Internal and External Environments Continue to Lesson 16: Plant Vascular Systems →