IB Biology HL Theme D: Continuity and change questions

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Question 1

Paper 2Hard16 marks

Researchers investigated how active sodium-ion transport in the ascending limb of the loop of Henle relates to water reabsorption in the collecting ducts. They compared three groups of mammals: normal transport, reduced transport, and transport restored after being reduced. Each group contained eight animals. Water intake, the volume of fluid entering the collecting ducts (2.0 mL h⁻¹ per animal), and collecting-duct permeability to water were the same in all groups. The treatment used to reduce transport acted selectively on active sodium-ion transport in the ascending limb. Values are group means.

ConditionRelative rate of active Na⁺ transport in ascending limb (normal = 100)Osmotic concentration at cortex (arbitrary units)Osmotic concentration at outer medulla (arbitrary units)Osmotic concentration at inner medulla (arbitrary units)Water reabsorbed from collecting ducts (%)Urine flow (mL h⁻¹)
Normal transport1003006001000700.60
Reduced transport40300420560421.16
Transport restored90300580960680.64
(a)

Calculate the percentage decrease in osmotic concentration at the inner medulla when transport is reduced compared with normal transport.

[2]
(b)

Compare the osmotic concentration profiles and collecting-duct outcomes in the three conditions.

[3]
(c)

Explain how active sodium-ion transport in the ascending limb can affect water reabsorption from the collecting ducts and urine flow.

[4]
(d)

In a separate test, collecting-duct segments with identical water permeability were used. The osmotic concentration of the fluid inside each segment was 300 units. The surrounding solution had the concentration shown below; its solute could not cross the duct wall. The percentage of the initial water volume inside the segment that crossed out after 10 minutes was measured.

Osmotic concentration of surrounding solution (arbitrary units)Water crossing out of segment (%)
3000
60024
100048

Deduce what this test adds to the interpretation of the first experiment.

[2]
(e)

A researcher concludes: “These results establish that active sodium-ion transport in the ascending limb is the only factor controlling water reabsorption from the collecting ducts.” Evaluate this conclusion using both experiments.

[5]

Question 2

Paper 2Hard16 marks

Scientists investigated the effects of carbon dioxide concentration and water temperature on coral fragments. Fragments were kept for 10 weeks under four conditions. Other conditions were kept constant, and each condition was tested in six replicate tanks. The table gives mean results.

CO₂ conditionSeawater pHTemperature / °CMean calcification rate / mg CaCO₃ g⁻¹ day⁻¹Fragments showing bleaching / %
Ambient8.1274.05
Elevated7.8272.66
Ambient8.1303.948
Elevated7.8302.550
(a)

Calculate the percentage decrease in mean calcification rate when CO₂ is elevated at 27 °C.

[2]
(b)

Compare the effects of elevated CO₂ and increased temperature on calcification and bleaching.

[4]
(c)

A separate survey was carried out at one reef in Year 0 and again in Year 12. No comparison reef was surveyed. Fish species were grouped as those recorded using living coral for shelter and those not recorded doing so; the two groups together make up total fish species richness.

MeasureYear 0Year 12
Mean seawater pH8.17.8
Maximum summer water temperature / °C2730
Live coral cover / %5219
Algal cover / %1037
Total reef fish species2818
Fish species using living coral for shelter167
Fish species not recorded using living coral for shelter1211

Compare the changes in the two fish groups from Year 0 to Year 12, using data.

[2]
(d)

Evaluate the claim that rising CO₂ and water temperature could cause coral loss and lead to collapse of the reef ecosystem. Use evidence from both studies and give one limitation of the evidence.

[8]

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