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.

Condition	Relative 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 transport	100	300	600	1000	70	0.60
Reduced transport	40	300	420	560	42	1.16
Transport restored	90	300	580	960	68	0.64

(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 (%)
300	0
600	24
1000	48

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

Multiple choiceHard1 mark

A record for one region covers four consecutive intervals, each 1 million years long. In each interval, the record identifies the ecosystem type and 10 species groups. The final column shows how many groups are also recorded in the immediately preceding interval. Which conclusion is supported by the complete record?

Time interval (million years ago)	Ecosystem type	Species groups shared with preceding interval
4–3	Forest	—
3–2	Forest	8
2–1	Forest	3
1–0	Forest	6

Show answer and explanation

Answer: A. All four consecutive intervals are identified as forest, spanning four million years. Fewer than 10 groups are shared at each transition, so species-group composition changed while the ecosystem type remained forest.
B: Confuses the same number of groups in each interval with the same groups; the overlap values are below 10.
C: Uses species-group turnover to infer ecosystem-type change, although every interval is identified as forest.
D: Stops at the most recent two intervals and overlooks the forest classifications in the earlier intervals.
Remember: continuity of ecosystem type does not require identical species composition.

Question 3

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 condition	Seawater pH	Temperature / °C	Mean calcification rate / mg CaCO₃ g⁻¹ day⁻¹	Fragments showing bleaching / %
Ambient	8.1	27	4.0	5
Elevated	7.8	27	2.6	6
Ambient	8.1	30	3.9	48
Elevated	7.8	30	2.5	50

(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.

Measure	Year 0	Year 12
Mean seawater pH	8.1	7.8
Maximum summer water temperature / °C	27	30
Live coral cover / %	52	19
Algal cover / %	10	37
Total reef fish species	28	18
Fish species using living coral for shelter	16	7
Fish species not recorded using living coral for shelter	12	11

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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