IB Biology HL B4.2 Ecological niches questions

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Question 1

Multiple choiceHard1 mark

A prokaryote was cultured with identical mineral nutrients and no organic compounds; CO₂ availability was controlled as shown. The table gives the net change in biomass after 24 h. Which conclusion is supported?

Light	CO₂ supplied	Net biomass change / mg in 24 h
Yes	Yes	+12
Yes	No	−2
No	Yes	−2
No	No	−2

Show answer and explanation

Answer: B. Photosynthetic nutrition uses light energy to make organic matter from inorganic carbon. Biomass increases only when both light and CO₂ are supplied (+12 mg in 24 h); each other condition gives a decrease. With no organic compounds supplied, this supports photosynthetic nutrition.

A: treats CO₂ as sufficient without light; the dark, CO₂-supplied culture loses biomass.
C: treats light as sufficient without CO₂; the illuminated culture without CO₂ loses biomass.
D: assumes either factor alone is sufficient; both single-factor conditions show biomass loss.
Remember: photosynthetic nutrition uses light energy and an inorganic carbon source.

Question 2

Paper 2Hard16 marks

A unicellular alga, A, and a photosynthetic prokaryote, P, were cultured separately or together. Each species present in a culture had an initial density of 100 cells mL⁻¹. Cultures were kept at the same temperature and pH, with inorganic carbon and mineral nutrients maintained at non-limiting concentrations. Light treatments received the same illumination for 12 hours each day; dark treatments received no light. The table shows mean cell densities after 10 days. Each mean is based on five replicate cultures.

Culture	A at start / cells mL⁻¹	A after 10 days / cells mL⁻¹	P at start / cells mL⁻¹	P after 10 days / cells mL⁻¹
A alone, light	100	800	—	—
A alone, dark	100	100	—	—
P alone, light	—	—	100	600
P alone, dark	—	—	100	100
A and P together, light	100	200	100	500
A and P together, dark	100	100	100	100

A dash indicates that the species was not present.

(a)

State the mode of nutrition shared by A and P, and explain how this mode supports population growth.

[3]

(b)

Calculate the percentage decrease in A’s mean density after 10 days when grown with P in light, compared with A grown alone in light.

[2]

(c)

Compare the changes in density of the two species when grown alone in light and in darkness. Use the data.

[3]

(d)

In the light treatments, the mean percentage of incident light reaching the base of the culture after 10 days was:

Culture	A alone	P alone	A and P together
Light reaching base / %	76	72	24

Evaluate the claim that P reduces the growth of A by competing with it for light. Use the population and light data, link the light data to photosynthetic nutrition, and identify and explain up to two limitations before reaching a conclusion.

[8]

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