IB Biology HL Theme C: Interaction and interdependence questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type

Multiple choice Written

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Question 1

Paper 2Medium16 marks

A stimulus initiates an action potential at site A of an axon. Recording sites B and C are further along the same axon. The threshold potential is −55 mV.

Recording site	Distance from A / mm	Time at which peak potential is reached after the stimulus / ms	Peak potential / mV
A	0	1.0	+30
B	4	4.2	+30
C	8	7.4	+30

(a)

State the threshold potential for an action potential in this axon.

[1]

(b)

Calculate the propagation speed of the action potential between B and C. Show your working and include a unit.

[3]

(c)

Describe how the time at which the peak potential is reached changes with distance from A. Use the data to support your answer.

[2]

(d)

Explain how local currents arising at A can bring the adjacent region of the axon to threshold and initiate an action potential there. Include the movement of sodium ions inside and outside the axon.

[5]

(e)

The experiment is repeated with the same stimulus, but with a reduced concentration of sodium ions outside the axon. An action potential is still initiated at A in the reduced-sodium condition. The maximum potential reached at B and whether an action potential is detected there are recorded.

Condition	Maximum potential at B / mV	Action potential detected at B
Normal external sodium-ion concentration	+30	Yes
Reduced external sodium-ion concentration	−58	No

Using the data and the threshold potential, explain why reducing the external sodium-ion concentration affects propagation to B.

[5]

Question 2

Paper 2Medium4 marks

Twenty-four participants learned to choose one of two buttons to obtain a reward. On each trial, one of two shapes appeared on a screen while one of two tones was played. There were four shape–tone combinations, and the correct button depended on the combination: neither the shape alone nor the tone alone predicted the correct button. Participants received feedback during four training blocks. Twenty-four hours later, they completed a test without feedback or further practice. With two possible buttons, chance performance is 50%.

The graph shows the mean percentage of correct choices in each block and at the 24-hour test. Means are rounded to the nearest whole percentage point.

(a)

Using Figure 1, explain what the results suggest about learning and memory.

[2]

(b)

Explain why successful performance in this task involves the brain processing information from both inputs.

[1]

(c)

State how this differs from the role of the spinal cord as an integrating centre.

[1]

Question 3

Paper 2Medium4 marks

Researchers added the same concentration of an antibiotic to bacterial cultures and to cultures of human lymphocytes, some of which were infected with HIV. Figure 1 shows measurements after 24 hours, expressed relative to the corresponding culture without antibiotic.

(a)

Compare the effect of the antibiotic on bacterial cells and uninfected human lymphocytes, using Figure 1.

[1]

(b)

Using Figure 1 and your knowledge of antibiotics, explain why the antibiotic does not control HIV.

[1]

(c)

Explain how HIV infection can result in opportunistic infections.

[2]

Question 4

Paper 2Medium16 marks

Researchers investigated factors affecting plant density in a meadow community. Plots received either ambient nutrient levels or additional nutrients. Herbivores either had access to the plots or were excluded. The table shows the mean density of established plants after 10 weeks.

Nutrient level	Herbivores had access (plants m⁻²)	Herbivores excluded (plants m⁻²)
Ambient	120	180
Additional nutrients	210	300

In a separate investigation, researchers tested juglone, released by the roots of black walnut trees (Juglans nigra), and penicillin, released by Penicillium mould.

Source and substance tested	Response measured	Control (no substance)	Substance present
Black walnut; juglone	Germination of competing plant seeds after 5 days (%)	80	40
Penicillium; penicillin	Bacterial growth index (control set to 100)	100	25

(a)

For the plant population, state which type of control is tested by (i) adding nutrients and (ii) excluding herbivores.

[2]

(b)

Calculate the mean increase in plant density associated with (i) nutrient addition and (ii) herbivore exclusion. Show your working.

[4]

(c)

Which type of control appears stronger in these plots? Justify your answer using your results from part (b).

[2]

(d)

Calculate the percentage decrease in the response for each substance, relative to its control.

[4]

(e)

Identify the process shown by each example in the second table. State one similarity and one difference between the processes.

[4]

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