IB Biology HL Theme C: Interaction and interdependence questions

Question 1

Multiple choiceMedium1 mark

A purified enzyme forms a complex with each of four substrates. Shape-change scores compare each molecule’s shape in the complex with its unbound shape; 0 means no change, and a positive score means a change.

Substrate	Enzyme shape-change score	Substrate shape-change score
P	2	1
Q	2	0
R	0	1
S	0	0

Which substrate’s results are consistent with induced-fit binding?

Show answer and explanation

Answer: A. Induced-fit binding involves a change in shape of both the enzyme and the substrate. For P, both shape-change scores are positive, so both molecules change shape during complex formation.
B: uses the enzyme’s positive score but overlooks that the substrate’s score is zero.
C: uses the substrate’s positive score but overlooks that the enzyme’s score is zero.
D: shows no shape change in either molecule and therefore is not induced-fit binding.
Remember: induced-fit binding involves shape changes in both interacting molecules.

Question 2

Paper 2Hard16 marks

The reaction S → P occurs in a cell and is catalysed by enzyme E. To investigate the reaction, equal volumes containing the same initial amount of S and no P were held at the same temperature and pH, either with E or without E. Table 1 shows the measured concentration of P at different times. Values are reported to the nearest 0.1 mmol dm⁻³.

Time / min	[P] with E / mmol dm⁻³	[P] without E / mmol dm⁻³
0	0.0	0.0
1	2.8	0.1
2	5.1	0.2
4	6.9	0.4
8	7.8	0.8
16	8.0	1.7
32	7.9	3.2
64	8.0	5.5
128	8.0	7.8

Figure 1 shows schematic energy profiles for the conversion of S to P.

(a)

Calculate the mean rate of formation of P during the first 2.0 min, with E and without E. Include units.

[3]

(b)

Describe the change in [P] in each mixture during the first 8 min.

[2]

(c)

Using Figure 1, calculate how much E lowers the activation energy.

[1]

(d)

Explain how E causes the faster formation of P and state one benefit of this increase in reaction rate for cells.

[4]

(e)

A cell process Q requires a supply of P at an average rate of at least 1.2 mmol dm⁻³ min⁻¹ during the first 2.0 min. Using your results from (a), determine whether the reaction with E and the reaction without E meet this requirement.

[2]

(f)

A student concludes that “E increases the total amount of P that can be produced from a fixed starting amount of S, not just the rate at which P appears”. Evaluate this conclusion using the data at 2 min and 128 min, and any other relevant data in Table 1.

[4]

Question 3

Multiple choiceMedium1 mark

During aerobic respiration, a 3C pyruvate is processed before its acetyl group enters a cycle containing 4C oxaloacetate. Which description correctly links pyruvate processing to the carbon product and outputs of one cycle turn?

Show answer and explanation

Answer: D. Oxidative decarboxylation of 3C pyruvate leaves a 2C acetyl group. CoA transfers it to 4C oxaloacetate, forming 6C citrate; Krebs cycle oxidations and decarboxylations yield ATP and reduced NAD while regenerating oxaloacetate. A: gives the link reaction and citrate formation but stops before stating the cycle’s outputs and regeneration of oxaloacetate. B: reverses the redox direction; pyruvate is oxidized, not reduced. C: treats oxaloacetate as a net reactant, rather than a molecule regenerated by the cycle. Remember: acetyl groups enter the Krebs cycle by combining with oxaloacetate, which is then regenerated.

Question 4

Multiple choiceMedium1 mark

After epinephrine is added to a responsive cell, the concentration of cAMP in the cytoplasm rises. Which sequence explains this response?

Show answer and explanation

Answer: D. Epinephrine binds its transmembrane receptor, which activates a G protein. The activated G protein stimulates adenylate cyclase to produce cAMP inside the cell; cAMP acts as the second messenger that relays the signal to intracellular targets.
A: omits the G protein, incorrectly making the receptor activate adenylate cyclase directly.
B: gives the pathway correctly but misidentifies cAMP as the first messenger; epinephrine is the first messenger.
C: reverses the order of G protein and adenylate cyclase activation.
Remember: the first messenger binds the receptor; the second messenger relays the signal inside the cell.

Question 5

Multiple choiceMedium1 mark

In a neuron, ATP availability is reduced while the membrane remains intact. Assume this affects sodium–potassium pumping only. After sufficient time, which paired change is most likely?

Show answer and explanation

Answer: C. ATP supplies energy for sodium–potassium pumping, which establishes and maintains ion concentration gradients. With less ATP, pump activity decreases; over time, the gradients weaken and the resting potential becomes less negative.

A: predicts weaker gradients but reverses their effect on the resting potential. B: assumes the pump becomes more active despite reduced ATP availability. D: treats the pump as dissipating, rather than establishing and maintaining, concentration gradients.

Remember: ATP-driven pumping maintains the ion gradients that support the resting potential.

Question 6

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 7

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 8

Multiple choiceHard1 mark

During a controlled observation, blood CO₂ is raised briefly and then allowed to fall. The measurements are:

Stage	Blood CO₂ (relative units)	Blood pH	Ventilation rate (dm³ min⁻¹)
Baseline	1.0	7.40	6
After CO₂ rises	1.3	7.32	12
Recovery	1.1	7.38	7

Which statements are consistent with the data and feedback control of ventilation?

I. The initial CO₂ increase can lower blood pH by increasing blood acidity.
II. The fall in pH causes brainstem chemoreceptors to signal the diaphragm and intercostal muscles to reduce ventilation.
III. Increased ventilation can remove CO₂ from the blood, raising pH and reducing chemoreceptor stimulation so ventilation falls.

Show answer and explanation

Answer: B. Increased blood CO₂ can lower pH. Brainstem chemoreceptors respond to the pH change by signalling the diaphragm and intercostal muscles to increase ventilation; greater CO₂ removal then helps pH recover and reduces the stimulus.

A: stops at the initial pH change and omits the feedback described in III.
C: reverses the response in II and omits the supported CO₂–pH relationship in I.
D: includes II, which incorrectly says ventilation decreases when pH falls.
Remember: increased ventilation removes CO₂, helping blood pH rise.

Question 9

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 10

Multiple choiceMedium1 mark

A microbial population grows with a fixed supply of the nutrient shown; it is the only limiting resource, and no predators or pathogens are present. As population density rises, which paired identification and population response is expected?

Show answer and explanation

Answer: A. The Fischer projection shows an aldehyde at C1 and the characteristic OH arrangement of D-glucose. As population density rises with a fixed supply of the only limiting resource, competition reduces the amount available per individual and slows population growth, tending to return the population towards carrying capacity.

B: reverses the effect of increasing density on the resource available per individual. C: misidentifies glucose as fructose; the displayed structure has a terminal aldehyde group. D: treats a fixed resource supply as density-independent, although its availability per individual changes with population density.

Remember: density-dependent factors tend to counter changes in population size relative to carrying capacity.

Question 11

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]

Question 12

Multiple choiceMedium1 mark

The diagrams show energy transfers per 100 kJ of chemical energy entering cell respiration in representative autotrophic and heterotrophic cells. Which statement gives the total heat released in each cell type and the stages at which it is released?

Show answer and explanation

Answer: D. Heat is released at both stages shown, so add the heat outputs for each cell type: autotrophic cells, 40 + 25 = 65 kJ; heterotrophic cells, 30 + 20 = 50 kJ per 100 kJ input.

A: counts only the heat released during cell respiration. B: counts only the heat released when ATP is used. C: gives the totals but reverses the cell-type labels.

Remember: energy transfers are not 100% efficient, including when ATP is produced and when it is used.

Time / min	[P] with E / mmol dm⁻³	[P] without E / mmol dm⁻³
0	0.0	0.0
1	2.8	0.1
2	5.1	0.2
4	6.9	0.4
8	7.8	0.8
16	8.0	1.7
32	7.9	3.2
64	8.0	5.5
128	8.0	7.8

Time / min	[P] with E / mmol dm⁻³	[P] without E / mmol dm⁻³
0	0.0	0.0
1	2.8	0.1
2	5.1	0.2
4	6.9	0.4
8	7.8	0.8
16	8.0	1.7
32	7.9	3.2
64	8.0	5.5
128	8.0	7.8

IB Biology HL Theme C: Interaction and interdependence questions

The best questions are still waiting

Time / min	[P] with E / mmol dm⁻³	[P] without E / mmol dm⁻³
0	0.0	0.0
1	2.8	0.1
2	5.1	0.2
4	6.9	0.4
8	7.8	0.8
16	8.0	1.7
32	7.9	3.2
64	8.0	5.5
128	8.0	7.8