IB Biology SL Theme C: Interaction and interdependence questions

Question 1

Multiple choiceEasy1 mark

A cell’s metabolic reactions must occur quickly enough to support its activities. Which role of enzymes provides this benefit?

Show answer and explanation

Answer: D. Enzymes are catalysts that increase the rates of reactions. Faster metabolic reactions can support the activities of the cell.

A: confuses catalysis with increasing the supply of reactants. B: treats enzymes as an energy source rather than catalysts. C: treats enzymes as reactants that are converted into products.

Remember: enzymes increase reaction rates without being used up.

Researchers cultured equal numbers of the same type of animal cell under identical conditions, supplying a different carbon compound to each group. They measured the rates of ATP synthesis and oxygen uptake.

Carbon compound supplied	ATP synthesis rate (relative units min⁻¹ per 10⁶ cells)	Oxygen uptake rate (µmol min⁻¹ per 10⁶ cells)
None added	2.0	1.0
Glucose	8.0	4.0
Fatty acids	6.0	5.0

(a)

Calculate the percentage increase in ATP synthesis rate when glucose is supplied compared with when no carbon compound is added. Show your calculation.

[3]

(b)

State which of glucose or fatty acids gives the higher ATP synthesis rate, and state the difference between their rates.

[2]

(c)

Explain how energy from cell respiration can be transferred to active transport through the interconversion of ATP and ADP.

[4]

(d)

Distinguish between gas exchange and cell respiration.

[2]

(e)

A student claims that oxygen uptake is a direct measure of the rate of ATP synthesis. Evaluate this claim using the data, and identify two limitations of the evidence.

[5]

Question 3

Multiple choiceMedium1 mark

Band X from a leaf extract is yellow-green. The solvent front travelled 8.0 cm and X travelled 3.6 cm. Reference chromatographic values and the absorption spectrum of X are shown.

Reference pigment	Appearance	Rf
Chlorophyll a	Blue-green	0.60
Chlorophyll b	Yellow-green	0.45
Xanthophyll	Yellow	0.70
Carotene	Orange	0.95

Wavelength / nm (colour)	450 (blue)	550 (green)	680 (red)
Relative absorption by X	0.85	0.10	0.70

Which conclusion about X is supported by the results?

Show answer and explanation

Answer: B. The Rf value is distance travelled by the pigment divided by distance travelled by the solvent: 3.6/8.0 = 0.45. This matches chlorophyll b and is consistent with the yellow-green appearance. At 550 nm (green), X has its lowest relative absorption, so fewer electrons in X are excited and less light energy is available to support conversion to chemical energy.

A: reverses the spectrum reading; green light has the lowest, not the highest, absorption. C: reads the low green absorption correctly but assigns chlorophyll a, whose reference Rf is 0.60. D: mislabels 550 nm as red; the table identifies this wavelength as green.

Remember: wavelengths absorbed by a pigment can excite its electrons and contribute to photosynthesis.

Question 4

Multiple choiceEasy1 mark

When an impulse reaches a neuron terminal, the presynaptic membrane depolarizes and neurotransmitter is released. Which ion movement acts as the intracellular signal for this release?

Show answer and explanation

Answer: C. Depolarization of the presynaptic membrane causes Ca²⁺ uptake into the neuron. Ca²⁺ acts as a signalling chemical inside the neuron and promotes neurotransmitter release. A: confuses K⁺ efflux, associated with repolarization, with the signal for neurotransmitter release. B: confuses Na⁺ influx, associated with depolarization, with the ion that signals release. D: reverses the direction: Ca²⁺ enters rather than leaves the presynaptic neuron. Remember: Ca²⁺ entry at the presynaptic membrane signals neurotransmitter release.

Question 5

Paper 2Medium16 marks

An electrical impulse was initiated at a point on the axon of a neuron. Detectors recorded the time at which the impulse reached different distances along the same axon. Figure 1 shows the results. Times and distances were recorded to the nearest 0.1 ms and 0.1 cm, respectively.

(a)

State what forms the cell body of a neuron, and give two differences between an axon and dendrites.

[3]

(b)

Use Figure 1 for parts (i) and (ii).

(i)

Describe the relationship between distance from the stimulation point and time of detection shown in Figure 1.

[2]

(ii)

Determine the average speed of the impulse along the axon. Give your answer in m s⁻¹.

[3]

(c)

Use the best-fit line in Figure 1 to estimate the time at which the impulse would be detected 7.0 cm from the stimulation point.

[2]

(d)

Explain how the arrangement of a neuron’s fibres can enable it to conduct impulses over long distances and along multiple branches.

[3]

(e)

A student concludes that Figure 1 proves that all neurons conduct electrical impulses at the same speed. Evaluate this conclusion using Figure 1.

[3]

Question 6

Paper 2Easy5 marks

A healthy adult’s pineal gland secretes melatonin. The table shows relative melatonin secretion over 24 hours. A student walks along a narrow beam while catching a ball.

Time	Relative melatonin secretion / arbitrary units
08:00	2
14:00	1
20:00	5
02:00	9
08:00 next day	2

(a)

Describe the pattern of melatonin secretion shown in the table.

[1]

(b)

Explain how this pattern helps establish a cycle of sleeping and waking.

[2]

(c)

State two roles of the cerebellum in the student’s movements.

[2]

Question 7

Multiple choiceMedium1 mark

A sharp object touches a hand, which is then withdrawn. The table gives the first time after contact at which activity is detected at each site. Which pathway and response description is consistent with the data?

Site	First activity after contact / ms
Sensory neuron entering spinal cord	2
Skeletal muscle of arm	9
One interneuron in grey matter of spinal cord	4
Free sensory nerve ending in hand	0
Motor neuron leaving spinal cord	6

Show answer and explanation

Answer: D. The activity occurs in sequence at the free sensory nerve ending, sensory neuron, spinal interneuron, motor neuron and skeletal muscle. This is an involuntary pain reflex pathway, with the skeletal muscle as the effector.
A: reverses the order of the interneuron and motor neuron, contrary to their recorded activity times.
B: gives the correct pathway but misclassifies the reflex response as voluntary.
C: substitutes smooth muscle for the skeletal muscle recorded as active.
Remember: a pain reflex passes from a sensory neuron through an interneuron in spinal grey matter to a motor neuron and skeletal muscle.

Question 8

Multiple choiceMedium1 mark

A patient is diagnosed with a viral infection, and an antibiotic is considered. Which explanation accounts for why the antibiotic will not control the infection?

Show answer and explanation

Answer: B. Antibiotics block processes that occur in bacteria but not in eukaryotic cells. Viruses rely on host-cell processes to reproduce, so they do not carry out the bacterial processes targeted by antibiotics.
A: correctly identifies viral reliance on human cells, but incorrectly claims the antibiotic target is shared with human cells.
C: identifies the bacterial target, but incorrectly states that viruses reproduce independently of host cells.
D: correctly describes viral reliance on host cells, but reverses the selectivity of antibiotics.
Remember: antibiotics target bacterial processes, not viral reproduction in host cells.

Question 9

Paper 2Medium16 marks

A group of 100 people with HIV was followed from the time their infection was confirmed. Researchers recorded the mean count of a particular type of lymphocyte and the percentage of participants who had at least one opportunistic infection.

Years after HIV infection was confirmed	Mean lymphocyte count (cells per μL)	Participants with at least one opportunistic infection (%)
0	900	2
3	690	6
6	420	24
9	180	50

(a)

Calculate the percentage decrease in mean lymphocyte count between year 0 and year 9.

[3]

(b)

Describe the changes in both measurements between year 0 and year 9, using the data.

[4]

(c)

Identify the type of lymphocyte infected by HIV and state what HIV does to these cells.

[2]

(d)

Explain how the decline in these lymphocytes can lead to AIDS.

[3]

(e)

Explain two mechanisms by which HIV can be transmitted in body fluids: one involving blood and one during sexual intercourse.

[4]

Question 10

Multiple choiceMedium1 mark

A simplified density-index series for the documented Canadian lynx–snowshoe hare case is shown; higher values indicate greater population density. Which interpretation is supported by the data?

Year	Hare density index (arbitrary units)	Lynx density index (arbitrary units)
1	20	5
2	50	12
3	90	28
4	40	45
5	15	22

Show answer and explanation

Answer: C. The hare density index is highest in year 3 (90), while the lynx index is highest in year 4 (45). This lag is consistent with predator abundance responding to prey availability; predation can then contribute to a decline in hare density, a density-dependent effect.

A: reverses the timing of the two population peaks.
B: reads the peak timing correctly but misclassifies a predator response to prey density as density-independent.
D: treats the peaks as simultaneous, although the hare peak occurs one year earlier.
Remember: a delayed predator increase following a rise in prey density is consistent with density-dependent predator–prey control.

Question 11

Paper 2Medium16 marks

Two rabbit populations were monitored in separate fenced grassland enclosures, each with an area of 2.0 ha. The enclosures had similar vegetation and water supplies. At year 0, each enclosure contained 20 rabbits. From year 1 onwards, supplementary food was added to enclosure B each year; no supplementary food was added to enclosure A. Other management was kept the same.

Year	Rabbits in enclosure A	Rabbits in enclosure B
0	20	20
1	39	44
2	55	67
3	65	85
4	70	96
5	69	101
6	71	99

(a)

Calculate the approximate rabbit population density at carrying capacity in enclosure A. Give your answer with a unit.

[2]

(b)

Describe the changes in rabbit population size in enclosures A and B over the seven years.

[3]

(c)

Define carrying capacity and give two examples of resources that may limit it.

[3]

(d)

Explain how competition for limited resources affects a rabbit population as it approaches carrying capacity.

[4]

(e)

Use the data to explain what the comparison between enclosures A and B suggests about the effect of food availability on carrying capacity. State one limitation of this comparison.

[4]

Question 12

Multiple choiceEasy1 mark

The diagram shows the inputs used by four organisms that synthesize carbon compounds. Which organism is an autotroph?

Show answer and explanation

Answer: C. An autotroph uses an external energy source to synthesize carbon compounds from simple inorganic substances. Organism C receives external light energy and its sole carbon-containing input is CO₂, a simple inorganic substance.

A: treats glucose, an organic carbon source, as a simple inorganic substance.
B: treats amino acids, which are organic, as a simple inorganic carbon source.
D: treats starch, an organic carbon source, as a simple inorganic substance.
Remember: autotrophy depends on the source of carbon, not only on the energy source.

Question 13

Paper 2Medium3 marks

A bacterial culture was grown in darkness in a medium in which carbon dioxide was the only source of carbon. The same amount of iron was supplied in each culture, but in different forms. All other conditions were identical.

Form of iron supplied	Change in bacterial biomass after 48 hours
Iron(II) compounds	Increase
Iron(III) compounds	No increase

(a)

Classify these bacteria as photoautotrophs or chemoautotrophs.

[1]

(b)

Explain how the bacteria can produce carbon compounds in the condition where biomass increased. Refer to both the carbon source and the energy source.

[2]

IB Biology SL Theme C: Interaction and interdependence questions

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