IB Biology SL Theme C: Interaction and interdependence questions

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.

Answer: D. The solute moves from a compartment with fewer particles to an equal-sized compartment with more particles, so it moves against the concentration gradient and requires energy. At X, ATP is hydrolysed to ADP and phosphate, releasing energy that can drive this transport. At Y, ATP is synthesized from ADP and phosphate, which requires an energy input.
A: reverses the energy changes for both ATP hydrolysis and ATP synthesis.
B: correctly identifies energy release at X but reverses the energy requirement for ATP synthesis.
C: correctly identifies the energy input for ATP synthesis but treats ATP hydrolysis as requiring energy input.
Remember: ATP hydrolysis releases energy; ATP synthesis requires energy.

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.

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.

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.

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.

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.

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.