HSC Exam Practice

Sample response. A limiting factor is the environmental variable in shortest supply that restricts the rate of a process, even if other variables are increased. Example: at high light intensity and normal temperature, increasing light intensity further has no effect on photosynthesis rate; temperature (affecting the enzyme-driven light-independent reactions) has become the limiting factor instead.

Marking notes. 1 mark for defining limiting factor as the variable in shortest supply that restricts the rate; 1 mark for a correct, contextualised example from photosynthesis (light, CO₂, temperature, or water all acceptable).

Sample response. Melvin Calvin (along with Benson and Bassham) determined the carbon fixation pathway in the 1950s. The technique used was paper chromatography to separate and identify radioactively labelled compounds produced from ¹⁴C-labelled CO₂.

Marking notes. 1 mark for Calvin (and/or team), 1950s; 1 mark for ¹⁴C tracer / radioactive labelling and paper chromatography (accept either or both). “X-ray crystallography” or “NMR” alone, 0.

Sample response. Reliability refers to the consistency of results across repeated trials under the same conditions, reliable data are reproducible. Validity refers to whether the experimental method actually measures what it claims to measure and whether the conclusion logically follows from the evidence. Van Helmont’s willow experiment illustrates the difference: it is relatively reliable (quantitative, clearly described, could be replicated) but has limited validity because an uncontrolled variable, CO₂ uptake from the atmosphere, means the conclusion “mass arose from water only” is not fully supported by the data collected.

Marking notes. 1 mark for defining reliability (reproducibility/consistency); 1 mark for defining validity (method measures what it claims; conclusion follows from data); 1 mark for correctly applying both concepts to a named historical example.

Sample response. (1) Pressure probe measurements: probes inserted into xylem of transpiring plants record pressures as low as −2 MPa, directly demonstrating the negative pressure (tension) the theory predicts. (2) Acoustic detection of cavitation: ultrasonic detectors placed on stems detect clicking sounds as water columns break under tension during drought stress, confirming that cohesion is the force that can be overwhelmed at the limits of the mechanism. (3) Dendrometer measurements: tree trunks contract slightly during peak transpiration hours and expand at night, consistent with tension in the xylem pulling the vessel walls inward. Accept also: heavy water (D₂O) isotope tracing; transpiration-uptake correlation (covering leaves stops water uptake).

Marking notes. 1 mark per correctly named and briefly explained evidence line. Naming alone without description, 0 per entry.

Sample response. Van Helmont’s experiment is valid in one specific respect: his data correctly show that the soil did not supply the tree’s mass (the soil lost less than 2 oz while the tree gained 164 lbs), so the rejection of soil as the primary source is a conclusion that follows from the evidence. However, his experiment had an uncontrolled variable that was also unmeasurable with 17th-century technology: atmospheric CO₂, which had not yet been discovered. In reality, carbon dioxide from the atmosphere is the primary source of the carbon that makes up plant biomass (as de Saussure later showed). Van Helmont’s conclusion is incomplete because he only measured two of the three inputs (soil and water) and was unable to detect the third (CO₂).

Marking notes. 1 mark for explaining what van Helmont’s data validly show (soil is not the mass source; data supports this); 1 mark for explaining why the conclusion is incomplete (CO₂ was unmeasured and undiscovered); 1 mark for identifying CO₂ as the actual source of plant carbon (de Saussure reference optional).

Sample response. Priestley (1771) showed that plants could “restore” air consumed by a burning candle, demonstrating for the first time that plants produce a gas (later identified as O₂) that supports combustion. However, Priestley could not always reproduce his results and did not know that light was required, when he inadvertently used plants in low light, the results were inconsistent. Ingenhousz (1779) built on Priestley’s work by systematically comparing plants in sunlight versus plants in complete darkness. He showed that only green parts of plants in light produce O₂, while plants in darkness actually consume O₂ and release CO₂ (cellular respiration). This extended Priestley’s finding in two ways: it explained Priestley’s inconsistent results (light was the uncontrolled variable) and it distinguished photosynthesis from cellular respiration as two separate processes for the first time.

Marking notes. 1 mark, Priestley: plants produce O₂ (or restore air); 1 mark, Ingenhousz: light is essential, green parts in light only; 1 mark, Ingenhousz explains Priestley’s inconsistency (light was the uncontrolled variable); 1 mark, Ingenhousz distinguishes photosynthesis from respiration. All four required for full marks; 3/4 for missing one.

Sample response (a). At 0.04% CO₂, the rate of O₂ evolution increases with increasing light intensity from 0 to approximately 3 000 lux, after which the rate approaches a plateau of approximately 12 units/hr and increasing light intensity produces no further increase. This plateau indicates that light intensity is no longer the limiting factor at that CO₂ concentration.

Sample response (b). At 6 000 lux, the rate at 0.08% CO₂ is substantially higher (approximately 20 units/hr) than at 0.04% CO₂ (approximately 12 units/hr, which has plateaued). According to Blackman’s concept of limiting factors, at high light intensities the light-dependent stage of photosynthesis is no longer limiting. The Calvin cycle (light-independent, temperature- and substrate-dependent stage) becomes the bottleneck. With more CO₂ available at 0.08%, the Calvin cycle can run at a higher rate because the substrate for carbon fixation is more abundant, so the overall rate of photosynthesis increases.

Sample response (c). Temperature. The Calvin cycle (light-independent reactions) is enzyme-catalysed, so its rate is temperature-dependent. At high light and high CO₂, if temperature is low, the enzymatic reactions of the Calvin cycle will limit the overall rate of photosynthesis even though light and CO₂ are not limiting. Accept also: water availability; chlorophyll concentration; enzyme concentration / activity (RuBisCO saturation).

Marking notes. (a) 1 mark, increase in rate with increasing light; 1 mark, plateau identified and linked to light no longer being limiting. (b) 1 mark, correctly notes that rate is higher at 0.08% CO₂ at 6 000 lux; 1 mark, Blackman’s limiting factor concept applied (CO₂ becomes the new limiting factor at high light); 1 mark, links CO₂ to the Calvin cycle / light-independent reactions as the substrate-limited step. (c) 1 mark, names a valid limiting factor; 1 mark, justification that correctly links the factor to a specific stage of photosynthesis.

Sample response. This extract is a popular science article from 1990, a secondary source written for a general audience, not for domain experts. Its credibility is therefore lower than a peer-reviewed journal article; popular science articles are often simplified and may not reflect current scientific consensus accurately.

In terms of currency, the article is approximately 35 years old. While the cohesion-tension theory had been well established by 1990, the description of root pressure as the primary mechanism for water transport to the tops of tall trees was already understood to be incorrect by that date: pressure probe measurements showing negative xylem pressures were available from the 1970s onwards.

Regarding claim vs evidence, the extract makes two scientifically inaccurate claims. First, root pressure is generated by osmosis at root cells and acts like a pump. Root pressure does exist and does generate some upward force via osmosis, but it is only sufficient to raise water a few metres at most, it cannot account for the ascent of water in tall trees (which may be 50–100 m). The claim that root pressure is “strong enough to push water up to the tops of the tallest trees” is not supported by any experimental evidence; it contradicts direct pressure probe measurements showing that xylem pressure in tall trees is deeply negative (−1 to −2 MPa), not positive as a pressure-driven mechanism would require. Second, the extract calls the process “entirely uncontroversial.” While the cohesion-tension theory is now the dominant explanation and is supported by multiple independent evidence lines, there have been and continue to be scientific debates about the exact contribution of root pressure, osmotic mechanisms, and living cell involvement, so claiming the matter is “entirely uncontroversial” overstates the consensus.

The correct mechanism is cohesion-tension theory: transpiration at leaf surfaces creates negative pressure (tension) in the xylem that is transmitted through the continuous water column to the roots via the cohesive hydrogen bonds between water molecules. This has been confirmed by pressure probe measurements (xylem at −2 MPa in tall trees), acoustic detection of cavitation at the limits of the mechanism, dendrometer trunk-diameter measurements, and heavy water isotope tracing.

Overall, this source is unreliable and invalid as a current description of plant water transport. Its source type (popular press), low currency relative to established evidence, and two specific scientific inaccuracies about the mechanism reduce its value for HSC study. It retains only historical value for showing the state of popular understanding in 1990.

Marking criteria.

• 1 markSource type correctly identified as secondary (popular science article) and credibility assessed relative to peer-reviewed sources.
• 1 markCurrency assessed: 1990 publication; notes that evidence against root pressure as the primary mechanism was available before 1990 (or simply that the source is 35 years old and may lag current understanding).
• 1 markIdentifies the first scientific inaccuracy: root pressure cannot push water to the tops of tall trees (only generates a few metres of pressure; xylem pressures are measured as negative, not positive).
• 1 markCorrectly describes the cohesion-tension mechanism as the actual explanation for water ascent in tall trees, with reference to transpiration pull and negative pressure.
• 1 markNames at least one specific line of experimental evidence supporting cohesion-tension theory (pressure probes, acoustic cavitation, dendrometers, isotope tracing).
• 1 markReaches a justified overall evaluative conclusion about the source’s reliability and validity for current use (not just lists errors, must evaluate the source as a whole).