Biology • Year 11 • Module 2 • Lesson 19
Secondary Source Analysis, Photosynthesis and Plant Transport Models
Apply the four-point evaluation framework to real source extracts, interpret a graph of photosynthesis rate data, and reason about how historical experiments build on each other.
1. Interpret photosynthesis rate data, two experimental conditions
A student measured the net O2 production of an aquatic plant at three light intensities and two temperatures, using a controlled CO2 concentration. The data are shown below. 7 marks
| Light intensity (lux) | O2 produced at 20°C (mL/hr) | O2 produced at 35°C (mL/hr) |
|---|---|---|
| 500 | 0.8 | 0.9 |
| 2 000 | 2.1 | 3.4 |
| 5 000 | 2.2 | 4.8 |
1.1 Describe the effect of increasing light intensity on O2 production at 20°C. 2 marks
1.2 Using Blackman’s concept of limiting factors, explain why increasing light intensity from 2 000 to 5 000 lux produces only a small increase at 20°C but a larger increase at 35°C. 3 marks
1.3 Predict what would happen to O2 production at 35°C if light intensity were increased to 10 000 lux but CO2 concentration was kept constant and low. Justify your prediction. 2 marks
2. Interpret a graph, O2 production over time by Priestley’s mint
The stylised figure below represents what Priestley might have observed if he could measure O2 concentration inside his sealed bell jar over 10 days. One line shows a jar with the candle and plant both in light; the other shows the same setup but with the plant shaded from light. 7 marks
Figure 2. Stylised O2 concentration inside sealed bell jars, illustrative model after Priestley (1771). Both jars start with 50% O2.
2.1 Describe the trend shown by the light curve from day 0 to day 10. 2 marks
2.2 Using lesson content, explain why the dark curve falls rather than rises. Name the process responsible. 2 marks
2.3 Explain what Ingenhousz’s contribution was, in terms of what the comparison between these two curves represents. 2 marks
2.4 Identify one limitation of Priestley’s original experiment (not shown in this graph) that reduced his confidence in the results. 1 mark
3. Apply the four-point framework, evaluate a source extract
Read the following adapted extract from a 1965 secondary school biology textbook, then answer the questions below using the lesson’s four-point evaluation framework. 7 marks
Extract 3.1, adapted from a 1965 NSW secondary biology textbook
“Photosynthesis is the process by which green plants use sunlight to manufacture their food from water and carbon dioxide. The oxygen released during this process is a by-product of the reaction. The rate of photosynthesis depends entirely on the intensity of light available; higher light always means faster photosynthesis. Plants absorb all the water they need for photosynthesis through their roots, and the water travels to the leaves by osmosis.”
3.1 Source type and currency. Identify the type of source and assess how current the information is likely to be. 2 marks
3.2 Claim vs evidence. Identify two scientific inaccuracies in the extract. For each, state the correct information from the lesson. 4 marks (2 per error)
3.3 Based on your evaluation, explain whether this source would be reliable for an HSC student studying photosynthesis today. 1 mark
4. Sequence the steps, how photosynthesis understanding developed
The eight events below are shuffled. Write their correct chronological order in the “Order” column (1 = earliest). 6 marks (1 per correct position, first 6 correct score full marks)
| Order | Event |
|---|---|
| Calvin’s team uses 14C tracer to map the full carbon fixation cycle. | |
| Ingenhousz identifies light as essential; distinguishes photosynthesis from respiration. | |
| De Saussure shows CO2 and water are both consumed; plant carbon comes from atmospheric CO2. | |
| Van Helmont plants a willow sapling and concludes mass comes from water, not soil. | |
| Blackman demonstrates two distinct stages, light-dependent and temperature-dependent. | |
| Priestley shows plants “restore” air consumed by a candle and can support combustion. | |
| Mitchell publishes the chemiosmotic theory, explaining ATP synthesis in the light-dependent reactions. | |
| The first gas thermometer is invented, enabling measurement of gas volumes in biological experiments. |
Q1.1, Trend at 20°C (2 marks)
O2 production increases as light intensity increases from 500 to 2 000 lux (0.8 to 2.1 mL/hr) [1], but then shows almost no further increase from 2 000 to 5 000 lux (2.1 to 2.2 mL/hr), suggesting a plateau or light saturation point [1].
Q1.2, Blackman’s limiting factor explanation (3 marks)
Blackman showed that photosynthesis has two stages: a light-dependent stage (limited by light) and a temperature-dependent stage (enzymatic; limited by temperature) [1]. At 20°C and high light, the light-dependent stage is no longer limiting, but the temperature-dependent (enzymatic) stage becomes the bottleneck, enzyme reaction rates are slower at lower temperatures [1]. At 35°C the enzymatic stage runs faster, so increasing light continues to increase overall O2 production, temperature was the limiting factor at 20°C once light was abundant [1].
Q1.3, Prediction at very high light + low CO2 (2 marks)
O2 production would plateau or stop increasing despite more light [1], because CO2 concentration (not light) would become the new limiting factor for the Calvin cycle, the plant cannot fix any more carbon without CO2, so it cannot use the additional light energy [1].
Q2.1, Light curve trend (2 marks)
The O2 concentration rises from 50% at day 0, increasing more steeply in the first few days, then levelling off and approaching an apparent maximum near day 7–10 [1]. The rate of increase slows as O2 accumulates, suggesting a limiting factor (e.g. CO2 availability in the sealed jar) [1].
Q2.2, Why dark curve falls (2 marks)
The plant in darkness cannot photosynthesise because light is unavailable, so it can only carry out cellular respiration [1]. Cellular respiration consumes O2 and releases CO2, so the O2 concentration inside the sealed jar falls over time [1].
Q2.3, Ingenhousz’s contribution (2 marks)
Ingenhousz’s contribution was to make exactly this comparison, showing that light-exposed plants produce O2 (rising curve) while dark plants consume O2 (falling curve) [1]. This demonstrated that light is the essential condition for O2 production, and that the two processes (photosynthesis in light vs. respiration in dark) are distinct and separable [1].
Q2.4, Limitation of Priestley’s experiment (1 mark)
Accept any one of: Priestley did not control for light availability, meaning that when he repeated the experiment the plant may have been in lower light and he could not reproduce the result; he made no quantitative gas measurements so could not confirm how much O2 was produced; he could not rule out microorganisms consuming O2 in the jar; there was only one plant with no replication (n = 1).
Q3.1, Source type and currency (2 marks)
This is a secondary source (a textbook interpreting scientific knowledge for a student audience) rather than a primary research report [1]. It was published in 1965, approximately 60 years before today’s HSC, meaning it predates significant refinements in understanding of limiting factors, the two-stage biochemical model at a molecular level, and the mechanism of water transport, so currency is low [1].
Q3.2, Two inaccuracies (4 marks)
Error 1. The claim that “the rate of photosynthesis depends entirely on the intensity of light available; higher light always means faster photosynthesis” is incorrect [1]. Blackman’s work (1905) showed that once light is no longer the limiting factor, further increases in light intensity produce no increase in rate, temperature or CO2 concentration becomes limiting instead [1].
Error 2. The claim that “water travels to the leaves by osmosis” is incorrect [1]. Water in the xylem moves primarily by cohesion-tension (transpiration pull), not by osmosis, which operates across semi-permeable membranes and cannot account for the ascent of water to the tops of tall trees at the rates observed [1]. Accept also: O2 is not merely a “by-product” in the colloquial dismissive sense, it is the direct product of photolysis of water in the light-dependent reactions, which is a specific and essential step.
Q3.3, Reliability for HSC use (1 mark)
This source would not be reliable for HSC study today. It contains scientific inaccuracies about limiting factors and water transport, both of which are examinable HSC content. Its low currency (1965) and secondary textbook status (no primary data) reduce its credibility further. A student using this source would risk learning and reproducing incorrect information in exams.
Q4, Sequence (1 mark each, first 6 correct)
Correct chronological order: 1, Van Helmont plants a willow sapling (1648) • 2, Priestley shows plants restore air (1771) • 3, Ingenhousz identifies light requirement (1779) • 4, The gas thermometer invention (late 1700s; enabled de Saussure’s quantitative work) • 5, De Saussure shows CO2 and water consumed (1804) • 6, Blackman demonstrates two stages (1905) • 7, Calvin uses 14C tracer (1950s) • 8, Mitchell publishes chemiosmotic theory (1961). Note: the gas thermometer entry is debatable in placement; accept it between Priestley and de Saussure.