Biology • Year 11 • Module 2 • Lesson 19
Secondary Source Analysis, Photosynthesis and Plant Transport Models
Lock in the timeline of six key photosynthesis experiments, the vocabulary of secondary source evaluation, and how scientific models change in response to new evidence.
1. Match the scientist to their discovery
The six scientists below made discoveries that built the modern understanding of photosynthesis. In the right-hand column, write the letter of the matching description from the list below the table. 6 marks (1 each)
| # | Scientist (year) | Your answer (A–F) |
|---|---|---|
| 1.1 | van Helmont (1648) | |
| 1.2 | Priestley (1771) | |
| 1.3 | Ingenhousz (1779) | |
| 1.4 | de Saussure (1804) | |
| 1.5 | Blackman (1905) | |
| 1.6 | Calvin (1950s) |
Description bank (shuffled):
- A. Used radioactive 14C-labelled CO2 and paper chromatography to trace the complete carbon fixation pathway; awarded the Nobel Prize in Chemistry 1961.
- B. Showed that plants release a gas (O2) that supports combustion, using a sealed bell jar with a mint sprig; did not know light was required.
- C. Demonstrated that plants gain most of their mass from water rather than soil by measuring the weight of a willow tree and pot of soil over five years.
- D. Used quantitative measurements to show that CO2 and water are both consumed during photosynthesis, identifying atmospheric CO2 as the source of plant carbon.
- E. Showed that photosynthesis has at least two distinct stages, one limited by light intensity and one limited by temperature, introducing the concept of limiting factors.
- F. Systematically compared plants in sunlight versus darkness, demonstrating that light is essential for O2 production and distinguishing photosynthesis from cellular respiration.
2. Term–definition match, secondary source evaluation
The ten definitions below are shuffled. Write the matching term from this list: secondary source, reliability, validity, limiting factor, cohesion-tension theory, cavitation, claim, currency, model, light-independent reactions. 10 marks
| # | Definition | Matching term |
|---|---|---|
| 2.1 | The explanation that water rises in xylem because transpiration at leaves creates tension that pulls a continuous column of cohesive water molecules upward. | |
| 2.2 | A representation of a system or process that can be revised when new evidence emerges; all scientific models are provisional. | |
| 2.3 | The environmental variable in shortest supply that restricts the rate of a process, even if other variables are increased. | |
| 2.4 | Information that interprets, analyses or summarises primary sources rather than reporting original experimental data. | |
| 2.5 | The extent to which an experiment measures what it claims to measure; whether the conclusion logically follows from the data. | |
| 2.6 | How up-to-date a source is; older sources may describe models that have since been revised. | |
| 2.7 | The formation of air bubbles in a xylem vessel when tension in the water column exceeds the cohesive strength of water. | |
| 2.8 | The stage of photosynthesis (Calvin cycle) in which CO2 is fixed into organic molecules using ATP and NADPH; does not directly require light. | |
| 2.9 | A statement or conclusion drawn from data; may be broader, narrower, or equivalent to what the evidence actually supports. | |
| 2.10 | The extent to which results can be reproduced under the same conditions; consistent, repeatable data from multiple trials. |
3. True or false, with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 10 marks (1 T/F, 1 correction where needed)
3.1 Van Helmont correctly identified that atmospheric CO2 is the main source of plant carbon. T / F
3.2 Priestley's bell jar experiment showed that plants release a gas that supports combustion, but he did not know light was required. T / F
3.3 Ingenhousz demonstrated that all parts of a plant, whether in light or darkness, release O2 at the same rate. T / F
3.4 Scientific models are permanent once they are widely accepted by the scientific community. T / F
3.5 The cohesion-tension theory is supported by multiple independent lines of evidence, including pressure probe measurements and acoustic detection of cavitation. T / F
4. Function recall, what did each experiment fail to explain?
Each question below names a scientist and states what their experiment revealed. In 1–2 sentences, state one important question that each experiment left unanswered. 10 marks (2 each)
4.1 Van Helmont (1648) showed that plants gain most of their mass from water. What did his experiment fail to explain?
4.2 Priestley (1771) showed that plants produce a gas that supports combustion. What did his experiment fail to explain?
4.3 Ingenhousz (1779) showed that light is essential for O2 production. What did his experiment fail to explain?
4.4 De Saussure (1804) showed that CO2 and water are both consumed during photosynthesis. What did his experiment fail to explain?
4.5 Blackman (1905) showed that photosynthesis has two stages. What did his experiments fail to explain?
5. Build a concept map, how models evolve
Draw labelled arrows between the five terms below to show how they connect in the process of scientific model revision. Each arrow must carry a linking phrase (e.g. “leads to”, “is revised by”, “generates”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: new experimental evidence · current scientific model · new technology · revised model · unanswered questions.
6. Cloze, complete the paragraph
Fill each blank with the correct word or phrase from the word bank below. Use each word or phrase once. 8 marks
Word bank: Calvin cycle • two stages • negative pressure • water • CO2 • light • cohesion • provisional
The understanding of photosynthesis developed over three centuries as each new experiment revealed one piece of the mechanism while leaving others unexplained. Van Helmont showed that plants absorb (6.1) , but he did not know that atmospheric (6.2) is also consumed. Ingenhousz demonstrated that (6.3) is required for O2 production. Blackman’s experiments established that photosynthesis occurs in (6.4) , one driven by light and one driven by temperature. Calvin and colleagues later traced the carbon fixation pathway known as the (6.5) . All scientific models are (6.6) subject to revision as new evidence emerges. Similarly, the cohesion-tension theory of plant water transport proposes that water molecules are held together by (6.7) forces and are pulled upward by (6.8) created at the leaf surface during transpiration.
Q1, Scientist–discovery match
1.1 C (van Helmont, willow/water experiment) • 1.2 B (Priestley, bell jar O2) • 1.3 F (Ingenhousz, light requirement) • 1.4 D (de Saussure, CO2 and water quantified) • 1.5 E (Blackman, two stages / limiting factors) • 1.6 A (Calvin, 14C tracer / Calvin cycle / Nobel Prize).
Q2, Term–definition matches
2.1 cohesion-tension theory • 2.2 model • 2.3 limiting factor • 2.4 secondary source • 2.5 validity • 2.6 currency • 2.7 cavitation • 2.8 light-independent reactions • 2.9 claim • 2.10 reliability.
Q3, True/False with correction
3.1 False. Van Helmont did not know about CO2 at all, it had not yet been discovered. He concluded mass came from water alone; de Saussure (1804) was the first to show atmospheric CO2 is the actual source of plant carbon.
3.2 True.
3.3 False. Ingenhousz showed the opposite: only green parts of plants in light release O2, while all parts of plants in darkness actually consume O2 and release CO2 (cellular respiration).
3.4 False. Scientific models are provisional, always subject to revision when new experimental evidence emerges. The history of photosynthesis understanding (van Helmont to Calvin) is a direct example.
3.5 True.
Q4, What each experiment failed to explain
4.1 Van Helmont could not account for the role of atmospheric CO2 (undiscovered) as the actual source of plant carbon. His experiment had no way to measure gas uptake from the air, so the origin of the carbon in plant biomass remained entirely unknown.
4.2 Priestley did not know that light was required for O2 production, which explains why he could not always reproduce his results. He also could not explain the biochemical mechanism, what the plant was actually doing to produce the gas.
4.3 Ingenhousz could not identify CO2 as a substrate consumed during photosynthesis. He could not explain where the carbon in plant material came from, or what role water played in the reaction at a chemical level.
4.4 De Saussure could not explain the biochemical mechanism, how CO2 and water were combined at the molecular level, or what role light played in the reactions. The “dark reactions” (Calvin cycle) were entirely unknown.
4.5 Blackman could not identify the specific chemical pathway of carbon fixation or the molecular identity of his two stages. The structure of the light-dependent and light-independent reactions at the biochemical level was not clarified until the 1950s–1960s.
Q5, Sample concept map
A correct map should include arrows such as:
- current scientific modelleaves → unanswered questions
- unanswered questionsmotivate development of → new technology
- new technologyenables collection of → new experimental evidence
- new experimental evidencechallenges or extends → current scientific model
- new experimental evidenceleads to → revised model
- revised modelitself becomes the next → current scientific model
Award full marks for at least 5 correctly labelled arrows that respect causal direction. Concrete example check: van Helmont’s balance (new technology) → willow experiment (new evidence) → “mass from water” model (revised model) → de Saussure’s gas measurement (newer technology) → CO2 role identified (further revision).
Q6, Cloze answers
6.1 water • 6.2 CO2 • 6.3 light • 6.4 two stages • 6.5 Calvin cycle • 6.6 provisional • 6.7 cohesion • 6.8 negative pressure.