Biology • Year 11 • Module 2 • Lesson 8
Photosynthesis, Products, Movement and Function
Apply transpiration-cohesion-tension, phloem pressure-flow, and the historical development of photosynthesis models to new data and scenarios.
1. Interpret transpiration rate data
A student used a potometer to measure the rate of water uptake (as a proxy for transpiration) in a leafy shoot under four conditions. Each condition was maintained for 20 minutes. Results are shown below. 8 marks
| Condition | Avg. water uptake (mm³ min⁻¹) |
|---|---|
| Still air, 20 °C | 12 |
| Moving air (fan), 20 °C | 31 |
| Still air, 35 °C | 28 |
| Moving air (fan), 35 °C | 54 |
1.1 Describe the effect of moving air (fan) on water uptake rate at both temperatures. Quote data in your answer. 2 marks
1.2 Using the transpiration-cohesion-tension theory, explain why moving air increases the rate of water uptake through the plant. 3 marks
1.3 A student suggests that the highest water uptake rate (54 mm³ min⁻¹) is advantageous for the plant because more water reaches the leaf for photosynthesis. Evaluate this suggestion, what does the plant gain and potentially lose at this rate? 3 marks
2. Phloem transport, source-to-sink reasoning
The diagram below (represented as a simple data table) shows sucrose concentration measurements in a bean plant at three locations during midday on a sunny day. 6 marks
| Location | Role (source or sink?) | Sucrose concentration in phloem sap (mg mL⁻¹) |
|---|---|---|
| Mature photosynthetic leaf | ? | 120 |
| Mid-stem phloem | 95 | |
| Developing fruit (growing) | ? | 40 |
2.1 Identify whether the mature leaf and the developing fruit are sources or sinks. Justify your answer using the sucrose concentration data. 2 marks
2.2 Using the pressure-flow hypothesis, explain how the sucrose concentration gradient in the table drives phloem transport from leaf to fruit. 3 marks
2.3 At night, photosynthesis stops. Predict what would happen to the sucrose concentration in the leaf phloem, and explain the effect on transport direction and rate. 1 mark
3. Historical experiments, identify the scientist and the gap
Each description below matches an experiment from lesson Card 5's historical timeline. For each one: (a) name the scientist, (b) state what it revealed, and (c) identify one thing it did NOT explain. 9 marks (3 per experiment)
Experiment A (date clue: 1648)
A researcher grew a willow tree in a pot for five years. The tree gained 74 kg but the soil lost only 57 g. The researcher concluded that most plant mass must come from water alone.
(a) Scientist:
(b) What it revealed:
(c) What it failed to explain:
Experiment B (date clue: 1779)
A researcher repeated previous gas experiments using sealed jars with plants, but varied whether jars were placed in sunlight or kept in darkness. In sunlight, a candle could burn in the jar after the plant was added; in darkness, the plant did not restore the air.
(a) Scientist:
(b) What it revealed:
(c) What it failed to explain:
Experiment C (date clue: 1950s)
A researcher supplied algae with radioactively labelled CO₂ for very short time intervals, then immediately stopped the reaction and separated the products by paper chromatography to trace which molecules had incorporated the label.
(a) Scientist:
(b) What it revealed:
(c) What it failed to explain:
4. Apply to a new scenario, a potato plant during prolonged darkness
A potato plant is kept in a darkroom for 72 hours. During this time, the plant continues to carry out cellular respiration but cannot photosynthesise. 5 marks
4.1 Identify which of the five fates of glucose would not be occurring during this period, and explain why. 2 marks
4.2 Explain what would happen to the starch stored in the tubers during the 72-hour dark period. Link your answer to one of the five fates. 2 marks
4.3 Would phloem transport of sucrose continue during the dark period? Justify your prediction using lesson content. 1 mark
Q1.1, Effect of moving air (2 marks)
Moving air increased water uptake rate at both temperatures. At 20 °C, rate increased from 12 to 31 mm³ min⁻¹ (a 2.6× increase). At 35 °C, rate increased from 28 to 54 mm³ min⁻¹ (a ~1.9× increase). In both cases, moving air roughly doubled or more than doubled the rate [1]. The effect was larger in absolute terms at the higher temperature, suggesting temperature and air movement have additive effects [1].
Marking criteria: 1 mark for correctly describing the direction of effect (increased) at both temperatures with reference to at least one data value; 1 mark for noting the magnitude of the difference (quoting a second data value or describing it as roughly double).
Q1.2, Why moving air increases water uptake (3 marks)
Moving air removes water vapour from around the stomata, maintaining a steeper water vapour concentration gradient between the moist leaf interior and the drier air outside [1]. This accelerates transpiration, water evaporates more rapidly from mesophyll cell walls and exits through stomata [1]. Faster transpiration lowers water potential in leaf cells more quickly, drawing more water from the xylem, which increases tension in the xylem column and pulls water up more rapidly from the roots, hence a higher water uptake rate [1].
Marking criteria: 1 mark for identifying that moving air removes water vapour, steepening the concentration gradient; 1 mark for linking this to faster evaporation (transpiration) at the leaf surface; 1 mark for connecting increased transpiration to increased tension in the xylem and therefore higher water uptake from roots.
Q1.3, Evaluate high transpiration rate (3 marks)
Gains: More water reaches the leaf via xylem, supplying H₂O as a reactant for the light-dependent reactions of photosynthesis, and providing the cooling effect of evaporation [1]. The high transpiration also delivers dissolved mineral ions (via xylem) to photosynthetic cells, supporting enzyme function and biosynthesis [accept any one gain].
Losses / risks: The plant risks losing water faster than it can be absorbed from the soil, if stomata cannot close quickly enough, the plant may experience water deficit, causing stomata to close, which would then reduce CO₂ entry and slow photosynthesis, or in extreme cases cause wilting and cell damage [1]. There is also the energetic cost of maintaining the water column under high tension, and the risk of cavitation (air bubbles entering xylem) if tension exceeds the tensile strength of the water column [1].
Marking criteria: 1 mark for identifying a valid gain (water for photosynthesis, mineral delivery, cooling); 1 mark for identifying the risk of water deficit / stomatal closure limiting CO₂ uptake; 1 mark for either linking stomatal closure to reduced photosynthesis or discussing cavitation risk.
Q2.1, Source and sink identification (2 marks)
The mature leaf is the sourceit has the highest sucrose concentration (120 mg mL⁻¹) because it is actively photosynthesising and loading sucrose into the phloem [1]. The developing fruit is the sinkit has the lowest sucrose concentration (40 mg mL⁻¹) because it is consuming sucrose for growth and converting it to starch and other compounds, removing it from the phloem [1].
Marking criteria: 1 mark for correctly identifying leaf as source with concentration-based justification; 1 mark for correctly identifying fruit as sink with concentration-based justification.
Q2.2, Pressure-flow from leaf to fruit (3 marks)
At the leaf (source), sucrose is actively loaded into phloem sieve tubes by companion cells, creating a high solute concentration inside the phloem [1]. Water enters the phloem from adjacent xylem by osmosis (moving from higher to lower water potential), increasing turgor pressure at the source end of the phloem [1]. This creates a pressure gradient from the high-pressure leaf end (120 mg mL⁻¹ sucrose) to the lower-pressure fruit end (40 mg mL⁻¹), the sucrose solution flows by bulk flow from high pressure toward the developing fruit (sink), where sucrose is unloaded and used for growth, maintaining the low pressure at the sink end [1].
Marking criteria: 1 mark for active loading at source creating high solute / high turgor pressure; 1 mark for osmosis drawing water in from xylem increasing turgor; 1 mark for bulk flow down the pressure gradient from source (high) to sink (low) driven by sucrose unloading at the sink.
Q2.3, Night effect on leaf phloem (1 mark)
The sucrose concentration in the leaf phloem would decrease overnight because photosynthesis has stopped and no new sucrose is being loaded. The concentration gradient between leaf and fruit would shrink, reducing the pressure gradient and slowing (or possibly temporarily reversing) phloem flow toward the fruit. Accept also: the fruit or other sinks still consuming sucrose may briefly maintain a gradient, but the rate of transport would fall significantly.
Marking criteria: 1 mark for identifying that sucrose concentration in leaf phloem falls (no new loading) and linking this to a reduced pressure gradient / reduced transport rate.
Q3, Historical experiments
Experiment A: (a) Jan Baptist van Helmont. (b) It revealed that plant mass does not come from the soil, soil mass barely changed, so the tree's 74 kg gain must have another source, which van Helmont (incorrectly) attributed solely to water. (c) It failed to explain the role of CO₂ as a reactant; it also failed to explain the role of light. The conclusion that water was the only source was incorrect.
Experiment B: (a) Jan Ingenhousz. (b) It revealed that light is essential for the production of O₂ by plants, the process that "restores" air only occurs in sunlight; in darkness, plants actually "corrupt" the air by producing CO₂ (cellular respiration). It was the first experiment to experimentally separate photosynthesis from respiration. (c) It did not identify CO₂ as a reactant, did not identify glucose as a product, and did not explain the internal biochemical mechanism.
Experiment C: (a) Melvin Calvin. (b) It revealed the biochemical pathway (Calvin cycle / light-independent reactions) by which CO₂ is fixed into organic molecules, the radioactive ¹⁴C label allowed the intermediates of the pathway to be identified for the first time. This work won the Nobel Prize in Chemistry (1961). (c) It did not fully elucidate the electron transport chain or the mechanism of photosystem II water-splitting.
Q4, Potato plant in darkness
4.1: The fates that would not be occurring are sucrose transport (via phloem from leaf as source), starch synthesis, cellulose synthesis, and biosynthesis of new compounds, because these all require a supply of new glucose from photosynthesis. The only fate actively occurring would be cellular respiration, consuming stored glucose [1]. Award full marks for identifying "any fate that requires new glucose production from photosynthesis is not occurring" with correct explanation (no photosynthesis = no new glucose = no new phloem loading, starch making, etc.).
Marking criteria: 1 mark for identifying at least one fate not occurring (e.g. sucrose transport / starch synthesis / cellulose synthesis / biosynthesis); 1 mark for explaining that the reason is the absence of new glucose from photosynthesis.
4.2: Starch stored in the tubers would be broken down to glucose by amylase (this is the reversal of the starch storage fate, fate 2 is reversible). The glucose released would then be used as the substrate for cellular respiration to supply ATP to the plant's cells during the dark period [1 + 1].
Marking criteria: 1 mark for identifying that starch stores are broken down (reversed) to glucose; 1 mark for linking the released glucose to cellular respiration (fate 1) to supply ATP.
4.3: Phloem transport of sucrose from leaves to sinks would effectively stop (or reduce to near zero) during the dark period because the leaf is no longer photosynthesising and producing sucrose to load into the phloem. Without active loading at the source, no high-turgor pressure is generated at the leaf end, so there is no pressure gradient to drive bulk flow. Accept also: some residual transport may occur briefly from any sucrose still present in leaf cells until it is depleted.
Marking criteria: 1 mark for stating that phloem transport from leaf would stop/greatly reduce, with a correct reason (no new sucrose loading → no pressure gradient).