Biology • Year 11 • Module 2 • Lesson 9
Gas Exchange in Plants
Apply stomatal regulation and net gas exchange principles to real data and environmental scenarios.
1. Interpret CO₂ concentration data in a sealed chamber
A plant was placed in a sealed, transparent chamber. The CO₂ concentration inside the chamber was measured every two hours over a 24-hour period, beginning at midnight. The results are shown in the table below. 8 marks
| Time | CO₂ concentration (ppm) | Conditions |
|---|---|---|
| 00:00 (midnight) | 415 | Dark |
| 02:00 | 432 | Dark |
| 04:00 | 448 | Dark |
| 06:00 | 461 | Dark |
| 08:00 | 450 | Light, dawn |
| 10:00 | 388 | Light, bright |
| 12:00 (noon) | 312 | Light, bright |
| 14:00 | 295 | Light, bright |
| 16:00 | 308 | Light, dimming |
| 18:00 | 351 | Light, dim (compensation zone) |
| 20:00 | 398 | Dark |
| 22:00 | 410 | Dark |
1.1 Describe the trend in CO₂ concentration between midnight and 06:00. Explain this trend using your knowledge of plant gas exchange. 3 marks
1.2 Between 08:00 and 14:00 the CO₂ concentration falls sharply despite the plant performing cellular respiration throughout. Explain why CO₂ still falls during this period. 2 marks
1.3 Identify the time period most likely to represent the compensation point and justify your answer using the data. 2 marks
1.4 Predict what would happen to the CO₂ concentration if the chamber were heated to 42 °C at noon. Justify your prediction with reference to stomatal regulation. 1 mark
2. Stomatal triggers, apply to scenarios
For each scenario, state whether stomata would open or close, name the trigger involved, and explain the mechanism (refer to K⁺ ions, turgor pressure, and/or ABA where relevant). 6 marks, 2 per scenario
2.1 A plant is moved from bright sunlight into complete darkness.
Stomata will: __________ Trigger: __________
2.2 The CO₂ concentration inside the leaf drops well below atmospheric levels during intense photosynthesis.
Stomata will: __________ Trigger: __________
2.3 Soil moisture drops to near zero in a drought. The plant's leaf cells begin to lose turgor.
Stomata will: __________ Trigger: __________
3. Apply structure–function thinking, aquatic vs terrestrial plants
A botanist is comparing two plant species: Species A is a submerged aquatic plant; Species B is a drought-adapted terrestrial plant from an Australian woodland. The table below shows observed structural features. For each feature, explain the adaptive advantage for that species. 8 marks, 2 each
| Species | Observed feature | Adaptive advantage, your explanation |
|---|---|---|
| Species A (submerged aquatic) | Thin or absent waxy cuticle on leaf surface | |
| Species A (submerged aquatic) | Extensive aerenchyma tissue running from leaves through stems to roots | |
| Species B (drought-adapted terrestrial) | Very thick waxy cuticle; stomata densely concentrated on lower leaf surface | |
| Species B (drought-adapted terrestrial) | Stomata capable of rapid closure when ABA is detected |
4. Applied scenario, rice farming in flooded paddies
Rice is grown in flooded paddies where the roots are permanently submerged in anaerobic (oxygen-free) sediment. Without structural adaptations, the roots cannot perform aerobic respiration and die. 6 marks
4.1 Identify the structural adaptation that allows rice roots to survive in anaerobic sediment, and explain how it works. 2 marks
4.2 In which direction does O₂ move through aerenchyma, and what drives this movement? Name the process. 2 marks
4.3 A farmer drains the paddy fields for three weeks to apply fertiliser. Predict whether the aerenchyma adaptation would still be essential during this period and justify your answer. 2 marks
Q1.1, Night trend (3 marks)
Between midnight and 06:00, CO₂ concentration rises steadily from 415 ppm to 461 ppm [1, trend described with data]. During darkness, stomata are closed and photosynthesis cannot occur [1]. Cellular respiration continues in all plant cells at all times, releasing CO₂ into the sealed chamber as a waste product of aerobic respiration. With no photosynthesis consuming CO₂, it accumulates [1].
Q1.2, Daytime CO₂ fall (2 marks)
During bright light (08:00–14:00), both photosynthesis and respiration occur simultaneously [1]. The rate of photosynthesis greatly exceeds the rate of respiration, so the plant consumes CO₂ faster than respiration produces it. The net result is a reduction in CO₂ concentration in the chamber, even though respiration is continuously producing CO₂ [1].
Q1.3, Compensation point (2 marks)
The compensation point is most likely around 18:00 (or in the 16:00–18:00 range) [1]. At this time the CO₂ concentration is no longer falling but begins to level off and then rise, suggesting photosynthesis rate has fallen to equal the respiration rate, no net CO₂ is consumed or released [1]. Accept any time in the 16:00–20:00 window with an appropriate justification referencing the change in gradient.
Q1.4, Heat stress at noon (1 mark)
CO₂ concentration would rise (or the rate of decrease would slow significantly). At 42 °C, the water deficit stress triggers ABA release, which causes K⁺ efflux from guard cells, reducing turgor and closing stomata. With stomata closed, CO₂ can no longer enter from the atmosphere, so photosynthesis falls while respiration continues, net CO₂ accumulates. [1]
Q2, Stomatal triggers (6 marks)
2.1 Stomata will close. Trigger: darkness. In darkness, guard cell chloroplasts cannot produce ATP, so H⁺-ATPase pumps stop. K⁺ is no longer pumped in and passively moves out (efflux), raising water potential inside guard cells. Water exits by osmosis, cells lose turgor and become flaccid, and the pore closes. [2]
2.2 Stomata will open further (or remain open and widen). Trigger: low CO₂ concentration. Low leaf CO₂ signals that photosynthesis is limited by CO₂ supply, guard cells respond by pumping more K⁺ in, increasing turgor and widening the aperture to allow more CO₂ to diffuse in. [2]
2.3 Stomata will close. Trigger: water stress / ABA (abscisic acid). Stressed cells release ABA, which binds to receptors in guard cells and triggers K⁺ efflux through ion channels. Guard cells lose water by osmosis, turgor falls, cells become flaccid, and the stoma closes to conserve water. [2]
Q3, Adaptive advantages (8 marks)
Species A, thin/absent cuticle: The plant is submerged in water, so there is no evaporative water loss, the waxy cuticle's waterproofing function is not needed. A thin or absent cuticle maximises the direct diffusion of dissolved CO₂ and O₂ through the leaf surface from the surrounding water, where gases diffuse ~10 000× slower than in air. [2]
Species A, aerenchyma: The roots are submerged in anaerobic sediment with no dissolved O₂ available. Aerenchyma creates large internal air channels from the above-water photosynthetic leaves down through stems to the roots, allowing O₂ produced by photosynthesis to diffuse down and supply root cells with O₂ for aerobic respiration. [2]
Species B, thick cuticle, lower-surface stomata: The terrestrial environment risks severe water loss through leaf surfaces, particularly in drought. The thick waxy cuticle is hydrophobic and blocks evaporation from the leaf surface. Stomata on the lower surface are shaded from direct sunlight, reducing leaf temperature and the rate of water vapour evaporation through open pores, minimising transpiration. [2]
Species B, rapid ABA closure: During drought, rapid stomatal closure in response to ABA prevents runaway water loss that could be fatal. By closing quickly when water stress is detected, the plant limits transpiration at the cost of temporarily reducing CO₂ uptake for photosynthesis, a trade-off that prioritises survival over growth rate. [2]
Q4, Rice aerenchyma (6 marks)
4.1 Aerenchyma, large air channels running through stem and root tissue [1]. O₂ produced by photosynthesis in the above-water leaves diffuses down through these channels to the submerged roots, where it is used for aerobic respiration, preventing anaerobic conditions from killing the roots [1].
4.2 O₂ moves downwardfrom the above-water leaves (where it is produced by photosynthesis and where atmospheric O₂ is also available) to the submerged roots (where O₂ concentration is very low). The driving force is diffusionpassive movement from high concentration (leaves) to low concentration (roots) down the diffusion gradient [1 for direction, 1 for diffusion].
4.3 During the drained period, the roots are no longer surrounded by anaerobic sediment. O₂ from the soil air spaces can now diffuse directly into the roots [1]. Aerenchyma would be less essential for root survival during this period, although it would still be present structurally. Aerobic respiration in roots could be sustained by soil O₂ rather than relying entirely on the aerenchyma pathway [1]. Accept any answer that correctly identifies the change in O₂ availability when the soil is drained and adjusts the functional importance accordingly.