Biology · Year 11 · Module 2 · Lesson 9
HSC Exam Practice
Gas Exchange in Plants
Short answer
1.Short answer
Distinguish between stomata and lenticels as gas exchange structures in plants. Refer to location, the presence or absence of regulatory cells, and whether each structure is permanently open or actively regulated.
Explain why the net gas exchange of a plant during the day shows CO₂ uptake and O₂ release, even though the plant is also performing cellular respiration continuously.
Identify two stimuli that cause stomata to close, and for each, state the mechanism by which closure occurs.
Define the term compensation point and explain what is happening to gas exchange in the plant at this point.
Data response
2.Data response, stomatal aperture under experimental conditions
A researcher measured the stomatal aperture (width of the pore opening) in leaf discs from a well-watered plant under four experimental treatments. Each treatment was applied for 30 minutes before measurement. Results are shown in the table below.
| Treatment | Mean stomatal aperture (µm) | Standard deviation |
|---|---|---|
| Bright white light | 8.4 | 0.6 |
| Complete darkness | 1.1 | 0.4 |
| Bright light + 1 mmol/L ABA solution applied to leaf surface | 1.8 | 0.5 |
| Bright light + CO₂ concentration reduced to 100 ppm (vs normal ~420 ppm) | 11.2 | 0.7 |
(a) Compare the stomatal apertures in bright light and complete darkness. Account for the difference using your understanding of guard cell physiology. (3 marks)
(b) In the ABA treatment, stomata were exposed to both bright light and ABA. Despite the light stimulus that would normally open stomata, the mean aperture was only 1.8 µm. Explain this result with reference to the mechanism by which ABA causes stomatal closure. (3 marks)
(c) Explain why reducing CO₂ to 100 ppm resulted in the largest stomatal aperture across all four treatments. (2 marks)
Extended response
3.Extended response
Explain the mechanism by which guard cells open stomata in response to light, and evaluate how this mechanism serves as an adaptation to the fundamental gas exchange challenge faced by terrestrial plants. In your response, refer to the role of K⁺ ions, osmosis, cell wall structure, and the CO₂–water trade-off.
Biology · Year 11 · Module 2 · Lesson 9
Answer Key & Marking Guidelines
Section 1 · Short answer · 3 marks · Band 3
Sample response. Stomata are located in the leaf epidermis (predominantly the lower surface) and are flanked by pairs of guard cells that actively regulate their aperture in response to light, CO₂ concentration, and water stress, they open and close. Lenticels are located in the woody stem bark (periderm) and have no guard cells; they consist of loosely arranged parenchyma cells that allow passive gas diffusion and are permanently open without active regulation.
Marking notes. 1 mark for location of each (leaf epidermis vs woody stem bark/periderm). 1 mark for regulatory cells (guard cells present vs absent in lenticels). 1 mark for aperture control (actively regulated and can open/close vs permanently open, passive diffusion only).
Section 1 · Short answer · 3 marks · Band 4
Sample response. During the day, both photosynthesis and cellular respiration occur simultaneously in plant cells. Photosynthesis consumes CO₂ and releases O₂; respiration consumes O₂ and releases CO₂. In bright light the rate of photosynthesis greatly exceeds the rate of respiration. This means photosynthesis consumes CO₂ faster than respiration produces it, and produces O₂ faster than respiration consumes it. The net result, the sum of both processes, is apparent uptake of CO₂ and release of O₂, even though the plant is producing CO₂ through respiration the entire time.
Marking notes. 1 mark for identifying that both processes occur simultaneously during the day. 1 mark for explaining that the rate of photosynthesis exceeds the rate of respiration during bright daylight. 1 mark for correctly deriving the net outcome: CO₂ uptake and O₂ release result from photosynthesis rate > respiration rate, not from the absence of respiration.
Section 1 · Short answer · 4 marks · Band 3–4
Sample response.
Stimulus 1, Darkness: In darkness, guard cell chloroplasts cannot produce ATP; without ATP the H⁺-ATPase pumps stop. K⁺ is no longer actively pumped in and diffuses out of guard cells. Water potential inside guard cells rises above that of surrounding cells; water leaves by osmosis, guard cells lose turgor and become flaccid, and the pore closes.
Stimulus 2, Water stress / drought (ABA release): When leaf water potential falls (drought), stressed cells release abscisic acid (ABA). ABA binds to guard cell receptors and activates ion channels that allow K⁺ to flow out of guard cells. The loss of K⁺ raises water potential inside guard cells; water exits by osmosis, turgor falls, cells become flaccid, and the stoma closes to prevent further water loss through transpiration.
Marking notes. Award 1 mark per stimulus identified (max 2), plus 1 mark per mechanism correctly described linking K⁺ efflux and osmosis to flaccid guard cells. Total 4 marks (2 stimuli × 2 marks). Accept also: high CO₂ concentration, high temperature. Each valid stimulus + mechanism pair scores 2 marks.
Section 1 · Short answer · 3 marks · Band 3
Sample response. The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of cellular respiration in a plant. At this point, every molecule of CO₂ produced by respiration is immediately consumed by photosynthesis, and every molecule of O₂ produced by photosynthesis is immediately consumed by respiration. There is no net uptake or release of either gas from the plant's perspective, apparent net gas exchange with the atmosphere is zero. However, both processes are occurring at full rate simultaneously; the compensation point does not mean the plant is metabolically inactive.
Marking notes. 1 mark for defining compensation point as the light intensity at which PS rate = respiration rate. 1 mark for explaining that gases produced by one process are fully consumed by the other, resulting in no net gas exchange with the atmosphere. 1 mark for clarifying that both processes are still actively occurring (i.e., gross exchange is not zero, only net exchange is zero).
Section 2 · Data response · 3 marks · Band 4–5
Sample response. In bright light the mean stomatal aperture is 8.4 µm, compared with only 1.1 µm in complete darkness, a difference of 7.3 µm (stomata are approximately 7.6-fold wider in light). In bright light, guard cell chloroplasts perform photosynthesis and produce ATP. ATP powers H⁺-ATPase pumps that drive K⁺ into guard cells, lowering water potential and drawing water in by osmosis. Increased turgor causes guard cells to bow outward (due to unequal wall thickness) and the pore opens. In darkness, ATP production stops; K⁺ diffuses out; water follows by osmosis; guard cells lose turgor and the pore closes.
Marking notes. 1 mark for comparing the two values using data from the table. 1 mark for explaining the light-induced K⁺ influx and osmotic water entry in bright light leading to high turgor and open pore. 1 mark for contrasting with darkness: absence of ATP → K⁺ efflux → water loss → flaccid guard cells → pore closes.
Section 2 · Data response · 3 marks · Band 4–5
Sample response. ABA overrides the light signal. Although light is present and guard cell chloroplasts can produce ATP, ABA binds to receptors in the guard cell membrane and activates outward K⁺ channels, causing K⁺ to leave the guard cells despite the ongoing light-driven K⁺ influx attempt. The net effect is K⁺ efflux, intracellular K⁺ concentration falls, water potential inside guard cells rises, water exits by osmosis, turgor falls, and the cells become flaccid and close the pore. The ABA signalling pathway is dominant over the light-opening pathway, reflecting the plant's priority to conserve water under stress conditions.
Marking notes. 1 mark for identifying that ABA activates K⁺ efflux channels in guard cells. 1 mark for linking K⁺ efflux to rising water potential and osmotic water loss from guard cells. 1 mark for concluding that ABA-driven closure overrides the light-opening mechanism, resulting in near-closed stomata despite the presence of light.
Section 2 · Data response · 2 marks · Band 4–5
Sample response. At 100 ppm CO₂ (well below normal atmospheric levels of ~420 ppm), the leaf's internal CO₂ concentration is very low, signalling that photosynthesis is severely limited by CO₂ supply. Guard cells respond to low CO₂ by maximising stomatal aperture, this is in addition to the light stimulus already driving K⁺ influx. Both signals (light and low CO₂) act together to maximise K⁺ influx and turgor, producing the largest aperture (11.2 µm) across all treatments.
Marking notes. 1 mark for identifying that low CO₂ is a signal to guard cells that CO₂ supply is limiting photosynthesis, triggering maximal aperture widening. 1 mark for explaining that both the light stimulus and the low-CO₂ stimulus are acting simultaneously and additively, producing a greater aperture than light alone.
Section 3 · Extended response · 5 marks · Band 5–6
Sample response. Guard cells are the only epidermal cells containing chloroplasts. In light, these chloroplasts perform photosynthesis and generate ATP. ATP powers H⁺-ATPase pumps that expel H⁺ ions from guard cells, creating an electrochemical gradient that drives K⁺ ions in through ion channels. K⁺ accumulation lowers water potential in guard cells below that of surrounding cells; water enters by osmosis, increasing turgor pressure. Guard cells swell but, due to their unequally thick walls, thick inelastic inner wall (facing the pore) and thinner, more elastic outer wall, they cannot expand uniformly. The outer wall stretches while the inner wall resists, forcing the cells to bow outward and widening the pore between them. This light-driven opening mechanism is an exquisitely tuned adaptation to the fundamental challenge of terrestrial plant gas exchange: plants need CO₂ for photosynthesis, but CO₂ can only enter through the same pores through which water vapour simultaneously exits (transpiration). By opening stomata in response to light, exactly when CO₂ is needed for photosynthesis, guard cells ensure that the trade-off is paid only when it is productive. Closing stomata in darkness, drought, or heat prevents water loss when the benefit (CO₂ uptake for photosynthesis) is either absent or outweighed by the cost (fatal dehydration). The guard cell mechanism therefore does not maximise gas exchange at all times, it optimises the CO₂–water trade-off in real time.
Marking criteria:
- 1 markDescribes K⁺ influx mechanism correctly: light → ATP → H⁺-ATPase pump → K⁺ enters guard cells.
- 1 markLinks K⁺ accumulation to lower water potential and osmotic water entry; turgor increases.
- 1 markExplains unequal wall thickness: thick inner wall resists, thin outer wall stretches, cells bow outward, pore opens.
- 1 markIdentifies the CO₂–water trade-off as the fundamental terrestrial challenge: stomata must be open for CO₂ uptake, but this inevitably allows water vapour loss.
- 1 markEvaluates the mechanism as an adaptation: opening in light links gas exchange to photosynthetic demand; closure in drought/darkness prevents unnecessary water loss, optimising survival vs. growth trade-off.