Biology • Year 11 • Module 2 • Lesson 9
Gas Exchange in Plants
Lock in the core vocabulary, stomatal structures, and the gas pathways that underpin every higher-order question on this topic.
1. Term–definition match
The seven definitions below are shuffled. In the right-hand column write the matching term from this list: stomata, guard cells, transpiration, gas exchange, diffusion gradient, mesophyll, cuticle. 7 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The movement of CO₂ and O₂ between the leaf's internal air spaces and the atmosphere through pores. | |
| 1.2 | Microscopic pores in the leaf epidermis through which CO₂ enters and O₂ and water vapour exit the leaf. | |
| 1.3 | A waxy layer secreted by epidermal cells covering the leaf surface that reduces water loss by evaporation. | |
| 1.4 | Paired kidney-shaped cells that flank each pore and control its opening and closing through changes in turgor pressure. | |
| 1.5 | The evaporation of water from leaf surfaces, primarily through open pores, which drives the water stream in xylem. | |
| 1.6 | The internal photosynthetic tissue of a leaf, comprising palisade and spongy layers. | |
| 1.7 | The concentration difference that drives the passive movement of gas molecules from a region of higher to lower concentration. |
2. Complete the gas pathway table
The table below shows gas movements through a plant. Complete the missing information in each cell. 10 marks
| Gas | Process | Direction | Structure(s) used |
|---|---|---|---|
| CO₂ | Photosynthesis (input) | _______ into leaf | |
| O₂ | Photosynthesis (output) | Out of leaf | _______, spongy mesophyll air spaces |
| O₂ | _______ (input) | In, all cells | Stomata (leaves), lenticels (stems), aerenchyma (aquatic plants) |
| CO₂ | Respiration (output) | Out, all cells | |
| H₂O vapour | _______ | Out, leaf | Stomata (primary), lenticels (minor) |
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 provided. 8 marks (1 for T/F, 1 for the correction where needed)
3.1 Stomata are found predominantly on the upper surface of leaves to maximise sunlight absorption. T / F
3.2 Guard cells are the only epidermal cells that contain chloroplasts. T / F
3.3 Lenticels are found in leaf epidermis and are actively regulated by guard cells. T / F
3.4 At the compensation point, both photosynthesis and respiration are occurring simultaneously at equal rates, so there is no net gas exchange with the atmosphere. T / F
4. Guard cell mechanism, sequence the steps
The six steps below describe stomatal opening, but they are out of order. Write the correct order (1–6) in the boxes provided, then briefly explain what drives each step. 6 marks
| Order | Step (shuffled) | Your explanation (one phrase) |
|---|---|---|
| Stoma opens, pore between guard cells widens. | ||
| K⁺ ions are actively pumped into guard cells through ion channels. | ||
| Guard cells bow outward because the thick inner wall resists expansion while the thinner outer wall stretches. | ||
| Light is absorbed by chloroplasts in guard cells; ATP is produced by photosynthesis. | ||
| Water potential inside guard cells falls below that of surrounding epidermal cells. | ||
| Water enters guard cells by osmosis; turgor pressure increases and cells swell. |
5. Net gas exchange, day, compensation point, night
Complete the table below. For each time period, state the active processes, whether stomata are open or closed, and the net movement of CO₂ and O₂. 9 marks
| Time period | Active processes | Stomata | Net CO₂ | Net O₂ |
|---|---|---|---|---|
| Bright daylight (PS rate > R rate) | ||||
| Compensation point (dim light; PS rate = R rate) | ||||
| Night (complete darkness) |
Q1, Term–definition matches (7 marks)
1.1 gas exchange • 1.2 stomata • 1.3 cuticle • 1.4 guard cells • 1.5 transpiration • 1.6 mesophyll • 1.7 diffusion gradient.
Q2, Gas pathway table (10 marks, 2 each)
Row 1 (CO₂ in for PS): Direction = In; Structures = stomata, spongy mesophyll air spaces.
Row 2 (O₂ out from PS): Structures = stomata.
Row 3 (O₂ in for respiration): Process = Respiration.
Row 4 (CO₂ out from respiration): Structures = stomata (leaves), lenticels (stems).
Row 5 (H₂O vapour): Process = Transpiration.
Q3, True / false (8 marks)
3.1 False. Correction: Stomata are found predominantly on the lower (abaxial) surface of leaves to reduce water loss by avoiding direct sun exposure.
3.2 True. Guard cells are the only epidermal cells containing chloroplasts, these produce the ATP needed to power K⁺ pumps.
3.3 False. Correction: Lenticels are found in woody stem bark (periderm), not leaf epidermis, and they have no guard cellsthey are permanently open and not actively regulated.
3.4 True. At the compensation point both processes operate simultaneously at exactly equal rates; the gases produced by one are consumed by the other, so no net exchange occurs with the atmosphere.
Q4, Correct sequence (6 marks, 1 each)
Correct order: 4 → 2 → 5 → 6 → 3 → 1
- Step 1: Light absorbed by guard cell chloroplasts → ATP produced.
- Step 2: ATP powers H⁺-ATPase pumps → K⁺ actively pumped in.
- Step 3: K⁺ accumulation lowers water potential inside guard cells.
- Step 4: Water enters by osmosis → turgor pressure rises → cells swell.
- Step 5: Unequal wall thickness forces cells to bow outward (thick inner wall resists; thin outer wall stretches).
- Step 6: Stoma opens, pore widens.
Q5, Net gas exchange table (9 marks, 3 per row)
Bright daylight: Processes = photosynthesis + respiration (PS rate > R rate); Stomata = open; Net CO₂ = net IN (absorbed); Net O₂ = net OUT (released).
Compensation point: Processes = photosynthesis + respiration (equal rates); Stomata = open; Net CO₂ = no net change; Net O₂ = no net change.
Night: Processes = respiration only; Stomata = closed; Net CO₂ = net OUT (released); Net O₂ = net IN (absorbed).
Marking note: Award 1 mark per correct cell (processes, stomata state, and correct net for both gases together count as 3 marks per row).