Plants need to exchange gases for both photosynthesis and respiration — but they have no lungs, no pump, and no circulatory system. Understanding how gas moves into and out of plant tissues, and how plants regulate that movement, is the focus of this lesson.
Content from this lesson that appears directly in HSC Biology exams
Explaining how guard cells open and close stomata using turgor pressure and ion pumping. Appears in almost every HSC paper — typically 3–4 marks in Section II. Must link structure (unequal wall thickness) to mechanism (turgor change) to function (aperture control).
Distinguishing between gross and net gas exchange during photosynthesis and respiration. A classic HSC trap question — tested in Section I (1–2 marks) and short answer (2–3 marks).
Comparing structural adaptations for gas exchange across environments. Appears in comparative questions worth 3–4 marks — must reference specific structures and explain why each environment requires different solutions.
Lenticels as the gas exchange structure of woody stems — tested as a 1–2 mark identification and function question. Frequently appears in questions about plant structures that students overlook.
Core Content
How gas enters and exits the leaf
Stomata (singular: stoma) are microscopic pores in the leaf epidermis, each flanked by a pair of guard cells. They are the primary route for gas exchange between the leaf interior and the atmosphere — allowing CO₂ to enter for photosynthesis and O₂ and water vapour to exit.
| Time Period | Processes active | Net CO₂ movement | Net O₂ movement |
|---|---|---|---|
| Daylight (bright) | Photosynthesis + Respiration (PS rate > R rate) | Into leaf (net uptake) | Out of leaf (net release) |
| Compensation point | Photosynthesis rate = Respiration rate | No net movement | No net movement |
| Night / darkness | Respiration only | Out of leaf (net release) | Into leaf (net uptake) |
Turgor-driven aperture control — the plant's gas exchange valve
Stomatal aperture is regulated by a pair of guard cells on either side of the pore. Guard cells are the only epidermal cells that contain chloroplasts. Their unusual kidney (bean) shape and unequally thick cell walls are the structural basis for the opening and closing mechanism.
Guard cells have thick, inelastic inner walls (facing the pore) and thinner, more elastic outer walls. When guard cells become turgid (swell with water), they cannot expand uniformly — the thick inner wall resists expansion while the thinner outer wall stretches. This forces the cells to bow outward, pulling the inner walls apart and opening the pore. When guard cells lose water and become flaccid, they straighten and the pore closes.
| Step | Opening (turgid) | Closing (flaccid) |
|---|---|---|
| 1. Signal | Light detected; CO₂ concentration low; ABA (abscisic acid) absent | Darkness; CO₂ concentration high; drought → ABA released from leaves |
| 2. Ion pumping | H⁺-ATPase pumps H⁺ ions OUT of guard cells using ATP → K⁺ ions flow IN from neighbouring cells through ion channels | K⁺ ions pumped OUT of guard cells → released back to neighbouring cells |
| 3. Water potential change | K⁺ influx lowers water potential inside guard cells | K⁺ efflux raises water potential inside guard cells |
| 4. Osmosis | Water enters guard cells by osmosis (high → low water potential) → cells become turgid | Water leaves guard cells by osmosis → cells become flaccid |
| 5. Aperture | Turgid guard cells bow outward due to unequal wall thickness → STOMA OPENS | Flaccid guard cells collapse inward → STOMA CLOSES |
| Factor | Effect on stomata | Reason |
|---|---|---|
| Light | Opens | Triggers K⁺ influx via guard cell photosynthesis; also signals photosynthesis demand for CO₂ |
| Low CO₂ | Opens | Low CO₂ in leaf signals that photosynthesis is limited by CO₂ supply — open to allow more in |
| High CO₂ | Closes | CO₂ is abundant — no need to keep pores open; closing reduces water loss |
| Water deficit / drought | Closes | Stressed leaves release ABA (abscisic acid) → ABA triggers K⁺ efflux → guard cells lose turgor → stomata close to conserve water |
| Darkness | Closes | No photosynthesis → no ATP from guard cell chloroplasts → K⁺ pumps stop → cells lose turgor |
| High temperature | Closes | Excessive water loss risk → ABA released → stomata close |
How stems breathe when covered in bark
Leaves use stomata for gas exchange, but woody stems are covered in bark (periderm) — an impermeable layer that cannot exchange gases through its surface. To allow gas exchange in stem cells, woody plants have evolved lenticels.
| Feature | Detail |
|---|---|
| What they are | Small, loosely packed pores or raised bumps in the bark of woody stems and roots; visible to the naked eye as small dots or ridges on tree bark |
| Structure | Loose, spongy parenchyma cells with large intercellular air spaces replacing the tightly packed cork cells of normal bark; no guard cells — always open |
| Function | Allow O₂ to diffuse in for cellular respiration in living stem cells, and CO₂ to diffuse out — passive diffusion along concentration gradients |
| Regulation | Unlike stomata, lenticels are not actively regulated — they are always open. Water loss through lenticels is low because stem cells have lower transpiration rates than leaf mesophyll cells |
| Where found | Woody stems, woody roots, fruit surfaces (visible as dots on apples and pears), young bark before it fully suberises |
Different environments require different structural solutions
The gas exchange challenge differs fundamentally between aquatic and terrestrial environments. Terrestrial plants risk water loss through open stomata; aquatic plants face slow gas diffusion through water and potential oxygen deficiency in waterlogged sediments. Each group has evolved specific structural adaptations.
| Feature | Terrestrial plants | Aquatic plants (submerged / emergent) |
|---|---|---|
| Primary challenge | Balancing gas exchange (needs open stomata) with water conservation (open stomata = water loss) | Obtaining CO₂ and O₂ from water, which diffuses gases ~10,000× slower than air; waterlogged roots may lack O₂ |
| Gas exchange surfaces | Stomata (predominantly lower epidermis); spongy mesophyll air spaces; lenticels in woody stems | Gases exchange directly across thin, uncoated leaf surfaces submerged in water; stomata may be on upper surface (floating leaves) or absent (submerged leaves) |
| Cuticle | Thick, waxy cuticle on upper surface reduces water loss | Thin or absent cuticle — waterproofing not needed; thin cuticle maximises gas diffusion directly through leaf surface |
| Stomata position | Mainly lower epidermis (reduces water loss by avoiding direct sun exposure) | Floating leaves: stomata on upper surface only (lower surface in contact with water); submerged leaves: stomata absent or non-functional |
| Aerenchyma | Not typically present | Many aquatic plants have aerenchyma — large air channels running from above-water leaves through stems to submerged roots; O₂ from photosynthesis diffuses down to supply roots; CO₂ from roots diffuses up and out |
| Root gas exchange | O₂ diffuses from soil air spaces; not usually limiting | Waterlogged sediments are anaerobic — no O₂ in soil. Aerenchyma channels O₂ from leaves to roots. Some species have pneumatophores (aerial roots that stick up above water surface to access atmospheric O₂) |
| Examples | Eucalypts, grasses, wheat, sunflower | Water lily (floating leaves), pondweed Elodea (submerged), mangrove (pneumatophores), rice (aerenchyma) |
Rice is grown in flooded paddies where the roots are permanently submerged in anaerobic (oxygen-free) sediment. Without oxygen, roots cannot perform aerobic respiration and die. Rice survives by developing extensive aerenchyma — large internal air channels that carry O₂ from above-water leaves down to the submerged roots. This structural adaptation is so efficient that rice roots can sustain aerobic respiration even when completely surrounded by anaerobic water. Understanding aerenchyma is central to research on flood-tolerant crop varieties — an increasingly important agricultural challenge.
Tracing every gas through every structure
This summary card integrates all gas exchange pathways across all plant structures. Use it to check your understanding before attempting the activities and assessment.
| Gas | Process | Direction | Path through plant | Structure used |
|---|---|---|---|---|
| CO₂ | Photosynthesis (input) | In → leaf | Atmosphere → stoma → sub-stomatal cavity → spongy mesophyll air spaces → into mesophyll cell cytoplasm → chloroplast stroma | Stomata, spongy mesophyll air spaces |
| O₂ | Photosynthesis (output) | Out ← leaf | Chloroplast thylakoids (produced) → cell → spongy mesophyll air spaces → sub-stomatal cavity → stoma → atmosphere | Stomata, spongy mesophyll air spaces |
| O₂ | Respiration (input) | In → all cells | Atmosphere → stomata/lenticels → intercellular air spaces → into cells → mitochondria | Stomata (leaves), lenticels (stems), aerenchyma (aquatic plants) |
| CO₂ | Respiration (output) | Out ← all cells | Mitochondria → cell → intercellular air spaces → stomata/lenticels → atmosphere | Stomata (leaves), lenticels (stems) |
| H₂O vapour | Transpiration | Out ← leaf | Xylem water → evaporates from mesophyll cell walls → vapour into air spaces → exits through stomata | Stomata (primary), lenticels (minor) |
Activities
In your book, draw two diagrams of a pair of guard cells — one showing the open state (turgid) and one showing the closed state (flaccid). Label: cell wall thickness difference, K⁺ direction, water movement, and aperture state. Then answer the questions below.
Type here or answer in your book.
For each scenario, identify which gases are moving in which direction, which processes are occurring, and what the net gas exchange of the leaf is. Be specific about the structures gases move through.
| Scenario | Processes active | Gases moving IN | Gases moving OUT | Net exchange |
|---|---|---|---|---|
| Bright midday sun, well-watered plant | ||||
| Midnight — complete darkness | ||||
| Dim light — compensation point | ||||
| Hot, dry afternoon — stomata closed by ABA |
Answer the following questions comparing gas exchange in aquatic and terrestrial plants.
Type here or answer in your book.
Assessment
Select the best answer — feedback shown immediately
1. Which of the following correctly explains why guard cells bow outward to open the stoma when turgid?
2. A plant is measured at the compensation point for photosynthesis and respiration. Which statement correctly describes gas exchange at this point?
3. Which of the following correctly describes lenticels?
4. Why do submerged aquatic plants typically lack a thick waxy cuticle on their leaves?
5. Abscisic acid (ABA) causes stomata to close. What is the sequence of events through which ABA achieves this?
Explain mechanisms — not just outcomes
6. Explain the mechanism by which guard cells open stomata in response to light. In your answer, refer to the role of chloroplasts, ion pumping, osmosis, and cell wall structure. 5 MARKS
Five distinct marking points — one per component listed.
7. During a bright sunny day, a plant's net gas exchange shows uptake of CO₂ and release of O₂. A student concludes that the plant is not producing CO₂ during the day. Evaluate this conclusion. 3 MARKS
8. Compare the gas exchange structures and strategies of terrestrial and submerged aquatic plants. In your answer, identify two structural differences and explain how each difference is an adaptation to the plant's environment. 4 MARKS
Two differences × two marks each — structure + environmental explanation
1. B — The thick, inelastic inner wall resists expansion while the thinner outer wall stretches when turgid. This forces the cells to bow outward, pulling the inner walls apart and opening the pore. The cells do not expand uniformly or contract.
2. D — At the compensation point, both processes occur simultaneously at equal rates. CO₂ produced by respiration is immediately consumed by photosynthesis, and O₂ produced by photosynthesis is immediately consumed by respiration. No net gas exchange occurs with the atmosphere — but both processes are fully active.
3. A — Lenticels are permanently open pores in woody bark with loosely packed parenchyma cells creating air spaces for passive gas diffusion. They have no guard cells and are not actively regulated. They are found in stems, not leaves.
4. C — Submerged leaves are surrounded by water, so there is no evaporative water loss — the cuticle's waterproofing function is unnecessary. A thin or absent cuticle allows dissolved gases (CO₂ and O₂) to diffuse more readily directly through the leaf surface from the surrounding water.
5. B — ABA triggers K⁺ efflux (potassium ions leave guard cells), raising water potential inside the guard cells above that of surrounding cells. Water leaves by osmosis, reducing turgor pressure. The flaccid guard cells lose their bowed shape and the stoma closes.
• Chloroplasts: Guard cells are the only epidermal cells that contain chloroplasts. In light, chloroplasts perform photosynthesis, producing ATP.
• Ion pumping: ATP powers H⁺-ATPase pumps in the guard cell membrane, which actively pump H⁺ ions out of the guard cells. This creates a charge gradient that drives K⁺ ions into the guard cells through specific ion channels.
• Water potential: The accumulation of K⁺ ions inside the guard cells lowers their water potential below that of surrounding epidermal cells.
• Osmosis: Water moves into the guard cells by osmosis (from higher water potential in surrounding cells to lower water potential in guard cells), increasing turgor pressure and causing the cells to swell.
• Cell wall structure: The inner wall of each guard cell (facing the pore) is thicker and less elastic than the outer wall. When turgid, the outer wall stretches while the inner wall resists, causing the cells to bow outward and pulling the pore open.
The conclusion is incorrect. The plant is producing CO₂ continuously during the day via cellular respiration, which occurs in all living cells at all times regardless of light availability.
The net uptake of CO₂ observed during the day does not mean CO₂ production has stopped — it means the rate of photosynthesis exceeds the rate of cellular respiration. Photosynthesis consumes CO₂ faster than respiration produces it, resulting in a net decrease in CO₂ from the leaf's perspective.
The correct interpretation is that both photosynthesis and respiration are occurring simultaneously, with photosynthesis dominant during bright daylight. Some of the CO₂ produced by respiration is immediately consumed by photosynthesis — never leaving the cell — while the remainder of the photosynthesis CO₂ demand is met by uptake from the atmosphere through stomata.
Difference 1 — Cuticle: Terrestrial plants have a thick, waxy cuticle covering the leaf surface, whereas submerged aquatic plants have a thin or absent cuticle. Terrestrial plants risk significant water loss by evaporation from leaf surfaces — the waxy cuticle is hydrophobic and reduces this evaporative loss. Submerged aquatic plants are surrounded by water and face no evaporative water loss; a thin cuticle instead maximises diffusion of dissolved CO₂ and O₂ directly through the leaf surface from the surrounding water.
Difference 2 — Stomata position: Terrestrial plants have stomata predominantly on the lower (abaxial) leaf surface, whereas floating aquatic leaves (e.g. water lily) have stomata only on the upper surface, and fully submerged leaves may lack functional stomata entirely. In terrestrial plants, lower surface stomata are shaded from direct sunlight, reducing leaf temperature and evaporative water loss through the open pores. In floating leaves, the lower surface is in contact with water — placing stomata there would block gas exchange with the atmosphere entirely, so stomata are on the upper (air-exposed) surface. Fully submerged leaves exchange gases directly through the leaf surface from dissolved gases in the water, making stomata non-functional or unnecessary.
Tick when you've finished all activities and checked your answers.