2.6.5Inquiry Process
Gas Exchange in Plants — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 67 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Leaf Features for Gas Exchange
Stomata
- Stomata Pores in the leaf epidermis, mostly on the lower surface, allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out.
Air spaces
- Air spaces The spongy mesophyll contains large connected air spaces, which expose the wet surfaces of many cells to the air inside the leaf.
Thin flat shape
- Thin flat shape A broad, flat leaf gives a large surface area and short distance between stomata and the cells that need carbon dioxide.
Moist cell walls
- Moist cell walls Gases dissolve in the water film on mesophyll cell walls before entering the cells.
Part 2 of 4Gas Flow Changes with Light
Gas Flow Changes with Light
In bright light
- In bright light Photosynthesis uses carbon dioxide faster than respiration produces it, so carbon dioxide enters the leaf and oxygen leaves.
In darkness
- In darkness Only respiration occurs, so oxygen enters and carbon dioxide leaves, as in animals.
Compensation point
- Compensation point At a certain low light intensity photosynthesis and respiration balance so there is no net gas exchange.
Concentration gradients
- Concentration gradients Photosynthesis lowers carbon dioxide inside the leaf, keeping the gradient from outside air steep.
| Condition | Net CO₂ movement | Net O₂ movement |
|---|---|---|
| Bright light | Into leaf | Out of leaf |
| Darkness | Out of leaf | Into leaf |
Part 3 of 4Guard Cells and the Water Loss Trade-off
1
Each stoma is bordered by two guard cells with unevenly thickened walls.
2
When guard cells take up water by osmosis they become turgid and bow outwards, which opens the pore.
3
When they lose water they become flaccid and the pore closes, reducing water vapour loss.
4
Open stomata allow carbon dioxide in for photosynthesis but also let water vapour escape by transpiration.
5
Plants therefore close stomata when water is scarce, using the hormone abscisic acid as a signal, even though photosynthesis slows.
Part 4 of 4Exam Checklist & Tips
2.6.5 – Key Points for the Exam
- Stoma
- A pore in the leaf surface that allows gas exchange;
- plural stomata.
- A pore in the leaf surface that allows gas exchange;
- Guard cell One of a pair of cells that changes shape to open or close a stoma.
- Spongy mesophyll Loosely packed leaf tissue with large air spaces.
- Transpiration The loss of water vapour from the aerial parts of a plant, mainly through stomata.
- Turgid Describes a cell swollen with water so that it presses firmly against its wall.
Common Mistakes
- Saying plants only photosynthesise and do not respire; respiration continues in all living cells.
- Stating that stomata open when guard cells are flaccid; they open when turgid.
- Forgetting that open stomata also cause water loss.
- Believing gas exchange occurs through the waxy cuticle; the cuticle is largely impermeable.
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