2.6.5Additional lessons
Gas Exchange in Plants — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 31 of 142
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 Tissues
Cuticle
- Cuticle A waxy waterproof layer covering the epidermis that reduces water loss, so it forms a barrier to gases.
Epidermis
- Epidermis A single layer of tightly packed cells that protects the leaf and contains stomata.
Palisade mesophyll
- Palisade mesophyll Tall cells packed with chloroplasts near the upper surface where most photosynthesis occurs.
Spongy mesophyll
- Spongy mesophyll Loosely arranged cells with large air spaces that allow gases to diffuse through the leaf.
Vascular bundles
- Vascular bundles Veins containing xylem, which brings water and minerals, and phloem, which carries sugars away.
Part 2 of 4Adaptations for Gas Exchange
Adaptations for Gas Exchange
Stomata
- Stomata Pores in the epidermis, mostly on the lower surface, through which carbon dioxide enters and oxygen and water vapour leave.
Guard cells
- Guard cells Pairs of cells that open and close each stoma by changing their turgor, controlling gas exchange and water loss.
Air spaces
- Air spaces Spaces among the spongy mesophyll provide a large moist surface and allow gases to move easily.
Thin leaf
- Thin leaf A flat, thin blade gives a high surface area to volume ratio and a short diffusion distance.
| Feature | Function |
|---|---|
| Stomata | Allow carbon dioxide in and oxygen out |
| Spongy mesophyll air spaces | Increase surface area and allow diffusion |
| Thin leaf blade | Short diffusion path |
| Waxy cuticle | Reduces water loss |
Part 3 of 4Transpiration
Transpiration can be measured with a potometer, which records the uptake of water by a cut shoot.
1
Open stomata allow carbon dioxide in, but water vapour also diffuses out from the moist mesophyll surfaces.
2
This loss of water vapour from the leaves is called transpiration.
3
Transpiration cools the plant by evaporation and pulls water and mineral ions up through the xylem.
4
Higher temperature, light intensity and air movement increase the rate, as they open stomata or remove humid air around the leaf.
5
Higher humidity decreases the rate, because the gradient of water vapour between leaf and air becomes smaller.
Part 4 of 4Exam Checklist & Tips
2.6.5 – Key Points for the Exam
- Stoma A pore in the leaf epidermis that allows gas exchange.
- Guard cell One of a pair of cells that control the opening of a stoma.
- Mesophyll The tissue between the upper and lower epidermis where photosynthesis takes place.
- Transpiration Loss of water vapour from the aerial parts of a plant, mainly through stomata.
- Potometer A device used to estimate the rate of transpiration by measuring water uptake.
Common Mistakes
- Saying stomata are in the upper epidermis of most plants;
- they are usually more numerous on the lower surface.
- Describing transpiration as the movement of water in the phloem; it relates to xylem.
- Stating that the cuticle allows gases through; it is waxy and stops most gas and water movement.
- Mixing up palisade mesophyll (many chloroplasts) with spongy mesophyll (air spaces).
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