This topic summary covers Knowledge Organiser within Transpiration for GCSE Biology. Transpiration process, stomatal control, factors affecting rate, plant adaptations, measuring transpiration, and practical investigations It is section 18 of 20 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 18 of 20
Practice
20 questions
Recall
25 flashcards
Knowledge Organiser
Key Terms
- Transpiration — loss of water vapour from leaves, mainly through stomata
- Transpiration stream — water pathway from root to leaf
- Stomata — pores in leaf epidermis
- Guard cells — control stomatal opening
- Potometer — measures water uptake rate
- Xerophyte — adapted to dry conditions
- Wilting — loss of cell turgor due to water deficit
- Boundary layer — humid air layer at leaf surface
Factors — THAW
- Temperature up → transpiration up (more kinetic energy)
- Humidity down → transpiration up (steeper gradient)
- Air movement up → transpiration up (removes boundary layer)
- Wavelength/light up → stomata open → transpiration up
- All four act by changing the concentration gradient or the area of the stomatal pore
Must-Know Facts
- 99% of water absorbed is lost by transpiration
- Stomata are mainly for CO₂ in / O₂ out — water loss is a trade-off
- Potometer measures uptake, NOT transpiration directly
- Cut shoot underwater — prevents air locks in xylem
- Guard cells open by K⁺ in → osmosis → turgid → pore opens
- Marram grass: rolled leaf, sunken stomata, hairs in grooves
Xerophyte Adaptations
- Thick waxy cuticle — reduces cuticular evaporation
- Sunken stomata — humid air trapped in pit
- Rolled leaves (marram grass) — encloses humid air
- Hairs in grooves — trap water vapour near stomata
- Reduced leaf area — less surface for evaporation
- CAM photosynthesis — stomata open at night only (Higher)
Common Mistakes
- Saying a potometer measures transpiration directly: A potometer measures water uptake by the cut shoot — this is an estimate of transpiration rate, not a direct measurement of water lost from leaves.
- Forgetting the shoot must be cut underwater: Cutting the shoot in air allows an air bubble to enter the xylem, blocking water flow. This explanation is a required mark point in potometer method questions.
- Listing xerophyte adaptations without explaining the mechanism: Always state the adaptation AND how it reduces transpiration — e.g., "sunken stomata trap a layer of humid air in the pit, reducing the water vapour concentration gradient out of the leaf."
- Getting the direction of factor effects wrong: Higher humidity REDUCES transpiration (smaller concentration gradient); higher wind speed INCREASES transpiration (removes the boundary layer). Students frequently reverse these relationships.
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Practice Questions for Transpiration
What is transpiration?
Describe the three stages of transpiration in a leaf.
Quick Recall Flashcards
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