This memory aid covers Memory Aids within Transpiration for GCSE Biology. Transpiration process, stomatal control, factors affecting rate, plant adaptations, measuring transpiration, and practical investigations It is section 15 of 20 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Topic position
Section 15 of 20
Practice
20 questions
Recall
25 flashcards
Memory Aids
Factors affecting transpiration — THAW:
"Plants THAW in good growing conditions — and transpire faster too."
- Temperature — higher = more transpiration
- Humidity — lower = more transpiration
- Air movement (wind) — more = more transpiration
- Wavelength of light (light intensity) — more light = stomata open = more transpiration
Guard cell mechanism — the inflation sequence:
"Potassium In → Water In → Cells swell → Pore Opens"
Think of guard cells as inflatable door hinges. When inflated (turgid), the thick inner wall bows the cell outward, pulling the pore open. When deflated (flaccid), the cells straighten and the pore closes.
Potometer limitation (guaranteed exam mark):
"A potometer measures water UP-take, not water OUT-go."
Always write: "The potometer measures the rate of water uptake by the shoot, which is approximately equal to the transpiration rate under steady conditions."
Xerophyte adaptations — 5 Ss:
Sunken stomata, Small leaves, Succulent stems, Spines, Sticky/waxy cuticle
Each S reduces transpiration by reducing surface area, trapping humid air near stomata, or blocking evaporation directly.
Quick Check: On a hot, sunny, windy day, a plant's leaves begin to wilt even though the soil is moist. Explain the chain of events from the environmental conditions to the wilting.
High temperature increases the kinetic energy of water molecules, raising the evaporation rate from mesophyll cell surfaces. Strong sunlight causes stomata to open fully (guard cells become turgid). Wind removes the humid boundary layer around leaves, maintaining a steep water vapour concentration gradient. The combination of these three factors causes the rate of transpiration to greatly exceed the rate of water uptake from the soil through the roots. As water leaves cells faster than it is replaced, the cells lose turgor pressure — they become flaccid. The plant cannot maintain the structural support that turgor provides, and the leaves wilt.
Quick Check: A student sets up a potometer experiment and measures a transpiration rate of 3.2 mm/min in still air. They then turn on a fan. Predict and explain what will happen to the rate, and identify one variable they must keep constant to make this a fair test.
The rate of water uptake (transpiration) will increase when the fan is turned on. Wind removes the humid boundary layer of air that builds up around the leaf surface. This layer partially reduces the water vapour concentration gradient between the inside of the leaf (saturated) and the air outside. By removing it, the wind restores a steeper gradient, so water vapour diffuses out more rapidly, driving faster water uptake through the xylem. To make this a fair test, the student must keep temperature constant (the fan motor could warm the air, which would also increase transpiration independently of the wind effect).
Quick Check: A scientist compares transpiration rates in marram grass and a broad-leaved plant on the same day. The marram grass has a much lower rate. Explain two structural features of marram grass that account for this difference.
1. Rolled leaves: Marram grass rolls its leaves inward in dry conditions. This encloses the stomata inside the roll, creating a small, humid microclimate. The water vapour concentration gradient between the stomata and the outside air is reduced, so diffusion of water vapour out of the leaf slows. 2. Sunken stomata with hairs in grooves: The stomata are recessed into grooves on the inner surface of the rolled leaf, and hairs in these grooves trap water vapour. This further reduces the concentration gradient immediately outside the stomata, slowing diffusion and reducing transpiration rate significantly compared to a broad-leaved plant with stomata exposed on a flat surface.