Transpiration

In the syllabus and the exam

Transpiration is studied within transport in plants as the loss of water vapour, mainly through the stomata of the leaves, and as the driving force behind the upward movement of water through the xylem. Candidates are expected to explain the mechanism in sequence: water evaporates from the moist walls of mesophyll cells into the internal air spaces of the leaf, and this water vapour then diffuses out through open stomata into the drier surrounding air. Exam questions frequently test the environmental factors affecting transpiration rate, requiring candidates to explain, rather than simply list, why higher temperature, lower humidity, greater wind speed, and higher light intensity each increase the rate of water loss, usually by increasing the concentration gradient for diffusion or causing more stomata to open. Paper 3 practical questions often involve using a potometer to measure the rate of water uptake as an estimate of transpiration rate, and candidates must know that a potometer measures water uptake by the shoot, not transpiration directly, though the two are closely linked. A common error is treating transpiration purely as a disadvantage or a wasteful process, when it is also credited with helping to cool the plant and drive the transport of water and dissolved minerals upward. Candidates should also know that transpiration is sometimes described as a necessary consequence of a leaf's structure being adapted for efficient gas exchange, since the same open stomata that let carbon dioxide in for photosynthesis inevitably also let water vapour escape, linking this term back to the dual role of the stoma.

Example

Explaining water loss, a candidate writes: 'Transpiration is the evaporation of water vapour from the mesophyll cells followed by its diffusion out through open stomata, and its rate increases on a hot, dry, and windy day.'

Related terms

Transpiration streamStomaCuticle

Source:SRC-DSKP-EN