By the end of this lesson
- Define transpiration accurately.
- Trace water from xylem to atmosphere.
- Explain benefits and costs of water loss.
BIG QUESTION
Why does a plant lose most of the water it absorbs?
Evaporation from moist mesophyll surfaces supports gas exchange but creates a continuous water demand.
Evaporation begins inside the leaf
Water moves from xylem into mesophyll cells and moistens their cell walls. It then evaporates into the leaf air spaces.
Water vapour diffuses through open stomata because the outside air is usually less humid.
Transpiration sustains a stream
transpirationLoss of water vapour from the surfaces of a plant, especially the leaves: water evaporates inside the leaf and diffuses out, mainly through the stomata. is evaporation of water from plant surfaces, especially leaves. The loss lowers leaf water potential and pulls replacement water through xylem.
This transpiration streamThe continuous movement of water from the roots, up the xylem, to the leaves, pulled by transpiration; it also carries mineral ions. also carries mineral ions and can cool leaves.
Stomata balance carbon gain and water loss
Open stomata allow carbon dioxide to enter for photosynthesis but also permit water vapour to leave.
Guard cells regulate stomatal aperture. During water stress, closure conserves water but can limit photosynthesis.
Stages of leaf water loss
Precise answers distinguish movement in liquid and gas phases.
| Stage | State/process | Location |
|---|---|---|
| Xylem to mesophyll | Liquid water movement. | Leaf veins and cells. |
| Cell wall to air space | Evaporation. | Moist mesophyll surface. |
| Air space to atmosphere | Diffusion of water vapour. | Through stomata. |
| Replacement from roots | Mass flow under tension. | Xylem. |
Language that earns precision
- transpiration
- Evaporation of water from plant surfaces.
- stoma
- Adjustable pore in the epidermis for gas exchange.
- guard cell
- Cell controlling stomatal opening.
- transpiration stream
- Continuous movement of water from roots through xylem to leaves.
Common misconceptions
Claim: Transpiration is water moving up xylem.
Correction: It is evaporation from plant surfaces; xylem flow replaces lost water.
Claim: Water leaves as liquid droplets through stomata.
Correction: It normally leaves as water vapour.
Claim: All water loss is useless.
Correction: It accompanies gas exchange, moves minerals and can cool leaves.
Match the answer to the command word
Use evaporation from the surface; do not define it as water uptake.
Write xylem → mesophyll wall → air space → stoma → atmosphere.
Recall, apply, then justify
Explain the pathway of water through a leaf during transpiration.
Hint
Begin at xylem and name the state change.
Suggested marking points
- Water moves from xylem into mesophyll cells/cell walls.
- It evaporates from moist cell walls.
- Water vapour enters leaf air spaces.
- It diffuses down a concentration gradient.
- It leaves through stomata.
Explain one benefit and one cost of stomatal opening.
Hint
Carbon dioxide and water.
Suggested marking points
- Open stomata allow carbon dioxide to diffuse in.
- This supports photosynthesis.
- They also allow water vapour to diffuse out.
- Excess loss can reduce turgor/cause wilting or stomatal closure.
Explain how transpiration can cool a sunlit leaf yet also reduce photosynthesis during drought.
Planning hint
Evaporation uses energy; drought changes stomata.
What a strong design includes
- Evaporation requires latent heat energy.
- Energy is removed from leaf tissue, lowering temperature.
- Drought lowers plant water potential/turgor.
- Guard cells close stomata to reduce water loss.
- Less carbon dioxide enters.
- Carbon dioxide can then limit photosynthesis.