Transpiration is the evaporation of water from the aerial parts of a plant, mainly through tiny pores called stomata on the underside of leaves. This constant water loss creates a continuous pulling force that draws water upward through the xylem from roots to leaves — a mechanism that requires no pumping energy from the plant at all.
What is transpiration and why does it happen?
Transpiration is the loss of water vapour from a plant, almost entirely through the stomata. Stomata must open to allow CO₂ in for photosynthesis, but every time they do, water vapour diffuses out along a concentration gradient (the inside of the leaf is humid; the outside air is drier).
A small amount of water is also lost through the thin waxy cuticle on the leaf surface, but this is typically less than 10 % of total water loss.
Transpiration has two useful side-effects:
- It helps cool the leaf by evaporative cooling.
- It drives the mass flow of minerals dissolved in water up from the roots.
How does water move from roots to leaves through the xylem?
The cohesion-tension mechanism
- Water evaporates from mesophyll cell walls into the air spaces inside the leaf and then out through the stomata.
- This lowers the water potential in the mesophyll cells.
- Water is drawn by osmosis from neighbouring xylem vessels into the mesophyll cells.
- Water molecules inside the xylem stick together by cohesion (hydrogen bonds between water molecules) and to the xylem walls by adhesion.
- The evaporation at the top therefore creates a continuous tension (negative pressure) that pulls the entire water column upward — like drinking through a straw.
This is called the cohesion-tension theory. No ATP is used; the driving energy comes from the sun evaporating water.
Structure of xylem vessels
| Feature | Detail | Why it matters |
|---|---|---|
| Dead cells | No cytoplasm or end walls | Uninterrupted hollow tube for water flow |
| Lignified walls | Reinforced with lignin | Prevents collapse under tension |
| Continuous column | Runs from root to leaf | Allows cohesion-tension to work |
Water enters the xylem from root hair cells via osmosis across the root cortex, driven by the lower water potential inside the plant compared with the soil water.
What factors affect the rate of transpiration?
| Factor | Effect on transpiration rate | Reason |
|---|---|---|
| Temperature increase | Increases | Faster evaporation; water molecules have more kinetic energy |
| Humidity increase | Decreases | Smaller concentration gradient between leaf interior and air |
| Wind speed increase | Increases | Removes humid air from around stomata, steepening the gradient |
| Light intensity increase | Increases | Stomata open wider for photosynthesis |
| Dry/drought conditions | Decreases (stomata close) | Guard cells lose turgor; stomata close to conserve water |
How do guard cells control the stomata?
Each stoma is flanked by a pair of guard cells. When guard cells take in water by osmosis they become turgid, and their asymmetric wall thickening causes them to bow outward, opening the pore. When they lose water they become flaccid and the pore closes.
Light stimulates guard cells to accumulate potassium ions (K⁺) by active transport, lowering their water potential so water enters by osmosis. This is why stomata are typically open by day and closed at night. In drought conditions the hormone abscisic acid (ABA) triggers potassium ions to leave guard cells, causing stomata to close regardless of light.
How do you use a potometer to measure transpiration?
A potometer measures water uptake by a cut shoot, which approximates the transpiration rate.
Step-by-step method
- Cut the shoot underwater to prevent air entering the xylem.
- Assemble the potometer without allowing air bubbles into the tubing.
- Introduce a small air bubble at the capillary tube end.
- Record the distance the bubble moves in a set time interval.
- Calculate rate: rate = distance ÷ time (in mm per minute).
- Change one variable at a time (e.g. place a fan beside the shoot) and repeat.
A wider tube means the same volume of water moves the bubble a shorter distance — use a narrow-bore capillary for accuracy.
What adaptations reduce transpiration in dry habitats?
Plants in arid environments (xerophytes) have evolved adaptations to limit water loss:
- Thick, waxy cuticle
- Sunken stomata (in pits, so humid air is trapped)
- Fewer stomata per unit area
- Stomata on the lower leaf surface only (away from direct sunlight)
- Rolled leaves (grasses) — trap humid air
- Reduced leaf surface area (cacti have spines instead of leaves)
Frequently asked questions
What is the difference between transpiration and evaporation?
Evaporation is a purely physical process by which any liquid water turns to vapour at a surface. Transpiration is a biological process specific to plants: it involves water vapour leaving through the stomata (which the plant can open and close) and through the cuticle. Transpiration is therefore regulated by the plant in a way that simple evaporation from a puddle is not.
Why does high humidity slow down transpiration?
Transpiration is driven by the difference in water vapour concentration between the humid interior of the leaf and the surrounding air. When the air is already very humid, that concentration gradient is small, so less water diffuses out per unit of time. In very high humidity — such as a tropical rainforest in still air — transpiration can drop almost to zero even though stomata are open.
Can a plant lose too much water through transpiration?
Yes. If transpiration exceeds water uptake by the roots, the plant wilts — cells lose turgor because they cannot maintain water potential. Prolonged wilting causes permanent wilting if cells desiccate severely. The plant's primary defence is stomatal closure, triggered by abscisic acid when water potential in the guard cells falls.
Why are xylem vessels made from dead cells?
Dead xylem cells have no cytoplasm and no end walls between them, creating a completely hollow, unobstructed tube from root to leaf. Living cells would impede the flow of water and could not maintain the structural rigidity needed to withstand the tension generated by transpiration pull. Lignin in the walls prevents the tube from collapsing inward under negative pressure.
For Socratic GCSE biology with Professor Darwin — following water from soil to stomata one step at a time — visit aitutors.me.