Transpiration = evaporation then diffusion
Transpiration is the loss of water vapour from a plant, in two stages: liquid water evaporates from the wet surfaces of spongy mesophyll cells into the leaf air spaces, then that vapour diffuses out through the stomata (mainly in the lower epidermis) down a water-vapour concentration gradient. Both stages are needed for full marks — "water loss" alone scores nothing. Hook: Evaporate inside → Diffuse out → Lost to air.
Four factors, one water-vapour gradient
Rate depends on the water-vapour concentration gradient between the saturated leaf air spaces and the outside air. Temperature ↑ — faster evaporation, more kinetic energy, steeper gradient, rate ↑. Wind speed ↑ — vapour swept away from the stomata, gradient kept steep, rate ↑. Humidity ↑ — more vapour outside, shallower gradient, rate ↓. Light intensity ↑ — guard cells turgid, stomata open wider, rate ↑.
Transpiration pull raises water up the xylem (Extended)
(Extended) Evaporation at the mesophyll surfaces lowers the water potential of those cells, so they draw water from neighbouring cells and from the xylem. Cohesion — hydrogen bonding between water molecules — holds the column together, so as molecules leave at the top the whole continuous column is pulled up under tension from root to leaf. Roots replace the loss by osmosis from the soil. This is the cohesion-tension mechanism, driven by transpiration pull.
Drawn from real examiner reports.
High humidity slows transpiration
Candidates repeatedly write that humid air speeds transpiration up because "there is more water about". The opposite is true. Humid air already holds plenty of water vapour, so the gradient between the saturated leaf air spaces and the outside air is shallower, diffusion through the stomata is slower, and the rate falls.
Flagged in three consecutive sittings: s23 P22 Q15 · s22 P12 Q18 · w22 P41 Q3c
At 100% humidity the rate is zero
At 100% relative humidity the water-vapour concentration inside and outside the leaf becomes equal, so the gradient disappears and there is no net diffusion — transpiration stops. Answering that it is merely "very slow" misses the point examiners wanted: state explicitly that inside and outside are equal.
Few candidates stated the equal-water-potential point: w22 P41 Q3c i
Evaporation happens in the mesophyll
Water evaporates from the wet surfaces of spongy mesophyll cells inside the leaf. The stomata are only the exit pores; the guard cells flanking them are not the evaporating surface either, and palisade cells are adapted for photosynthesis rather than water loss. "Evaporation through the stomata" is not credited.
Site of evaporation misidentified across sittings: s22 P11/12/21 Q16/Q18 · s23 P41 Q3
Water vapour diffuses out, not water
The substance leaving through the stomata is water vapour, a gas. Writing "water diffuses through the stomata" is imprecise and can cost the mark — liquid water is what evaporates inside the leaf. Track the state change: liquid water → evaporation → water vapour in the air spaces → diffusion out through the stomata.
"Diffusion of water" penalised where "diffusion of water vapour" was required: w22 P41 Q3b ii
Transpiration pull ≠ transpiration
(Extended) "Water moves up the xylem by transpiration" confuses the cause with the mechanism. Transpiration is the loss of water vapour at the leaf; the tension it generates — transpiration pull — is what drags the continuous column of liquid water up the xylem by cohesion-tension. Name the mechanism, not the process driving it.
Flagged directly: w22 P41 Q3a ii
A potometer measures uptake
A potometer records the water taken up by the cut leafy shoot, not the water transpired. The two are close, because most water absorbed is lost as vapour, but a little is used in photosynthesis and retained for growth — so uptake slightly overestimates transpiration. Write "rate of water uptake" unless the question treats it as a proxy.
Potometer: distance ÷ time, with a unit
Rate of water uptake = distance moved by the bubble ÷ time. If the capillary cross-sectional area is given, convert to a volume using . Always state a unit — mm/min, cm/min or cm³/hour. An unlabelled number loses the mark.
Answer factor questions in four steps
State how the factor changes, then what happens to the water-vapour concentration gradient, then the effect on the rate of diffusion through the stomata, then the transpiration rate. Four linked statements collect the four marks; jumping straight to "rate increases" collects one.
Light acts through the stomata
Light intensity is the odd one out — it does not change the gradient directly. Brighter light makes the guard cells turgid, so the stomata open wider and more water vapour can escape. Explain light via stomatal aperture, and temperature, wind and humidity via the gradient.
Extended xylem answers: use the terms
The marks sit on the vocabulary: a continuous column of water, cohesion between water molecules, tension created by evaporation at the leaf, and replacement by osmosis at the root. Vague answers about "suction" pulling the water up score little.
Transpiration: the loss of water vapour from a plant's leaves by evaporation from the surfaces of spongy mesophyll cells and diffusion of water vapour through the stomata.
Two-element precision (both are needed for full marks):
Stomata (singular: stoma): pores formed between pairs of guard cells in the leaf epidermis. When guard cells are turgid (full of water), the pore opens; when guard cells are flaccid, the pore closes. Light causes stomata to open.
Define transpiration. Name the two processes involved and where each occurs in the leaf.
(a) State the definition of transpiration.
(b) Name the process by which water becomes water vapour inside the leaf.
(c) Name the process by which water vapour passes through the stomata to the atmosphere.
