Transpiration is vapour loss from leaves
Transpiration = the loss of water vapour from the leaves of a plant: evaporation from mesophyll cell surfaces into the internal air spaces, then diffusion out through the stomata. Two words earn the marks. Vapour -- water leaves as a GAS, so a bare "water loss" is incomplete. Leaves -- not roots, stems or flowers. It is an unavoidable side effect of holding moist mesophyll surfaces and open stomata for gas exchange, not something the plant chooses to do.
Evaporation then diffusion, in that order
Two distinct physical processes, and both must be named. Evaporation: liquid water becomes vapour at the moist surfaces of mesophyll cells -- mainly spongy mesophyll, whose large irregular surfaces and interconnected air spaces give the greatest area -- lining the internal air spaces. Diffusion: the vapour then moves by random motion, from higher to lower concentration, through those air spaces and out through the stomata into the drier air outside.
(Extended) Gradient sets rate; wilting = lost turgor
Trace every rate factor to the water-vapour concentration gradient between the near-saturated air spaces and the air outside. Higher temperature: more kinetic energy, faster evaporation and diffusion, rate rises. Higher wind speed: humid air swept away, gradient stays steep, rate rises. Higher humidity: outside air holds vapour, gradient smaller, rate falls. Wilting: transpiration exceeds root uptake, cells lose turgor and go flaccid, so the plant droops.
Drawn from real examiner reports.
Evaporation happens inside the leaf
A genuine 50/50 split in past sittings. Water evaporates from the moist surfaces of mesophyll cells into the air spaces; only then does the vapour diffuse out through the stomata. Not credited: evaporation "from the leaf surface" or "off the stomata" -- these skip the internal evaporation step and the air-space diffusion step. Always give both steps, in order.
Flagged Jun 2022 P11 Q7; P31 Q10b
More wind means MORE transpiration
Reports flag the reversed version as common: candidates say low or still wind speed raises the rate. Higher wind speed increases it. Moving air sweeps away the humid layer that would otherwise build up just outside the stomata, keeping the water-vapour concentration gradient steep so diffusion stays fast. Think of wind as refreshing the drier air around the leaf.
Flagged Nov 2023 P12 Q5
(Extended) Humidity is not temperature
Two independent factors, muddled: some candidates claim higher humidity raises temperature. It does not -- humidity is how much water vapour the air already holds. Its own effect runs the other way: higher humidity decreases transpiration, because the outside air is nearer saturation, so the gradient out of the near-saturated air spaces is smaller and diffusion slows.
Flagged Jun 2022 P42 Q7a
Stomata are pores, not cells
Asked to name cells, candidates offer "stomata". A stoma is a pore in the epidermis, not a cell. The named cells here are the guard cells that open and close it, and the spongy mesophyll cells from whose surfaces the water evaporates. Keep the wording exact: evaporation happens at a cell surface, diffusion happens through a pore.
Flagged Jun 2022 P42 Q7c
Transpiration ≠ translocation
Transpiration is the loss of water vapour from leaves, by evaporation then diffusion through the stomata. Translocation is the movement of sucrose and amino acids through the phloem to where they are used or stored -- a different tissue, different substances, and often a different direction of travel. The two words are not interchangeable.
(Extended) Wilting ≠ plasmolysis
Wilting is the visible drooping of a leaf or whole plant when many cells lose turgor pressure because transpiration exceeds water uptake; those cells are alive and usually recover once watered. Plasmolysis is a far more extreme cell-level event, in which the membrane pulls away from the cell wall as osmosis removes enough water to shrink the cell contents.
Describe (Core) vs explain (Extended)
"Describe" wants the direction of the outcome only -- "increasing temperature increases the rate of transpiration". "Explain" wants the full mechanism, ending back at the concentration gradient. A bare direction gains no mechanism marks on an "explain" question.
Run the four-step gradient chain
Same chain every time: state the direction of change in the factor, and its effect on the water-vapour gradient (steeper or shallower), then the effect on the rate of diffusion out through the stomata, then the effect on transpiration rate. Match the links to the mark tariff.
Potometer: cut underwater, seal it
Cut and insert the shoot underwater so no air bubble blocks the xylem; make every joint airtight; introduce one bubble and record its starting position; vary only the factor under test; read square-on to avoid parallax. It measures water UPTAKE -- not exactly transpiration.
Say what you actually measured
Name the dependent variable as the distance the bubble moves per unit time, not just "transpiration". Divide distance by time, never time by distance. Justify repeats by saying a mean reduces the effect of random error -- "to be more accurate" on its own earns nothing.
Cambridge 0654 spec reference: Section B8 "Transport in plants", sub-topic B8.3 (Core + Extended). This leaf covers the definition of transpiration; the evaporation-then-diffusion mechanism inside the leaf; the Core effect (direction only) of temperature and wind speed on transpiration rate; and, for Extended, the mechanism-level explanation of the effects of temperature, wind speed and humidity, plus the explanation of wilting.
Out of scope for B8.3 (covered elsewhere): the structure and contents of xylem and phloem, and translocation of sucrose/amino acids through phloem (B8.1/B8.2); the transpiration-PULL mechanism that draws water up the xylem and the role of cohesion between water molecules (B8.1/B8.2); root-hair-cell structure and water uptake by osmosis at the root (B2/B3).
| Term | Precise definition |
|---|---|
| Transpiration | The loss of water vapour from the leaves of a plant, by evaporation (from mesophyll cell surfaces into internal air spaces) and diffusion (through the stomata into the surrounding air) |
| Evaporation (in this context) | Water changes from liquid to vapour at the moist surfaces of mesophyll cells lining the leaf's internal air spaces |
| Diffusion | Net movement of particles (here, water-vapour molecules), by random motion, from a region of higher concentration to a region of lower concentration, down a concentration gradient |
| Water-vapour concentration gradient | The difference in water-vapour concentration between the leaf's internal air spaces (usually close to saturated) and the air immediately outside the stomata (usually less saturated) -- this gradient drives diffusion out of the leaf |
| Wilting (Extended) | The drooping of leaves and stems that occurs when leaf/stem cells lose turgor pressure because the rate of water loss by transpiration is greater than the rate of water uptake by the roots |
The transpiration pathway (Core):
Core rate factors (direction only): increasing temperature increases the rate of transpiration; increasing wind speed increases the rate of transpiration.
Extended rate factors (with mechanism):
| Factor | Direction of change | Effect on water-vapour concentration gradient | Effect on transpiration rate |
|---|---|---|---|
| Temperature | Increases | Faster evaporation and diffusion; more water vapour enters air spaces | Increases |
| Wind speed | Increases | Sweeps humid air away from stomata, keeping the gradient steep | Increases |
| Humidity | Increases | Outside air already holds more water vapour, so gradient is smaller | Decreases |
Define transpiration.
Describe the pathway taken by water as it is lost as water vapour, starting from the surface of a mesophyll cell inside a leaf and ending in the air outside the leaf. (3 marks)
