Cells and organisation · GCSE Biology
Transpiration
Explain transpiration for GCSE Biology: water loss from leaves, the transpiration stream, xylem transport and factors that increase or decrease the rate.
Transpiration is evaporation of water from leaves, mainly through stomata. It pulls the transpiration stream in xylem. Light, wind, heat and low humidity speed it up.
The important bits
What you need to know
- 1
Transpiration is the loss of water vapour from plant leaves, mainly through stomata. It creates a pull that draws water up from the roots in the transpiration stream.
- 2
Water enters root hair cells by osmosis, moves across the root cortex, and enters xylem vessels. Xylem is dead, hollow tubes strengthened by lignin — well suited to withstand tension.
- 3
In the leaf, water evaporates from cell surfaces into air spaces and diffuses out through open stomata. This loss pulls more water up the xylem (cohesion-tension; cohesion of water molecules helps maintain a continuous column).
- 4
Stomata are pores mostly on the lower epidermis, surrounded by guard cells. When guard cells are turgid, the stoma opens for gas exchange in photosynthesis; when flaccid, it closes to reduce water loss.
- 5
Factors increasing transpiration rate: brighter light (stomata open), higher temperature (faster evaporation), faster wind (removes humid air), lower humidity (steeper water vapour gradient).
- 6
Factors decreasing rate: dark (stomata close), still air, high humidity, waxy cuticle, fewer stomata, rolled leaves (xerophyte adaptations).
- 7
A potometer measures water uptake by a cut shoot, which is approximately equal to transpiration rate when the plant is well hydrated.
- 8
Wilting occurs when water loss by transpiration exceeds uptake — cells become flaccid and the plant droops. Closing stomata reduces further loss but also limits carbon dioxide uptake.
Quotations worth analysing
Short evidence. Real method.
“Transpiration is the evaporation of water from a plant, especially through the stomata of leaves.”
Name leaves and stomata. Transpiration is not the same as translocation (sucrose in phloem).
“Light causes stomata to open, increasing transpiration.”
Link the factor to the mechanism: open stomata → more water vapour diffuses out. “More transpiration because it is hot” needs evaporation as the reason.
“Xylem vessels transport water and mineral ions from roots to leaves.”
Xylem is one-way, up the plant, in the transpiration stream. Phloem is for sucrose, both directions.
Go deeper
How does water get from soil to air?
Root hairs take in water by osmosis because cell sap is more concentrated than soil water. Water moves cell to cell or via the apoplast pathway to the xylem. Transpiration from mesophyll cells lowers water potential in the leaf; cohesion between water molecules and adhesion to xylem walls transmit the pull downwards. Mineral ions travel dissolved in the xylem water — some were taken up by active transport in root hairs. If a diagram shows arrows, xylem always goes root → stem → leaf. At night, stomata often close and transpiration falls, but respiration continues.
Go deeper
Stomata balance photosynthesis and water loss
Guard cells swell when water enters by osmosis (often after ion uptake), bowing apart to open the pore. Carbon dioxide enters for photosynthesis; water vapour leaves. In drought stress, abscisic acid triggers closure. Xerophytes may have sunken stomata, hairs to trap humid air, or rolled leaves to reduce exposed surface. In an explain question, tie each adaptation to reduced evaporation or reduced air movement over the stomatal surface. Do not write that plants “breathe out water like sweating” — it is evaporation and diffusion of water vapour.
Go deeper
Reading potometer and factor questions
A potometer records water uptake at the cut end of a shoot. Keep temperature, humidity, light and air movement controlled except the one factor you change. A bubble in a capillary moves faster when transpiration increases. Typical errors: air locks in the xylem, leaky seals, or equilibrating time not allowed. For six marks on factors, give two increases and two decreases with mechanisms. Wind works by replacing moist air at the leaf surface, maintaining a steep concentration gradient for water vapour.
See the idea in action
Explain why a plant on a windy windowsill loses water faster than one in still air (3 marks). Wind blows away water vapour that has diffused out of stomata. This keeps a steep concentration gradient between the humid air spaces inside the leaf and the air outside. More water evaporates from mesophyll surfaces and diffuses out, increasing transpiration rate and water uptake in the xylem.
Exam technique
Turn knowledge into marks
Separate transpiration (water in xylem, out of stomata) from translocation (sucrose in phloem). For each factor, state whether stomata or evaporation is affected, then the effect on rate.
Common mistakes
Do not give these marks away
- 01
Confusing transpiration with translocation, or saying phloem carries water.
- 02
Stating stomata open in the dark without linking to reduced transpiration.
- 03
Describing transpiration as active transport — it is evaporation and diffusion.
Which change would increase the rate of transpiration?
AHigher humidity around the leaf
BStomata closing in the dark
CIncreased wind speed past the leaf
DA thicker waxy cuticle
Show the answer
Increased wind speed past the leaf. Wind removes humid air from the leaf surface, maintaining a gradient for water vapour diffusion. High humidity, closed stomata and a thick cuticle all reduce transpiration.
Quick questions
If this is the bit you searched
What is transpiration?
The evaporation of water from a plant, mainly as water vapour diffusing out through stomata in the leaves.
What is the transpiration stream?
The movement of water from roots through xylem to leaves, pulled by water loss at the stomata.
How do guard cells control transpiration?
When turgid, they open the stoma for gas exchange; when flaccid, they close it to reduce water loss.
What factors increase transpiration rate?
Light, higher temperature, wind and low humidity. Each maintains open stomata or increases evaporation/diffusion.