Transpiration and Water Uptake: The Continuous Pull
Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Transpiration is the loss of water vapour from leaves through stomata. As water evaporates from leaf cell surfaces, it creates a pull that draws water upward through xylem from the roots, forming the transpiration stream.
Transpiration drives the movement of water through the entire plant. Students often describe it as "plants sweating" but the examiner wants the precise mechanism: evaporation from internal leaf surfaces, diffusion through stomata, and the resulting pull through xylem.
What transpiration actually is
Transpiration is defined as the loss of water vapour from the surface of a plant, primarily through stomata in the leaves. Inside the leaf, water evaporates from the surfaces of mesophyll cells into air spaces. This water vapour then diffuses out through open stomata along a concentration gradient: the air inside the leaf is saturated, while the external air usually is not.
Transpiration is not a metabolic process. It is a physical consequence of having stomata open for gas exchange during photosynthesis.
Stomata and guard cells
Stomata are small pores mainly on the lower epidermis of leaves. Each stoma is bordered by two guard cells that control its opening. When guard cells are turgid (full of water), their inner walls curve apart and the stoma opens. When they lose water and become flaccid, the stoma closes.
Stomata open during the day to allow carbon dioxide in for photosynthesis, which also permits water vapour to escape. At night, most stomata close, reducing water loss when photosynthesis is not occurring. This balance between gas exchange and water conservation is a central concept.
The transpiration stream
Water loss from leaves creates a transpiration pull that draws water upward through xylem vessels. This continuous column of water stretches from the roots to the leaves. At the root end, water enters root hair cells by osmosis because the soil solution has a higher water potential than the cell sap.
The transpiration stream also carries dissolved mineral ions from the soil to the rest of the plant. Without transpiration, these minerals would not reach the leaves where they are needed for processes like protein synthesis and chlorophyll production.
Try a focused question (5 marks)
Describe the pathway of water from the soil to the atmosphere through a plant.
Show the answer and marking guidance
Water enters root hair cells from the soil by osmosis, moving from higher to lower water potential.
Water passes from cell to cell across the root cortex to the xylem.
Water moves up through xylem vessels to the leaves.
Water evaporates from the surfaces of mesophyll cells into air spaces.
Water vapour diffuses out through stomata into the atmosphere.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying water is lost through the whole leaf surface rather than mainly through stomata.
Describing transpiration as the plant pumping water upward.
Forgetting that the transpiration stream also transports mineral ions.
Not linking stomatal opening to photosynthesis and carbon dioxide entry.