The Transpiration Stream: Tracing Water from Root to Leaf
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
The transpiration stream is the continuous movement of water from soil through root hair cells, across root cortex, up xylem vessels, into leaf mesophyll cells, and out through stomata as water vapour. It is driven by evaporation from leaf surfaces, not by the plant pumping.
Tracing the transpiration stream from start to finish is a classic extended-response question. Full marks require naming each structure the water passes through in the correct sequence and identifying the processes at each stage.
From soil to xylem
Water in the soil has a higher water potential than the root hair cell sap. It enters the root hair cell by osmosis through the partially permeable membrane. From there, water moves across the root cortex, passing from cell to cell by osmosis: each successive cell has a slightly lower water potential.
Eventually, water reaches the xylem vessels in the centre of the root. The xylem forms a continuous tube from root to leaf, with no end walls to obstruct flow.
From xylem to leaf
Water travels upward through xylem vessels, pulled by the transpiration pull created by evaporation from leaf mesophyll cells. The cohesion between water molecules maintains the water column under tension without breaking.
In the leaf, water leaves the xylem and moves into the spongy mesophyll cells. It evaporates from their surfaces into the air spaces within the leaf. The water vapour then diffuses through the stomata into the surrounding atmosphere.
Why the stream matters
The transpiration stream has three main benefits: it transports water to cells that need it for photosynthesis and other reactions; it carries dissolved mineral ions from roots to leaves; and it provides evaporative cooling that helps regulate leaf temperature.
However, excessive transpiration can lead to more water loss than the roots can replace, causing wilting. Plants balance this by closing stomata in hot, dry conditions: but this also reduces carbon dioxide entry and slows photosynthesis.
Try a focused question (6 marks)
Describe how water moves from the soil to the atmosphere through a plant. Include the names of the processes involved.
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 across the root cortex from cell to cell by osmosis.
Water enters xylem vessels in the root.
Water moves up through xylem to the leaf, pulled by transpiration.
Water evaporates from mesophyll cell surfaces into leaf 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.
Skipping the root cortex and going straight from root hair to xylem.
Saying the plant pumps water up rather than explaining the transpiration pull.
Writing that water escapes through the leaf surface rather than specifically through stomata.
Forgetting to name osmosis for water entry into root hair cells.