Transpiration and water transport in plants - TCE Biology (Tasmania)
“Investigate the transport of water from the roots via xylem (involving root pressure, cohesion of water molecules and transpiration) to the leaves and the factors that influence the rate of transpiration (light intensity, temperature, wind and humidity)”
Water enters root hair cells by osmosis, crosses the root to the xylem and moves up to the leaves. Three things move it: root pressure (a small push from the roots), cohesion (water molecules sticking together in an unbroken column) and transpiration (water evaporating from the leaf and diffusing out through stomata, which pulls the column upwards). Transpiration increases with light intensity (stomata open), temperature and wind, and decreases with humidity, because each factor changes the concentration gradient of water vapour between the leaf and the air.
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What this dot point is asking
Water balance in plants is part of criterion 6 (Section C of the TASC exam) under osmoregulation in plants. The external assessment specifications include the mechanism of transpiration, the factors affecting its rate and the role of stomata. CAM and C4 plants are excluded.
From soil to leaf
- Uptake: root hair cells have a large surface area. Water moves into them from the soil by osmosis, because the cell contents are more concentrated than the soil water.
- Across the root: water moves from cell to cell (and through cell walls) to the xylem in the centre of the root.
- Root pressure: mineral ions are actively moved into the xylem, so water follows by osmosis. This creates a small upward push.
- Up the stem: water travels up the xylem, dead, hollow tubes strengthened with lignin, as a continuous column. Cohesion (water molecules attracting each other) keeps the column unbroken, and adhesion to the xylem walls helps.
- Transpiration: in the leaf, water evaporates from the surfaces of mesophyll cells into the air spaces, and water vapour diffuses out through the stomata. The loss of water at the top pulls the water column up (transpiration pull), like drinking through a straw.
Transpiration pull, working with cohesion, moves most of the water. Root pressure contributes only a small push.
The role of stomata
Stomata are pores, mostly on the lower surface of the leaf, each surrounded by two guard cells. Open stomata let carbon dioxide in for photosynthesis but also let water vapour out. Guard cells usually open the stomata in the light and close them in the dark or when the plant is short of water, so the plant balances gas exchange against water loss.
Factors affecting the rate of transpiration
| Factor | Effect on rate | Why |
|---|---|---|
| Light intensity | Increases | Stomata open wider in the light for photosynthesis, so more water vapour escapes |
| Temperature | Increases | Water evaporates faster and water vapour molecules diffuse faster; warm air can hold more water, steepening the gradient |
| Wind | Increases | Removes the humid air next to the leaf, keeping a steep water vapour concentration gradient |
| Humidity | Decreases | Humid air already holds a lot of water vapour, so the gradient between the nearly saturated air spaces inside the leaf and the outside air is smaller |
TASC's 2025 assessment report noted that strong answers on humidity explained the concentration gradient: the air inside the leaf is close to saturated, so drier outside air makes the gradient steeper.
Measuring the effect of wind
A leafy shoot is sealed into a potometer, and an air bubble in a capillary tube moves as the shoot takes up water. The bubble moves 20 mm in 5 minutes in still air, and 45 mm in 5 minutes with a fan on.
- Rate in still air: 20 / 5 = 4 mm per minute. With the fan: 45 / 5 = 9 mm per minute.
- Explanation: the fan removes humid air around the leaves, steepening the water vapour gradient, so transpiration and water uptake increase.
- Controlled variables: the same shoot, light intensity and temperature; the shoot is cut and sealed under water so no air enters the xylem.
- Limitation: a potometer measures water uptake, which is close to, but not exactly equal to, water lost by transpiration, because some water is used by the plant.
Marker's note: calculate rates with units, and explain using the concentration gradient.
- Saying water is pumped up the stem
- Most of the movement is a passive pull caused by transpiration, not pumping.
- Explaining humidity without a gradient
- Say how humidity changes the water vapour concentration gradient between leaf and air.
- Confusing xylem and phloem
- Xylem carries water and minerals up from the roots; phloem carries sugars.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksExplain how water moves from the soil to the air around a leaf, referring to root hair cells, xylem, cohesion and stomata.Show worked answer →
One mark for each correct step, up to six.
- Water enters root hair cells from the soil by osmosis, because the cell contents have a higher solute concentration than soil water.
- Water moves across the root to the xylem.
- Root pressure, produced as mineral ions are moved into the xylem and water follows by osmosis, gives a small push upwards.
- Water molecules are attracted to each other (cohesion), forming a continuous column in the xylem.
- Water evaporates from mesophyll cells into the air spaces in the leaf and diffuses out through open stomata (transpiration).
- This loss pulls the column of water up the xylem from the roots (transpiration pull).
Original4 marksExplain why the rate of transpiration increases on a windy day and decreases on a humid day.Show worked answer →
Two marks for each factor, linked to the concentration gradient.
Wind. Wind removes the layer of humid air that builds up around the leaf, keeping the air outside the stomata dry. This maintains a steep concentration gradient of water vapour between the moist air spaces inside the leaf and the outside air, so water vapour diffuses out faster.
Humidity. When the outside air is humid, it already holds a lot of water vapour, so the concentration gradient between the air spaces inside the leaf (which are nearly saturated) and the outside air is smaller, and water vapour diffuses out more slowly.