F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and FunctionMoving Water: Osmosis, Root Pressure and the Cohesion-Tension Model
F3.1 F2.1 A1.1 A1.5 A1.6 A1.13About 70 minutes
Learning goals- I can describe the path of water from the soil, through the plant, to the air.
- I can explain root pressure and the cohesion-tension model of water transport.
- I can explain how light, temperature, humidity, wind and soil water affect the rate of transpiration.
- I can calculate a transpiration rate from potometer data.
Words to know
transpiration
The evaporation of water from the leaves (mainly through stomata), which pulls water up through the plant.
root pressure
Pressure in the xylem of roots caused by minerals being pumped into the xylem and water following by osmosis.
guttation
Droplets of xylem sap pushed out of special pores at leaf edges or tips by root pressure, usually at night or early morning.
cohesion
The attraction of water molecules to each other (through hydrogen bonds).
adhesion
The attraction of water molecules to other surfaces, such as the walls of xylem cells.
tension
A pulling force; in xylem, the pull on the water column created by evaporation from the leaves.
cohesion-tension model
The explanation that transpiration pulls a continuous, cohesive column of water up through the xylem from roots to leaves.
cavitation
The breaking of a water column in the xylem when an air bubble forms, blocking that tube.
potometer
A device that measures the rate at which a leafy shoot takes up water, which is close to its rate of transpiration.
Lifting water with no pump
On a hot July day, a large tree can move hundreds of litres of water from the soil to its leaves, some of them more than 30 m up. It has no heart and no pump, and xylem cells are dead. Yet the water rises. How?
The answer is that the plant doesn't really push water up. The air pulls it up, and the plant controls the pull with its stomata. In this lesson you'll build the full explanation, called the cohesion-tension model, and measure transpiration yourself.
Remember from Grade 10 and Lesson 2
In Grade 10 you learned that water evaporating from leaves creates a transpiration pull, like sipping through a long straw, and that transpiration is faster when it is warm, bright, windy and dry. In Lesson 2 you saw how water enters roots by osmosis and passes the endodermis. Now we look at each step in detail and at how scientists measure it.
The path of water
Water always moves from where it is more available to where it is less available. Moist soil has lots of available water; dry summer air has very little. The plant sits between them like a wick.
- Soil → root: water enters root hairs by osmosis and crosses to the xylem (Lesson 2).
- Root → leaf: water rises in the xylem as a continuous column.
- Leaf cells → air spaces: water evaporates from the wet walls of mesophyll cells into the air spaces of the spongy layer.
- Air spaces → atmosphere: water vapour diffuses out through the open stomata, into the drier outside air.
Only a small fraction of the water a plant absorbs is used in photosynthesis or kept in its cells; most of it is lost by transpiration.
Push from below: root pressure
At night, when stomata are closed, roots keep pumping mineral ions into the xylem by active transport. Water follows by osmosis, building up root pressure that pushes water a little way up the stem.
You can sometimes see it: on a cool, humid morning, the tips of grass blades and the edges of strawberry leaves carry tiny beads of liquid. That's guttation, xylem sap forced out by root pressure, not dew. Root pressure is useful, especially in spring before leaves open, but it is far too weak to lift water to the top of a tall tree, and many trees show little root pressure in summer. So something else must do most of the work.
Pull from above: the cohesion-tension model
The model (first proposed by Henry Dixon and John Joly in 1894) has three parts:
- Transpiration creates tension. As water evaporates from leaf cells, they draw water from the nearest xylem. This creates tension (negative pressure) at the top of the water column.
- Cohesion holds the column together. Water molecules are polar and form hydrogen bonds with each other. This cohesion makes a thin column of water act like a rope: when the top is pulled, the whole column rises, all the way down to the roots.
- Adhesion supports the column. Water molecules also stick to the hydrophilic cellulose walls of the very narrow xylem tubes (adhesion), which helps hold the column up and resist gravity.
Evidence: the diameter of a tree trunk shrinks slightly at midday, when transpiration is fastest; the xylem is under tension and pulls inward on its walls. And if you cut a stem under tension, air is sucked in, rather than sap spurting out.
The weak point: cavitation. If the tension gets too high (severe drought) or sap freezes and thaws, an air bubble can form and break the column, blocking that tube. Narrow tracheids, with pits that stop bubbles from spreading, limit the damage, which helps explain why northern conifers do well in cold climates.
What controls the rate of transpiration?
Anything that changes how fast water evaporates from the leaf, or how wide the stomata are open, changes the rate of transpiration.
| Factor | Effect on transpiration | Why |
|---|
| Light ↑ | Faster | Stomata open in light to take in CO₂ |
| Temperature ↑ | Faster | Water evaporates faster, and warm air can hold more water vapour |
| Humidity ↑ | Slower | Moist air reduces the difference in water vapour between the leaf and the air |
| Wind ↑ | Faster | Wind blows away the moist air layer next to the leaf |
| Soil water ↓ | Slower | The plant closes its stomata (signalled by ABA) to save water |
| Leaf surface area ↑ | Faster | More stomata and more evaporating surface |
Measuring transpiration: the potometer
A potometer holds a freshly cut leafy shoot in a sealed tube of water. As the shoot transpires, it draws water in, and an air bubble in a narrow, scaled capillary tube moves toward the shoot. The distance the bubble moves per minute gives the rate of water uptake, which is very close to the rate of transpiration (a small amount of water is used by the cells or stays in them).
To calculate the volume of water taken up, use the volume of a cylinder: V = πr² × distance moved, where r is the inside radius of the capillary tube.
Worked exampleLet's calculate a transpiration rate
In a potometer, the capillary tube has an inside radius of 0.50 mm. The air bubble moves 36 mm in 12 minutes. Calculate the rate of water uptake in mm³ per minute.
- Volume taken up: V = πr² × distance = π × (0.50 mm)² × 36 mm.
- (0.50)² = 0.25, so V = π × 0.25 × 36 ≈ 28.3 mm³.
- Rate = 28.3 mm³ ÷ 12 min ≈ 2.36 mm³/min.
- Round to 2 significant figures (the radius has 2): 2.4 mm³/min. Since 1000 mm³ = 1 mL, that is 0.0024 mL/min.
The shoot takes up water at about 2.4 mm³ per minute (2.4 × 10⁻³ mL/min).
Worked exampleLet's analyse an experiment
A student runs the same shoot in a potometer under four conditions, 10 minutes each, and records how far the bubble moves: still air at room temperature: 20 mm; fan blowing: 46 mm; plastic bag over the shoot: 8 mm; dark cupboard: 6 mm. Identify the independent and dependent variables, and explain each result.
- Independent variable: the condition (air movement, humidity, light). Dependent variable: distance the bubble moves (water uptake). Controlled: same shoot, same time, same tube, same temperature.
- Fan (46 mm): wind removes moist air from around the leaf, so evaporation is faster: rate more than doubles compared with still air.
- Bag (8 mm): the air inside the bag becomes very humid, so the difference in water vapour between leaf and air shrinks and transpiration slows.
- Dark (6 mm): stomata close in darkness, so very little water vapour can escape.
- Limitation: using one shoot in sequence means it may change over time (for example, wilting or cut end blocking). Repeating trials in a different order would improve reliability.
Wind speeds transpiration up; high humidity and darkness slow it down. The pattern supports the idea that transpiration is driven by evaporation and controlled by stomata.
Just for funThose sparkling drops on the tips of grass blades on a cool summer morning are often not dew at all. They are guttation: water pushed up and out of the grass by root pressure overnight. In other words, your lawn has been gently leaking.
Hands-on activityMeasure transpiration at home
Choose one investigation. Both are safe and use household materials.
Option A: the bag test (outdoors). You need: 2 clear plastic sandwich bags, 2 twist ties, a measuring spoon or a small syringe, a watch.
- Choose a leafy branch of a shrub or tree in the sun, and a similar branch on a plant of the same kind in the shade. (Ask permission if it isn't your plant.)
- Slip a bag over the leaves of each branch and close it gently around the stem with a twist tie.
- After 2 to 3 hours, carefully remove each bag and measure the water collected (tip it into a measuring spoon, or draw it up with a syringe).
Option B: the mass test (indoors). You need: 2 similar potted plants, plastic wrap, a kitchen scale.
- Water both plants, let them drain, then cover the soil (not the leaves) tightly with plastic wrap so only the plant can lose water.
- Weigh each pot. Put one in a bright, warm window and the other in a cool, dim room.
- Weigh again after 24 and 48 hours. Calculate mass lost (1 g of water ≈ 1 mL).
Write up: your question, a hypothesis ("If..., then..., because..."), independent, dependent and controlled variables, a data table, and a conclusion. Name one source of error.
What to look for: the sunny branch or the plant in the bright, warm window loses more water. Condensation on the inside of the bag is water vapour from transpiration. Possible errors: the two plants differ in leaf area; the plastic wrap leaks; the weather changed during the test.
Your turn
1List the four main steps in the path of water from the soil to the air.
- Enters root hairs by osmosis. 2. Rises in the xylem. 3. Evaporates from mesophyll cells into air spaces. 4. Diffuses out through the stomata.
2What causes root pressure?
Roots actively pump mineral ions into the xylem; water follows by osmosis, raising the pressure that pushes water upward.
3Define cohesion and adhesion, and explain the role of each in the xylem.
Cohesion: water molecules attract each other (hydrogen bonds), so the column holds together when pulled. Adhesion: water molecules stick to xylem walls, helping hold the column up against gravity.
4Why can't root pressure alone explain how water reaches the top of a tall tree?
Root pressure is too weak to push water that high, it is low or absent in many trees in summer when transpiration is greatest, and stems under tension suck in air when cut instead of oozing sap.
5How does wind affect transpiration, and why?
It increases transpiration: wind blows away the humid air next to the leaf, keeping the outside air drier, so water vapour diffuses out faster.
6A potometer tube has a radius of 0.40 mm. The bubble moves 25 mm in 5.0 min. Calculate the uptake rate.
V = π × (0.40)² × 25 = π × 0.16 × 25 ≈ 12.6 mm³. Rate = 12.6 ÷ 5.0 ≈ 2.5 mm³/min.
7What is cavitation, and why is it a problem?
An air bubble forms in a xylem tube and breaks the water column, so that tube can no longer carry water. Many bubbles reduce water supply to the leaves.
More practice: warm-up
Evaporation of water from leaves, mainly through the stomata.
2Does high humidity speed up or slow down transpiration?
Droplets of xylem sap pushed out of leaf tips or edges by root pressure.
4What kind of bonds cause cohesion between water molecules?
5What does a potometer measure directly?
The rate of water uptake by a leafy shoot (close to the rate of transpiration).
More practice: on your own
1Explain why the trunk of a tree is very slightly narrower at midday than at dawn.
At midday, transpiration is fastest, so the water in the xylem is under high tension. The tension pulls inward on the xylem walls, shrinking the trunk slightly. At dawn, stomata have been closed overnight, so the tension is low and the trunk is back to full width.
2A potometer reading overestimates transpiration slightly. Explain why.
A potometer measures water uptake. A small amount of the water taken up is used (in photosynthesis) or stored in growing cells, so not all of it is transpired.
3Why should you cut a shoot for a potometer under water?
If you cut it in air, air is sucked into the xylem (it is under tension), creating bubbles that break the water columns. Cutting under water keeps the columns continuous.
4Predict and explain how transpiration in a sugar maple changes from a sunny July afternoon to a cool, cloudy, humid evening.
It decreases a lot: less light means stomata start to close; cooler air means slower evaporation; humid air means a smaller difference in water vapour between the leaf and the air.
5Explain how transpiration helps a plant, besides moving water.
It carries dissolved minerals from the roots to the leaves in the xylem stream, and evaporation cools the leaves on hot days.
Challenge
1A student puts a leafy shoot in a potometer and records 30 mm in 10 min. She then coats the lower surface of every leaf with petroleum jelly and records 4 mm in 10 min. What does this show? Predict the result if she had coated the upper surfaces instead.
Coating the lower surfaces cut transpiration by about 87% [(30 − 4) ÷ 30], so most stomata are on the lower surface. If she had coated only the upper surfaces, the rate would drop only slightly (perhaps to about 25 to 28 mm), since few stomata are there and the cuticle already limits water loss.
2Use the cohesion-tension model to explain why a cut flower wilts faster if its stem sits out of water for an hour before going into a vase, even if the end is put in water afterwards.
When the stem is out of water, transpiration continues and air is pulled into the cut xylem tubes, causing cavitation. The broken water columns can't carry water even after the stem is put back in water. Cutting a few centimetres off the stem under water removes the air-blocked section and restores continuous columns.
3Two potometers have tubes of radius 0.50 mm and 0.25 mm. Both shoots take up water at exactly the same rate. Compare how far the bubble moves in each in the same time, and explain which is better for measuring a slow rate.
Volume = πr² × distance, so for the same volume, distance ∝ 1/r². Halving the radius divides the area by 4, so the bubble moves 4 times as far in the 0.25 mm tube. The narrower tube is better for slow rates, because the bubble moves a distance large enough to measure precisely.
Quiz
1. Which process provides most of the force that lifts water to the top of a tall tree?
2. Cohesion between water molecules is caused by:
3. Which condition would produce the slowest transpiration?
5. A potometer bubble moves 20 mm in 10 min in a tube of radius 0.50 mm. The uptake rate is about:
6. Why do cut flower stems last longer if re-cut under water?
Big idea: F3.1 includes transpiration and osmosis as transport mechanisms. Students should be able to explain why water rises (evaporation creates tension; cohesion and adhesion transmit it) and design and interpret simple transpiration experiments (A1.1, A1.5, A1.6).
How to help: The bag test works best on a sunny day; the mass test needs a kitchen scale and two days. Ask your teen to state the independent, dependent and controlled variables before starting. A bouquet of flowers is a nice real-life link: try re-cutting one stem under water and one in air and compare after two days.
Common difficulties:
- Thinking roots "pump" water up the tree. Most of the force comes from pulling at the top.
- Confusing guttation with dew.
- Using the diameter instead of the radius in πr².
- Forgetting that a potometer measures uptake, not loss directly.