DSE Biology | Transport in plants: root hairs, xylem, transpiration pull and rate calculations

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students say water goes up from the root to the leaf, then freeze when asked what pushes it. This lesson gives them the real driver: a negative pressure, the transpiration pull.

Objectives — By the end students can: 1) explain how root hairs enlarge the absorbing surface and how a water potential gradient drives uptake; 2) explain how cohesion and tension keep the xylem column continuous; 3) calculate transpiration rate from potometer data; 4) predict how light, humidity, wind and temperature change the rate.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Diffusion	Which way particles move	From high to low concentration
Water potential	The water potential of pure water	Zero; solutions are negative
Cell walls	What the plant cell wall does	Stops the cell bursting

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	The pathway	Root hair, cortex, xylem, leaf	Draw the four positions
8-20 min	Root hairs and water potential	Large surface, negative cell sap	Compute one hair area
20-34 min	Xylem and the pull	Cohesion, tension, continuous column	Straw analogy
34-46 min	Potometer arithmetic	Volume over time	Get 0.28 cm3 per hour
46-54 min	Rate factors	Light, humidity, wind, temperature	Design a control
54-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Root hairs: make the surface big

A root hair is an elongated epidermal cell. At 10 micrometres across and 200 micrometres long its surface area is about 2 pi r L, roughly 12566 square micrometres. A large surface speeds up diffusion and osmosis. Water moves from the soil, where water potential is near zero, into the hair cell, whose sap is negative because of solutes; mineral ions mostly need active transport against the gradient. Ask students to compare a bare root with the same root plus hairs: they do not need the number, only the reason.
Xylem vessels are dead, hollow and lack end walls, so they form a continuous pipe. When mesophyll cells lose water by transpiration their water potential falls and they draw water from the xylem, creating tension. Hydrogen bonds between water molecules give cohesion, letting the column survive that tension, while adhesion holds it to the wall, so the whole column is pulled upwards. Use a drinking straw as the analogy and stress that it is a pull, not a pump.

Potometer: measure how far the bubble travels, then volume equals pi r squared times distance. With a tube radius of 0.5 mm and 30 mm travelled in 5 minutes the volume is about 23.6 cubic millimetres, which is 0.28 cubic centimetres per hour. Light opens stomata, low humidity and wind raise the rate, and warmth raises it too. In a controlled experiment change one variable only. Ask what happens in front of a fan and why.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Transpiration rate	Volume over time, then convert units	0.28 cm3 per hour
Explanation	Why water reaches the leaf	Pull plus cohesion plus adhesion	A continuous column
Prediction	Which factor raises the rate	Look at stomata and vapour pressure deficit	Wind makes it faster
Experimental design	Setting a control	Change one variable, keep the rest equal	Light and dark pairs

Sanity check (teach this): Check units after every calculation: cubic millimetres to cubic centimetres divides by 1 000, minutes to hours multiplies by 60. A rate must be positive, and a negative value means the bubble moved backwards, usually a leak or a wilted shoot. Water always moves from higher to lower water potential, pure water is zero, and a value can never be positive.

05 Independent practice (3 questions)

Question	Answer	What to watch
Volume moved 30 mm in a tube of radius 0.5 mm	About 23.6 cubic millimetres	pi r squared times distance
Rate for the same reading over 5 minutes	About 0.28 cm3 per hour	Convert to cm3 then multiply by 12
Surface area of a hair 20 micrometres across and 200 long	About 1.26 x 10^4 square micrometres	Two pi r L
Effect of stronger wind on the rate	It increases	Removes humid air and steepens the gradient

06 Common mistakes and what to say

Mistake	Student writes	Say this
Calling the pull a pump	Answering that roots push water up	Root pressure only lifts short plants; tall trees are pulled from the leaves.
Mixing up xylem and phloem	Saying sugar travels in the xylem	Xylem carries water and minerals, phloem carries sugar, and the directions differ.
Wrong rate units	Answering 0.28 cm3 per minute	The 5-minute volume is about 0.024 cm3; multiply by 12 for an hourly rate.
Changing two variables in a control	One group windy, one group dark	With two changes you cannot tell which one moved the bubble.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Redo the two potometer questions
Tutor preparation guide hub — Pair with the photosynthesis chapter

08 Exit check (last 5 minutes)

Question	Answer	Decision
What does the xylem carry	Water and mineral ions	All three right: move to phloem
Rate for 30 mm in 5 minutes, radius 0.5 mm	About 0.28 cm3 per hour	One wrong: redo unit conversion
Why a tall water column does not break	Cohesion and adhesion	Two wrong: restart act one

09 Tutor FAQ

Does root pressure matter?

It helps short plants and at night, but tall trees rely on the transpiration pull.

When do stomata close?

In drought, darkness or high carbon dioxide, to cut water loss.

Why use the same leaf for a control?

Leaves of different age and area give rates that cannot be compared.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.