DSE Biology | Transpiration and Plant Transport Lesson-Prep Guide (for tutors) | potometer, transpiration pull, ringing experiment

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students say “water rises because the roots absorb it” and stop there. This lesson builds the full pull chain and turns transpiration rate into a number per unit leaf area.

Objectives — Students can: (1) explain one-way water movement from root to leaf using cohesion-tension; (2) calculate transpiration rate from a potometer and convert it per unit leaf area; (3) use the ringing and aphid-stylet evidence to separate xylem from phloem in direction and cargo.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Diffusion and osmosis	Water moves towards which solute concentration?	The higher one
Surface area to volume	Why are leaves thin and broad?	Fast gas exchange
Unit conversion	1 cm squared equals how many mm squared?	100

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Structure map	Root hair, cortex, endodermis, xylem, vein	Draw the whole route
8-22 min	Transpiration pull	Transpiration, water potential, cohesion-tension	Three causal steps
22-36 min	Potometer maths	54 mm in 4.5 min, then divide by leaf area	0.16 mm per min per cm squared
36-48 min	Xylem versus phloem	Direction, cargo, living or dead	Complete the table
48-56 min	Experiment questions	Ringing, aphid stylet, methylene blue	Predict each result
56-60 min	Exit check	Three questions collected and marked	Independent

03 Teaching script (read it out)

Scene 1: The pull: one directional chain

Mesophyll cells lose water by transpiration, their water potential falls, water enters from the leaf xylem, the xylem column is pulled up and the root takes water from the soil.
Hydrogen bonding between water molecules (cohesion) keeps the column continuous, while adhesion to the xylem wall stops it retracting.
Transpiration is therefore the power source, not just water loss: stop it and the upward flow nearly stops too.

Ask the student: what happens to xylem flow when stomata close at noon (it slows but does not stop, because root pressure remains).

Scene 2: Potometer: turn a bubble into a comparable number

The bubble moves 54 mm in 4.5 minutes, so the rate is 54 divided by 4.5, which is 12 mm per minute.
Dividing by the 75 cm squared leaf area gives 0.16 mm per minute per cm squared, which lets twigs of different sizes be compared.
For a volume rate, multiply by the bore area of 0.8 mm squared to get 9.6 mm cubed per minute.

Ask the student to compute: with a fan the bubble covers 40 mm in 2 minutes, so what is the new rate (20 mm per minute, about 1.7 times faster)?

Scene 3: Xylem and phloem: direction and cargo in one table

Xylem carries water and mineral salts upward from the root; its vessels and tracheids are dead and hollow at maturity and rely on tension, not a pump.
Phloem carries sucrose and amino acids from source to sink, upward or downward; sieve tube cells are alive and depend on companion cells for energy.
Ringing removes only phloem, so leaves still make sugar but cannot export it: the stem swells above the ring while water still rises through intact xylem.

Remind: “why do leaves wilt last after ringing” means the root is starved of sugar and fails first.

04 Question types and worked steps

Type	Example	Steps	Answer
Explain	How water reaches the top of a tall tree	Transpiration pull plus cohesion	No pump, just a tension chain
Calculate	Potometer rate	54 divided by 4.5	12 mm per minute
Calculate	Rate per leaf area	12 divided by 75	0.16 mm per min per cm squared
Experiment	Result of ringing	Phloem cut, xylem intact	Swelling above, root short of sugar

Sanity check (teach this): Transpiration rate always depends on temperature, wind, humidity and light, so there is no single fixed value; a potometer measures water uptake, which equals transpiration only at steady state. A rate per unit area must be smaller than the total rate, because dividing by area can only reduce the number.

05 Independent practice (3 questions)

Question	Answer	What to watch
Rate when the bubble moves 54 mm in 4.5 min	12 mm per minute	Did they divide by time?
Rate after dividing by 75 cm squared	0.16 mm per min per cm squared	Are the units complete?
Where sugar accumulates after ringing	Above the ring	Can they separate source from sink?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Explaining tall trees with root pressure	Only “roots push water up”	“Root pressure lifts a few metres; tall trees rely on transpiration pull.”
Forgetting to divide by leaf area	Stopping at 12 mm per minute	“To compare twigs you must divide by leaf area.”
Calling xylem cells living	Saying vessel cells are active	“Vessels are dead and hollow at maturity, which is why they transport well.”
Phloem only downward	Insisting sugar always moves down	“Source to sink: in spring it can move up from the root.”

07 Homework (paper, section and question numbers)

DSE Biology practice paper (plant transport) — Section B question 8: potometer and ringing
Secondary biology chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Rate when 54 mm is covered in 4.5 min	12 mm per minute	Three correct means move on to photosynthesis
What cohesion does	Keeps the water column unbroken	One wrong means reteach the chain
Which part starves first after ringing	The root	Two wrong means restart act one

09 Tutor FAQ

Is transpiration wasteful?

No: it provides the lifting force and cools the leaf, though excessive loss is limited by stomatal control.

Why use a leafy shoot in a potometer?

Leaves are the main transpiring organs, so a defoliated shoot gives readings too slow to compare.

Why cut the stem under water?

To stop air entering the xylem and breaking the water column with an air lock.

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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.