DSE Biology | Plant Transport Tutor Prep Guide | Xylem, Phloem and Transpiration

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Most students can recite that xylem carries water and phloem carries sugar, but ask why water goes up and they answer that the plant sucks it, mixing active pumping with a passive pull. This lesson follows one passive chain: root pressure, cohesion, and transpiration at the leaf.

Objectives — Students can: (1) distinguish xylem and phloem by structure, cargo and direction; (2) explain the rise of water with the transpiration pull; (3) compute a transpiration rate from potometer readings and compare conditions; (4) use girdling results to place the two pipe systems.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Cells and tissues	xylem dead, phloem alive	structure follows function
Diffusion and osmosis	water moves to lower water potential	root uptake is osmotic
Photosynthesis	gas exchange at the leaf	stomata also release water vapour

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Two pipe systems	draw a plant with two arrows	xylem upwards
8-20 min	Transpiration pull	leaf loss creates tension	a passive lift
20-32 min	Rate calculation	distance over time	2.6 mm/min
32-44 min	Factors	light, humidity, wind, heat	six leaves are 2.2 times three
44-54 min	Girdling	bark removed	swelling above the cut
54-60 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: First move: one plant, two lines, each labelled with cargo and direction

Xylem sits on the inside of the vascular bundle, is dead, hollow and lignified, carries water and mineral salts, and moves only upward. Phloem sits outside it, is alive, has sieve plates, carries sucrose and amino acids, and moves either way from source to sink.
Ask for two colours and a named cargo beside each arrow, never the vague word nutrients. Ask the student: why does dead xylem tissue help transport? Student task: mark on the drawing which pipe a girdling cut severs.

Scene 2: The transpiration pull: water is pulled, not pumped

Water evaporates from the moist mesophyll surfaces and leaves through the stomata, lowering water potential in the leaf, which draws water from the xylem in the veins. Cohesion between water molecules holds a continuous column, so the whole chain is pulled up from the root. Root pressure contributes only a little, and tall trees depend on the pull.
In numbers: 4.8 cm3 taken up in 20 minutes is 0.24 cm3 per minute. Ask the student: with no light at night, does water still rise? Student task: predict the bubble speed with a fan blowing across the shoot.

Scene 3: Comparing rates: read the potometer, calculate first, then explain

Three leaves: 12 mm in 10 minutes is 1.2 mm/min. Six leaves: 26 mm in 10 minutes is 2.6 mm/min, a factor of 2.17. More leaf area means more stomata, so faster transpiration.
Remember four factors: light opens stomata, humidity sets the diffusion gradient, wind removes the humid layer, and heat speeds evaporation. Ask the student: why are cactus leaves reduced to spines? Student task: if the six-leaf shoot moves only 9 mm in ten minutes in shade, find the new rate.

04 Question types and worked steps

Type	Example	Steps	Answer
Rate calculation	distance and time given	distance over time	2.6 mm/min
Explaining a factor	why wind speeds transpiration	steeper diffusion gradient	humid air removed
Girdling	bark removed	phloem cut	sugar piles up above the cut
Pipe identification	micrograph given	check cell status and position	xylem lies inside

Sanity check (teach this): A rate is always positive, and six leaves must exceed three (2.6 against 1.2); the reverse means the bubble direction was misread. Units must match: mm/min and cm3/min cannot be compared directly. Xylem transport is upward only, so two-way movement describes phloem. Swelling after girdling appears above the cut because sugar moving down is blocked. An explanation must mention water potential or a diffusion gradient, not just heat.

05 Independent practice (3 questions)

Question	Answer	What to watch
Rate when a six-leaf shoot moves 26 mm in 10 minutes	2.6 mm/min	distance over time
Rate when 4.8 cm3 is taken up in 20 minutes	0.24 cm3/min	volume over time
How many times faster six leaves are than three	2.2 times	divide the two rates

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying the plant has a heart that pumps	active pumping	Water is pulled by transpiration, not pumped.
Saying xylem carries sugar	two-way flow	Xylem carries water and salts, and only upward.
Saying transpiration stops at night	zero at night	It slows greatly but root pressure still pushes.
Mixing the rate units	mm/min treated as cm3/min	They measure different quantities and cannot be compared.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — questions 4 and 9 of the same type
Prep guide home — other chapters in the subject
DSE Biology practice papers — plant set, questions 1 to 4

08 Exit check (last 5 minutes)

Question	Answer	Decision
What xylem and phloem carry	water and sugar	three correct means move on to factors
Potometer rate with six leaves	2.6 mm/min	one wrong means reteach next lesson
Where swelling appears after girdling	above the cut	two or more wrong means back to act one

09 Tutor FAQ

Why does the water column not break?

Hydrogen bonding gives cohesion, and adhesion to the narrow lignified walls lets the column bear tension.

How is phloem direction decided?

Sugar flows from source leaves to sinks such as roots, fruit and growing tips, so both directions can run at once.

Why close stomata at night?

The pull is weak in darkness, so closing them limits pointless water loss, at the cost of carbon dioxide entry.

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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.