DSE Biology | Transport in Plants Lesson Prep (for tutors) | Water potential, transpiration pull, pressure flow

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students can recite that roots absorb water and leaves transpire, yet freeze when asked which way water potential points or why sugar gathers above a ring of removed bark. This lesson separates the three mechanisms: water potential gradient, transpiration pull, and pressure flow, with one calculation after each.

Objectives — Students can: (1) use water potential to explain water entering root hairs and rising in xylem; (2) use transpiration pull and cohesion to explain why the water column does not break; (3) use pressure flow to explain translocation from source to sink, and why sugar accumulates above a ring of removed bark.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Diffusion and osmosis	Which way does water move?	From higher potential, nearer zero, to lower
Cell structure	Which cell type is dead at maturity with only walls left?	Xylem vessel
Leaf structure	Which two cells surround a stoma?	Two guard cells

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three-question check	Osmosis, xylem, stomata	Answer orally
8-22 min	Water uptake	Water potential gradient and active ion uptake	Calculate the difference
22-38 min	Rise in xylem	Transpiration pull, cohesion, root pressure	Explain the unbroken column
38-52 min	Sugar transport	Pressure flow, source and sink	Draw a source-sink sketch
52-57 min	Timed practice	Transpiration rate and stomatal counts	Work alone
57-60 min	Exit check	Three checkpoints	Self-mark and circle slips

03 Teaching script (read it out)

Scene 1: Uptake: everything starts with a water potential difference

Water moves from higher potential, nearer zero, to lower, more negative. Soil sits near -0.05 MPa and a root hair cell near -0.45 MPa, a difference of 0.40 MPa, so water enters by osmosis. The root hair keeps its potential low by active transport of mineral ions inward: more solute means lower potential, and that step costs energy. Waterlogged, airless soil therefore slows uptake not through lack of water but because active transport stops.

Require both the direction and the reason; either one alone loses the mark.

Scene 2: Rise: the transpiration pull is a stretched water column

Stomata in the leaf transpire, mesophyll potential falls, water is drawn from the xylem, and a pull runs down the column. Hydrogen bonds between water molecules give cohesion, adhesion to the walls adds support, so the column survives considerable tension. Xylem vessels are dead, hollow and lignified at maturity, ideal as pipes. If a leaf has 20 square millimetres of surface with 300 stomata per square millimetre, it has 6,000 stomata; that transpiring area drives the pull and is the first thing closed when water runs short.

Three words must come out of the student: transpiration, cohesion, continuous column.

Scene 3: Sugar transport: pressure flow is not a by-product of transpiration

Phloem runs the other way, from source in the leaf to sink in root, fruit or growing bud. Companion cells load sucrose into sieve tubes by active transport, lowering their water potential; water follows from the nearby xylem, raising the pressure inside. At the sink sugar is unloaded, pressure falls, and the whole tube flows from high to low pressure: that is pressure flow. Measured in a capillary of radius 0.5 mm where the column moves 15 mm in 5 minutes, the volume is 11.78 cubic millimetres, the rate is 2.36 per minute, which is 141.4 per hour. Removing a ring of bark removes the phloem, so sugar cannot travel down, it gathers above the cut, and the roots below eventually starve.

Draw one tube, label two pressures, state the direction, and the two systems stop being confused.

04 Question types and worked steps

Type	Example	Steps	Answer
Water potential	Soil -0.05, root -0.45 MPa	Subtract the two, direction from high to low	0.40 MPa into the root
Transpiration rate	Radius 0.5 mm, 15 mm in 5 minutes	Volume is pi r squared h, divide by time	2.36 per minute
Stomatal count	300 per square millimetre over 20	Multiply	6,000 stomata
Ringed bark	A complete ring removed	Phloem cut, sugar above, roots starved	Swelling above, then death

Sanity check (teach this): Check magnitudes: a root hair is a tube 0.01 mm across and 0.5 mm long, so its own surface area is only about 0.0157 square millimetres. It is small, but there are enormous numbers of them, which is why the total surface is large; never answer simply that one hair has a big surface. Water potential is zero or negative, zero being pure water, so -0.05 is higher than -0.45. A rate of 2.36 cubic millimetres per minute looks small but reaches 141.4 in an hour; a student reporting hundreds per minute has used diameter for radius.

05 Independent practice (3 questions)

Question	Answer	What to watch
Soil -0.05 MPa, root hair -0.45 MPa: find the difference and direction	0.40 MPa into the root	Is the direction stated
Radius 0.5 mm, 30 mm in 10 minutes: find the rate	2.36 per minute	Is pi r squared used rather than pi d squared
Explain why roots die a month after ringing	Phloem cut, sugar cannot reach the root	Are source and sink named

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping xylem and phloem directions	Xylem carries sugar down to the root	Xylem moves water and minerals upward only; sugar travels in phloem, either way.
Treating transpiration as active pumping	Transpiration uses cell energy to pull water up	The pull is passive; the energy is spent on ion uptake in the root.
Getting the sign of water potential wrong	Calling -0.45 higher than -0.05	Zero is pure water; the more negative value is lower, so water moves to -0.45.
Stopping at the bark being removed	No mention of sugar gathering or roots starving	Say the phloem is cut, sugar stays above, and the root has no supply.

07 Homework (paper, section and question numbers)

Biology subject index — All chapters
Biology practice paper — Biology section: transpiration and transport

08 Exit check (last 5 minutes)

Question	Answer	Decision
Water potential	Which way water enters	Cannot say: reteach act one
Unbroken column	Name two forces	Only transpiration: add cohesion
Ringed bark	Why sugar gathers	Cannot say: back to act three

09 Tutor FAQ

Does root pressure matter?

Transpiration pull is usually far larger, but at night when transpiration is weak, root pressure explains guttation.

Why close stomata when water is short?

Guard cells lose water and go flaccid, closing the pore to cut transpiration, at the cost of less carbon dioxide and slower photosynthesis.

Are xylem and phloem cells both dead?

Xylem is dead at maturity; sieve tube cells stay alive but lose most organelles and depend on companion cells for energy.

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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.