Xylem & Phloem
A tall gum tree must lift water nearly 100 metres from its roots to its highest leaves, and at the same time send sugar back down to feed the roots that have no light of their own. It does this with two completely different sets of pipes, running in opposite directions, side by side. This lesson dissects the plant's plumbing: dead hollow xylem pulling water up, and living phloem pushing sugar wherever it is needed.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upA plant must move two very different things: water (taken up by roots) and sugar (made in leaves).
Before reading: predict whether one tube could carry both at once, or whether the plant needs separate tubes, and give one reason.
Learning Intentions
goalsKnow
- What xylem and phloem each transport, and in which direction
- The structural features of xylem vessels and phloem sieve tubes
- That xylem cells are dead and phloem cells are living
Understand
- How each tissue's structure suits its function
- Why xylem can be hollow and dead but phloem must be alive
- Why a variety of sources improves a comparison
Can Do
- Compare the structures and functions of xylem and phloem
- Link each structural feature to its role
- Interpret a labelled diagram or micrograph of a vascular bundle
Scan these before reading
vocabMisconceptions To Fix
watch outOdd one out: Three of these statements about the plant transport tissues are correct. Which one is wrong?
Xylem, Water Going Up
Dead, hollow, lignified pipes
Core Content
Xylem, Water Going Up
+5 XPDead, hollow, lignified pipes
Xylem is structured as a series of hollow, reinforced tubes precisely because its only job is to let water flow upward with as little resistance as possible.
Xylem transports water and dissolved mineral ions in one direction, upward, from roots to leaves. Its vessels are formed from dead, hollow cells joined end to end with their end walls removed, forming continuous pipes; their walls are thickened with lignin, which waterproofs them and stops them collapsing under tension.
Because the cells are dead and empty, water meets little resistance and the lignin gives the plant mechanical support (xylem is the "wood" of a tree). Xylem flow is one-way only: water never travels back down.
Pause, copy the highlighted description of xylem structure and function into your book.
The strong, waterproof substance that thickens xylem walls and stops them collapsing is _____.
Phloem, Sugar Going Anywhere
Living sieve tubes with companion cells
Phloem, Sugar Going Anywhere
+5 XPLiving sieve tubes with companion cells
We just saw xylem as dead, one-way pipes. That raises a question: why can't phloem be built the same way? This card answers it → moving sugar needs energy, so phloem must be alive.
Phloem is built from living cells because transporting sugar against gradients and in changeable directions requires active, energy-using cells rather than empty pipes.
Phloem transports sugars (as sap) in any direction, from sources (leaves) to sinks (roots, fruit, growing tips), wherever they are needed. It is made of living sieve-tube cells joined end to end by perforated sieve plates that let sap flow through; each sieve tube is supported by an adjacent companion cell that supplies the energy and metabolism the sieve tube lacks (the sieve tube has no nucleus).
Text description
- Xylem vessel (dead): lignified walls, hollow tube; water moves up only.
- Phloem sieve tube (living): sieve-tube cells joined by perforated sieve plates, with a companion cell alongside; sap moves in either direction.
Pause, copy the highlighted phloem description and note the companion-cell role.
Xylem vessels are made of dead, hollow cells, whereas phloem sieve tubes are living.
Both xylem and phloem transport their contents in one direction only, upwards.
Companion cells supply the energy and metabolism that phloem sieve tubes lack.
Compare Xylem and Phloem from a Variety of Sources
Do the activity in your book, then check what you have understood.
Activity 1
Compare Xylem and Phloem from a Variety of Sources
Pattern A, Structured comparison (WS04)
Drawing on the diagram in this lesson plus a labelled textbook diagram and a transverse-section micrograph, build a comparison table with the rows: what is transported, direction, living or dead, key structural feature, and how structure suits function. Finish with one sentence explaining why using more than one source gives a more reliable comparison than a single diagram.
Match each structural feature to the function it serves:
- Lignified walls
- End walls removed, cells dead
- Perforated sieve plates
- Companion cells
- Supply the ATP and metabolism the sieve tube lacks
- An unobstructed pipe so water flows with little resistance
- Stop the vessel collapsing under tension, and support the plant
- Let sap pass from one living cell to the next
Structure Suits Function
Why each design fits its job
Structure Suits Function
+5 XPWhy each design fits its job
We just compared the two tissues. That raises a question: why is each built exactly the way it is? This card answers it → every feature is matched to the transport task.
Each structural feature of xylem and phloem exists because it makes that tissue better at its particular transport job.
Xylem's hollow, end-wall-free vessels give an unobstructed pipe for fast water flow; its lignin waterproofs the tube and provides support; being dead means no living contents block the path. Phloem's sieve plates let sap pass between cells while keeping the column organised; its companion cells provide the ATP needed to load sugar; being alive lets it actively control transport and reverse direction as the plant's needs change.
This structure-function matching is a theme across the whole focus area, you will see it again in arteries, alveoli and nephrons. Always pair a feature with the job it does.
Add the highlighted structure-function links to your notes before the check below.
Why is it an advantage that xylem vessels are made of dead, hollow cells?
Why ring-barking kills a tree
Where this biology is doing real work outside the classroom.
Why ring-barking kills a tree
If you cut a complete ring of bark from around a tree trunk, "ring-barking", the tree dies, but slowly and from the bottom up. The reason lies in this lesson: phloem sits just inside the bark, while xylem is deeper in the wood. Removing the ring of bark severs the phloem but leaves the xylem intact, so water still reaches the leaves but sugar can no longer travel down to the roots. The roots, cut off from their food supply, eventually starve. Foresters, orchardists and even possums exploit (or accidentally cause) this, and it is direct evidence that the two transport tissues are separate and run in opposite directions.
Compare the structures and functions of xylem and phloem in the plant transport system. Refer to at least three structural differences and link each to its function.
Compare requires similarities and differences on shared criteria. The question then adds a condition: each structural difference must be linked to its function, so a table of features with no functions attached cannot reach full marks.
6 marks. Choose three or four criteria that apply to both tissues, state each for xylem and phloem in the same breath, and attach the function to every structural claim.
Xylem transports water and dissolved mineral ions in one direction only, upward from the roots to the leaves. Phloem transports dissolved sugars as sap in either direction, from a source such as a photosynthesising leaf to a sink such as a root, fruit or growing tip. Both are transport tissues in the same plant, but they carry different substances and one is unidirectional while the other is not.
Xylem vessels are formed from dead, hollow cells. Because the cells are dead and empty there are no living contents to obstruct the path, so water meets little resistance. Phloem sieve tubes are living cells, which they must be, because moving sugar requires energy and only a living cell can supply it.
Xylem cells are joined end to end with their end walls removed, forming continuous, unobstructed pipes, and their walls are thickened with lignin. Lignin waterproofs the vessels, stops them collapsing inward under the tension of the transpiration stream, and provides the plant with mechanical support — xylem is the wood of a tree. Phloem sieve tubes are joined end to end by perforated sieve plates, which let sap pass from one living cell to the next while keeping the column of cells organised.
A xylem vessel has no associated living cell, because it needs none: transport through it is passive. Each phloem sieve tube is accompanied by a companion cell, which supplies the ATP and the metabolic machinery the sieve tube lacks, since a mature sieve-tube cell has no nucleus. Sugar is actively loaded into the sieve tube using that energy.
The two designs are opposite solutions to two different problems. Xylem needs to move a large volume of water quickly in one direction with minimal resistance, so it is built as a dead, rigid, empty pipe. Phloem needs to move sugar to wherever the plant currently needs it, at a rate and in a direction that change through the season, so it is built as a living, energy-using system that can be actively controlled.
Xylem transports water and dissolved mineral ions in one direction only, upward from roots to leaves, whereas phloem transports dissolved sugars as sap in either direction, from a source such as a photosynthesising leaf to a sink such as a root, fruit or growing tip. Structurally, xylem vessels are formed from dead, hollow cells, so there are no living contents to obstruct the path and water meets little resistance, while phloem sieve tubes are living, which they must be because moving sugar requires energy that only a living cell can supply. Xylem cells are joined end to end with their end walls removed to form continuous unobstructed pipes, and their walls are thickened with lignin, which waterproofs them, prevents them collapsing under the tension of the transpiration stream and provides mechanical support to the plant; phloem sieve tubes are joined by perforated sieve plates that let sap pass from one living cell to the next while keeping the column organised. A xylem vessel has no associated living cell because transport through it is passive, whereas each phloem sieve tube is served by a companion cell that supplies the ATP and metabolism the sieve tube lacks, having no nucleus of its own, and that energy is used to load sugar actively into the tube. The two designs are therefore opposite solutions to different problems: xylem must move a large volume of water rapidly in one direction with minimal resistance, so it is a dead, rigid, empty pipe, while phloem must deliver sugar wherever the plant currently needs it at a rate and in a direction that change with the season, so it is a living, energy-using system that can be actively controlled.
Explain why xylem can carry out its function while its cells are dead, but phloem cannot.
Explain means cause and effect. The marks are for why one transport process needs a living cell and the other does not — which means explaining what each process actually costs in energy.
4 marks. Establish what drives movement in each tissue, then show that one requires energy from the cell itself and the other does not. Anchor both in a named structure.
Movement through xylem is passive. Water is pulled up the plant by the tension created when water evaporates from the leaves, so the energy that drives the flow comes from outside the xylem cells themselves — ultimately from the sun, through evaporation. The vessel contributes nothing but a path, so it does not need to be alive; being dead and empty is an advantage, because hollow lignified vessels with their end walls removed provide an unobstructed pipe through which water flows with little resistance.
Movement through phloem is not passive at the point that matters. Sugar must be actively loaded into the sieve tubes at the source, against its concentration gradient, and active transport requires ATP. Only a living cell can respire to supply that ATP, so the tissue must contain living cells. The mature sieve-tube cell has lost its nucleus, so the ATP and metabolic machinery are supplied by the adjacent companion cell.
The difference therefore follows from where the driving energy comes from. Xylem transport is powered from outside the tissue, so its cells can die and become mere pipes; phloem transport is powered from within, so its cells must remain alive. This is also why phloem can reverse its direction of flow as the plant's sources and sinks change, while xylem cannot: a dead pipe cannot choose which way to send its contents.
Movement through xylem is passive: water is pulled upward by the tension created as water evaporates from the leaves, so the energy driving the flow comes from outside the xylem cells themselves, ultimately from the sun. The vessel supplies only a path, so it need not be alive, and being dead is positively useful — hollow, lignified vessels with their end walls removed form an unobstructed pipe through which water flows with little resistance. Movement through phloem is different, because sugar must be actively loaded into the sieve tubes at the source against its concentration gradient, and active transport requires ATP, which only a living, respiring cell can supply. Since the mature sieve-tube cell has no nucleus, that ATP and metabolic support are provided by the adjacent companion cell. The difference therefore comes down to where the driving energy originates: xylem transport is powered from outside the tissue, so its cells can die and serve as pipes, whereas phloem transport is powered from within, so its cells must stay alive. That is also why phloem can reverse its direction of flow as sources and sinks change through the season, while xylem cannot.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 13 marks
+5 XPAnalyseBand 5(5 marks) 1. Compare the structures and functions of xylem and phloem in the plant transport system. Refer to at least three structural differences and link each to its function.
1 mark: xylem transports water/minerals upward; phloem transports sugar in any direction · 1 mark: xylem dead/hollow vs phloem living · 1 mark: lignin (support/no collapse) for xylem · 1 mark: sieve plates + companion cells for phloem · 1 mark: answer expressed as a comparison linking structure to function
UnderstandBand 4(3 marks) 2. Explain why xylem can function while its cells are dead, but phloem cannot.
1 mark: xylem transport is passive, water is pulled through an empty tube, so no living contents are needed · 1 mark: phloem requires ATP to actively load sugar, which only a living cell can supply · 1 mark: links this to a named structure in each (hollow lignified vessel; companion cell)
AnalyseBand 5–6(5 marks) 3. A complete ring of bark is removed from around a tree trunk. Over the following months the trunk swells just above the cut, the leaves stay green for some time, and the roots eventually die. Analyse this result using the structures and functions of xylem and phloem, and state what it shows about the plant transport system.
1 mark: identifies that phloem sits in the bark and xylem deeper in the wood · 1 mark: explains that xylem is intact, so water still reaches the leaves and they stay green · 1 mark: explains the swelling as sugar accumulating above the severed phloem · 1 mark: explains root death as loss of the sugar supply the roots cannot make themselves · 1 mark: concludes that the two tissues are separate and carry different substances in different directions
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): Xylem transports water and dissolved minerals in one direction, upward from roots to leaves, whereas phloem transports sugars (sap) in any direction, from sources such as leaves to sinks such as roots and fruit. Structurally, xylem is made of dead, hollow cells joined end to end with their end walls removed, forming an unobstructed pipe so water flows with little resistance; its walls are thickened with lignin, which waterproofs the vessels, prevents them collapsing under tension and provides support. Phloem, in contrast, is made of living sieve-tube cells joined by perforated sieve plates that let sap pass between cells, each supported by a companion cell that supplies the ATP and metabolism the sieve tube lacks. Thus xylem can be dead and hollow because water is simply pulled through, while phloem must remain alive because moving sugar requires energy and active control, each tissue's structure is matched to its transport function.
Q2 (3 marks): Xylem transport is passive: water is pulled upward by tension from transpiration, so the tissue only has to provide an open, strong pipe. Dead cells with their contents gone and their end walls removed do that better than living ones, because there is nothing left to obstruct the flow, and the lignin in the walls keeps the vessel from collapsing under the pull. Phloem transport is active: sucrose has to be loaded into the sieve tubes against its concentration gradient, and that requires ATP, which only a living cell can make. This is why each sieve tube is paired with a living companion cell that supplies the energy and metabolism the sieve tube itself lacks. So the difference in whether the cells are alive follows directly from the difference in how each substance is moved.
Q3 (5 marks): Phloem lies just inside the bark while xylem lies deeper in the wood, so ring-barking severs the phloem and leaves the xylem intact. Because xylem is undamaged, water and dissolved minerals continue to be pulled upward from the roots to the leaves, which is why the leaves stay green and photosynthesis continues for some time. The leaves keep loading sugar into the phloem, but the sap can no longer travel past the cut, so sugar accumulates in the tissue immediately above the ring and the trunk swells there. The roots cannot photosynthesise and depend entirely on sugar translocated down the phloem; with that supply cut they exhaust their stores, can no longer respire enough to power mineral uptake, and eventually die, after which the whole tree fails. The result is direct evidence that xylem and phloem are separate tissues carrying different substances, that phloem carries sugar downward from leaves to roots, and that the upward water pathway is independent of the downward sugar pathway.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now compare xylem and phloem and link each structural feature to its function. Next lesson explains how phloem actually moves sugar, the translocation theory.
↩ Back to focus-area hubHow did your thinking change?
At the start you predicted whether one tube or two were needed. The plant needs two separate tissues: dyed water rose through the dead, hollow xylem to colour the petals, while sugar travels (often downward) in the living phloem, which would not pick up the dye. The opposite directions and different cargoes are exactly why one tube could not do both jobs.
If you thought xylem and phloem were the same kind of tube, revisit the misconception box: one is dead and hollow, the other living and active.