In simple terms
A friendly intro before the formal notes — no formulas yet.
How a plant moves water and sugar without a pump
Water gets into a root by osmosis, crosses the root along cell walls or through the cells, is forced through a membrane at the endodermis, and is then pulled up the xylem by the water evaporating from the leaves. Sugar goes the other way: companion cells use energy to load sucrose into the phloem, water follows by osmosis, and the pressure that builds up pushes the sap to wherever sugar is needed.
Think of the xylem as a drinking straw with the top in the leaf: every bit of water that evaporates at the top pulls the whole column up, and it only works because water molecules hold hands (cohesion) and grip the sides (adhesion). The phloem is more like a water pistol: load sugar at one end, water rushes in behind it, and the pressure squirts the sap out at the other end.
- 1
Water enters root hairs by osmosis, then crosses the cortex by the apoplast (cell walls) or the symplast (cytoplasm and plasmodesmata).
- 2
The Casparian strip at the endodermis blocks the apoplast, so water must pass through a membrane before it reaches the xylem.
- 3
Evaporation in the leaf creates tension; cohesion and adhesion carry that pull all the way down the xylem — the transpiration stream.
- 4
Companion cells pump out H⁺ and cotransport sucrose in; water follows by osmosis, and sap moves by mass flow from source to sink.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 7.2.1
State that some mineral ions and organic compounds can be transported within plants dissolved in water
- 7.2.2
Describe the transport of water from the soil to the xylem through the: • apoplast pathway, including reference to lignin and cellulose • symplast pathway, including reference to the endodermis, Casparian strip and suberin
- 7.2.3
Explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
- 7.2.4
Explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
- 7.2.5
Make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
- 7.2.6
State that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
- 7.2.7
Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
- 7.2.8
Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
1 more simulation for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
1 simulation
- PhET9700 7.2
Diffusion
Gas particles behind a divider: choose how many there are and how hot they are, then remove the divider.
Try this
- Put 10 particles on the left and none on the right, tick Particle Flow Rate, remove the divider and note the flow.
- Reset with 50 particles on the left and compare the flow at the start.
- Reset again at a higher temperature and compare once more.
Look for The steeper the concentration gradient and the higher the temperature, the faster the net diffusion. Water vapour leaves the air spaces of a leaf the same way, so dry, moving or warm air raises the rate of transpiration.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
At a glance — side by side
Compare key properties side by side — ideal for exam contrasts.
Transport in xylem and phloem compared
| Feature | Xylem transport | Phloem translocation |
|---|---|---|
| What is carried | Water and dissolved mineral ions | Assimilates — sucrose and amino acids — dissolved in water |
| Direction | One way, upwards from roots to leaves | From source to sink, so up or down; bidirectional overall |
| What drives it | Transpiration pull: tension created by evaporation in the leaf | Hydrostatic pressure gradient created by osmosis after active loading |
| Energy from the plant | None — the sun evaporates the water; lignin resists the tension | ATP spent by companion cells to pump H⁺ and cotransport sucrose |
| Cells involved | Dead, hollow, lignified vessels with pits | Living sieve tube elements with sieve plates, kept alive by companion cells |
| Water enters by | Osmosis into root hairs, then apoplast or symplast to the endodermis | Osmosis into the sieve tube at the source, after sucrose is loaded |
What is carried
Xylem transport
Phloem translocation
Direction
Xylem transport
Phloem translocation
What drives it
Xylem transport
Phloem translocation
Energy from the plant
Xylem transport
Phloem translocation
Cells involved
Xylem transport
Phloem translocation
Water enters by
Xylem transport
Phloem translocation
Full topic notes
Formal explanation with the rigour you need for the exam.
From soil to xylem: two pathways across the root
Water enters a root hair cell by osmosis, down a water potential gradient from the soil solution, which is more dilute than the cell sap. Root hairs give the root a huge surface area for this. Mineral ions can enter passively by diffusion or, against their gradient, by active transport — and some mineral ions and organic compounds then travel through the plant dissolved in the water. Once inside the root, water has two routes to the xylem in the centre: along the cell walls (the apoplast) or from cytoplasm to cytoplasm (the symplast).
Apoplast pathway: water moves through the continuous network of cellulose cell walls and the intercellular spaces, never crossing a membrane. Movement is by diffusion down the water potential gradient (and by mass flow when transpiration is fast), so it is quick; most water takes this route when transpiration rates are high. The walls of xylem vessels are lignified, and lignin is waterproof, so water that reaches the xylem stays in the lumen.
Symplast pathway: water moves through the cytoplasm and vacuoles of cells and from cell to cell through the plasmodesmata. Every step crosses a partially permeable membrane, so the movement is osmosis, driven by the water potential gradient that the concentrated sap of inner cells maintains. It is slower than the apoplast route.
The endodermis and the Casparian strip: each endodermal cell has a band of suberin in its radial and transverse walls — the Casparian strip — which is impermeable to water. The apoplast route is blocked here, so all water (and every dissolved ion) must cross a cell surface membrane to enter the symplast. The plant can therefore control which mineral ions reach the xylem, and the active pumping of ions into the xylem contributes to root pressure.
Suberin deposits thicken as endodermal cells age, except in certain passage cells, through which water still passes freely. From the endodermis water moves into the xylem vessels through pits in their walls.
Transpiration: evaporation, then diffusion
Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through the stomata of the leaves — around 99 % of the water a plant absorbs leaves this way. It happens in two stages. Water first evaporates from the wet cell walls of the spongy mesophyll cells into the air spaces inside the leaf; water vapour then diffuses out through open stomata to the atmosphere, down a water potential gradient. Transpiration is an unavoidable consequence of gas exchange: stomata must be open for CO₂ to diffuse in for photosynthesis, and while they are open water vapour diffuses out.
Transpiration rate rises with light intensity (stomata open) and temperature (faster evaporation and diffusion), and with wind speed, which sweeps humid air away from the leaf surface and steepens the gradient.
It falls with humidity (a smaller water potential gradient between leaf and air) and when the water supply is short, because stomata close.
Transpiration is not just a loss: the transpiration stream carries mineral ions up the plant, and evaporation cools the leaf.
Cohesion-tension: how water climbs the xylem
Nothing pumps water up a xylem vessel. The column is pulled from the top. When water evaporates from the mesophyll cells, water is drawn out of the xylem in the leaf to replace it, which lowers the hydrostatic pressure at the top of the vessel below that at the bottom. Water moves up the vessel down this pressure gradient — the transpiration pull — and the column holds together because of two properties of water that both come from hydrogen bonding.
Cohesion: water molecules cling to one another by hydrogen bonds. The tension created at the top of the xylem is transmitted all the way down the column, so the whole column moves up as one — the continuous transpiration stream.
Adhesion: water molecules are also attracted to the cellulose in the walls of the xylem vessels. Adhesion stops the column pulling away from the walls, and helps the water creep up narrow vessels.
The result is the cohesion-tension theory: evaporation in the leaf creates tension, cohesion transmits it, adhesion keeps the column attached, and water is pulled up from the roots to replace what the leaf has lost. Xylem walls are strengthened with lignin so the vessels do not collapse under this tension.
Xerophytes: leaves built to keep water
Xerophytes (from the Greek xero, dry) are plants adapted to dry or windy habitats — marram grass on sand dunes, cacti, pines. Their leaves show structural (xeromorphic) and physiological adaptations that reduce transpiration, and the syllabus expects you to draw and annotate a transverse section of such a leaf and link each feature to a reduced water potential gradient or a smaller surface for evaporation.
Thick waxy cuticle on the epidermis, so little water is lost through the cells themselves (Pinus, Opuntia).
Sunken stomata in pits or crypts, and fine hairs (trichomes) on the leaf surface, trap a layer of moist air next to the stomata. This reduces the water potential gradient between the leaf and the air, so diffusion of water vapour is slower (Pinus, Nerium).
Rolled leaves: in marram grass (Ammophila arenaria) hinge cells lose water and shrink when the plant is short of water, rolling the leaf so the stomata — found only on the upper epidermis — open into a humid space inside the roll, while the thick cuticle of the lower epidermis faces the wind.
Reduced leaf area — leaves reduced to spines or needles, or curled and folded — gives less surface for evaporation; fewer stomata means fewer pores.
Succulent leaves or stems store water for dry periods; CAM plants (pineapple, agave) open their stomata at night and fix CO₂ then, so the stomata stay closed in the heat of the day.
For an annotated drawing, each label must do two things: name the feature and state its effect on water loss — "sunken stomata: trap humid air, reduce the water potential gradient, so less diffusion of water vapour". A feature on its own scores nothing; "reduces transpiration" on its own scores nothing.
Phloem: sources, sinks and loading sucrose
Assimilates — sucrose and amino acids made by the plant — move in the sieve tubes of the phloem, dissolved in water, from a source to a sink. A source is any part that releases sucrose into the phloem: photosynthesising leaves and green stems (glucose is converted to sucrose for transport because sucrose has less osmotic effect and is less reactive), storage organs such as tubers unloading their stores at the start of a growth period, and germinating seeds. A sink is any part that takes sucrose up: dividing meristems, growing roots absorbing ions, and developing seeds, fruits and storage organs. Because sinks lie above and below the sources, translocation in the phloem is bidirectional overall, though in any one sieve tube it runs one way.
Loading is active. A companion cell (a modified transfer cell beside each sieve tube element) uses ATP to pump hydrogen ions out of its cytoplasm by proton pumps in its cell surface membrane. This builds up an excess of H⁺ in the cell wall (apoplast) outside the cell.
H⁺ ions then diffuse back into the companion cell down their concentration gradient through a cotransporter protein, which carries a sucrose molecule in with each H⁺ — so sucrose enters the companion cell against its concentration gradient. This is secondary active transport.
Sucrose moves from the companion cell into the sieve tube element through the plasmodesmata that link them.
Companion cells are built for the job: infoldings of the cell surface membrane increase the surface area for the proton pumps, and many mitochondria supply the ATP.
Mass flow down a hydrostatic pressure gradient
Once sucrose is in the sieve tube, a concentration gradient is no longer what moves it. Sap moves by mass flow: the whole solution is pushed along the tube by a difference in hydrostatic pressure between the source end and the sink end. That pressure difference is created by osmosis, and it is sustained for as long as the source keeps loading and the sink keeps unloading.
At the source: loading sucrose lowers the water potential inside the sieve tube, so water enters by osmosis from the surrounding tissue (and from the xylem). The extra volume raises the hydrostatic pressure at the source end.
At the sink: sucrose is unloaded — used in respiration, or converted to starch for storage — so the water potential in the sieve tube rises, water leaves by osmosis and the hydrostatic pressure falls.
Flow: sap moves down the hydrostatic pressure gradient from source to sink, carrying all the dissolved assimilates with it. Water returns to the xylem at the sink, which is why phloem transport is often drawn as a loop alongside the xylem.
Sieve plates with their pores, and the living, largely empty sieve tube elements, offer little resistance to this flow; the companion cells keep the sieve tubes alive.
Two words decide most 7.2 marks: osmosis for every movement of water across a membrane (root hair, symplast, endodermis, into and out of sieve tubes) and mass flow for movement along a tube down a pressure gradient (xylem and phloem). Never write that sucrose "diffuses" along the phloem, and never say water is "sucked" up the xylem — it is pulled by the tension that evaporation creates.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Describe how water moves from the soil, across the root, to the xylem. [5 marks]
- 1
Uptake: water enters the root hair cell by osmosis, down a water potential gradient, because the soil solution has a higher (less negative) water potential than the cell sap. Root hairs provide a large surface area for this. [1]
Explain how the properties of water allow it to be moved up the xylem from the roots to the leaves. [4 marks]
- 1
Evaporation creates tension: water evaporates from the walls of mesophyll cells and diffuses out through the stomata, so water is drawn out of the xylem in the leaf and the hydrostatic pressure at the top of the vessel falls below that at the bottom — the transpiration pull. [1]
Explain how sucrose is loaded into phloem sieve tubes at a source, and how it is then transported to a sink. [6 marks]
- 1
Proton pumps: companion cells use ATP to actively transport hydrogen ions out of their cytoplasm into the cell wall, building a high concentration of H⁺ outside the cell. [1]
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- active transport
And some mineral ions and organic compounds then travel through the plant dissolved in the water.
- Apoplast pathway
Water moves through the continuous network of cellulose cell walls and the intercellular spaces, never crossing a membrane.
- Symplast pathway
Water moves through the cytoplasm and vacuoles of cells and from cell to cell through the plasmodesmata.
- Transpiration
Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through the stomata of the leaves — around 99 % of the water a plant absorbs leaves this way.
- transpiration pull
And the column holds together because of two properties of water that both come from hydrogen bonding.
- Cohesion
Water molecules cling to one another by hydrogen bonds. The tension created at the top of the xylem is transmitted all the way down the column, so the whole column moves up as one — the continuous transpiration stream.
- Adhesion
Water molecules are also attracted to the cellulose in the walls of the xylem vessels.
- cohesion-tension theory
Evaporation in the leaf creates tension, cohesion transmits it, adhesion keeps the column attached, and water is pulled up from the roots to replace what the leaf has lost.
- Xerophytes
Xerophytes (from the Greek xero, dry) are plants adapted to dry or windy habitats — marram grass on sand dunes, cacti, pines.
- Rolled leaves
In marram grass (Ammophila arenaria) hinge cells lose water and shrink when the plant is short of water, rolling the leaf so the stomata — found only on the upper epidermis — open into a humid space inside the roll, while the thick cuticle of the lower epidermis faces the wind.
- Reduced leaf area
Leaves reduced to spines or needles, or curled and folded — gives less surface for evaporation; fewer stomata means fewer pores.
- Assimilates
Sucrose and amino acids made by the plant — move in the sieve tubes of the phloem, dissolved in water, from a source to a sink.
- Mass flow in the phloem
Movement of the whole sap solution along a sieve tube down a hydrostatic pressure gradient, from a source (high pressure, water entering by osmosis) to a sink (low pressure, water leaving).
Sort it out
One statement at a time, say which heading it belongs under. Every statement comes from this lesson’s side-by-side comparison.
None — the sun evaporates the water; lignin resists the tension
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Apoplast pathway: water moves through the continuous network of cellulose cell walls and the intercellular spaces, never crossing a membrane. Movement is by diffusion down the water potential gradient (and by mass flow when transpiration is fast), so it is quick; most water takes this route when transpiration rates are high. The walls of xylem vessels are lignified, and lignin is waterproof, so water that reaches the xylem stays in the lumen.
Symplast pathway: water moves through the cytoplasm and vacuoles of cells and from cell to cell through the plasmodesmata. Every step crosses a partially permeable membrane, so the movement is osmosis, driven by the water potential gradient that the concentrated sap of inner cells maintains. It is slower than the apoplast route.
The endodermis and the Casparian strip: each endodermal cell has a band of suberin in its radial and transverse walls — the Casparian strip — which is impermeable to water. The apoplast route is blocked here, so all water (and every dissolved ion) must cross a cell surface membrane to enter the symplast. The plant can therefore control which mineral ions reach the xylem, and the active pumping of ions into the xylem contributes to root pressure.
Suberin deposits thicken as endodermal cells age, except in certain passage cells, through which water still passes freely. From the endodermis water moves into the xylem vessels through pits in their walls.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Describe the differences in the rates of nitrate uptake for 5.0 mmol dm⁻³ nitrate solution compared with 0.2 mmol dm⁻³ nitrate solution, between 2h and 5h
In the symplast pathway, water passes through the cells of the different tissues in the root before entering the xylem vessels of xylem tissue. Name the tissues of the root, in the correct sequence, through which water passes in the symplast pathway.
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9700/22 · Q4(d) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
Discuss Transport mechanisms
Ask, share and discuss with other Biology students