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IGCSE Biology 0610 · Chapter 7 notes · ~11 min read

Transport in plants

Xylem versus phloem, how a root hair cell takes in water, the transpiration stream, what speeds it up, potometer calculations, wilting and sources and sinks — every diagram you need.

Xylem & phloemWater uptakeTranspirationFactorsTranslocation
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Xylem carries water and mineral ions up from the roots and helps to support the plant. Phloem carries sucrose and amino acids (translocation) from sources to sinks.
  • Water path: soil → root hair cell → root cortex → xylem → mesophyll cells → evaporates → vapour leaves through stomata. Losing water vapour like this is transpiration.
  • Transpiration is faster when it is hot, windy, dry (low humidity) or bright. If water is lost faster than it is taken up, the plant wilts.

By the end you can

  • State the functions of xylem and phloem and find them in a root, stem and leaf.
  • Describe a root hair cell and say how its shape helps it absorb water and ions.
  • Describe the path taken by water through a plant and how to show it with a stain.
  • Define transpiration and describe how temperature, wind speed, light and humidity affect it.
  • Describe how a potometer is used and calculate a rate of water uptake.
  • Explain transpiration pull and wilting. EXT
  • Define translocation, source and sink. EXT

1Xylem and phloem

Two transport tissues

ROOTxylem in the centre(star-shaped), phloembetween the armsSTEMbundles in a ring:xylem on the inside,phloem outsideLEAF VEINxylem on the upperside, phloem on thelower sidexylemphloem
Where the two tissues sit in a root, a stem and a leaf (dicotyledonous plant).
XylemPhloem
carrieswater and mineral ionssucrose and amino acids
directionupwards only: roots → shoots and leavesup and down: from sources to sinks
cellsdead, hollow, no cytoplasmliving sieve tubes with companion cells
extra jobsupport: lignin makes it strongnone
Memory trickXylem = Xtra water going up. Phloem = Products of Photosynthesis (sugar) going everywhere.
  • In the root, xylem is in the centre, so the root can resist being pulled. In the stem, xylem and phloem are in bundles in a ring. In the leaf, the vein has xylem on the upper side and phloem on the lower side.

Built for the jobEXTENDED

XYLEM VESSELrings of lignin: strong,waterproof, give supportdead, hollow tube (no end walls)carries WATER andMINERAL IONS upPHLOEMsieve tube: living, holes inthe end walls (sieve plates)companion cell keeps it alivecarries SUCROSE andAMINO ACIDS up and downcompanioncellsieveplate
EXT The structure of each tissue matches what it does.
  • EXT Xylem vessels are long hollow tubes with no end walls and no cell contents, so water flows freely. Lignin in the walls is strong and waterproof: it supports the plant and stops the tubes collapsing under suction.
  • EXT Phloem sieve tubes have holes (sieve plates) in the end walls so sap can pass. Each is kept alive by a companion cell that makes the energy it needs.
Common mistake"Phloem carries food up and xylem carries it down." Both are wrong. Xylem carries water and only goes up. Phloem carries sugar and goes both ways.

2Taking in water

The root hair cell

thin cell wallvacuole: sugarand salt solutionnucleusmitochondria(release energy)LONG THIN HAIR= large surface areafor absorbing waterand mineral ions→ water: osmosis→ ions: active transport
A root hair cell: a long thin “hair” pushes between soil particles.
  • Root hair cells are on the surface of roots. They absorb water and mineral ions from the soil.
  • The hair gives a large surface area, and its wall is very thin, so water gets in quickly.
  • Water enters by osmosis, because the cell sap (sugars and salts) is more concentrated than the soil water. The cell has a lower water potential, so water moves in.
  • EXT Mineral ions such as nitrate are taken in by active transport, using energy from mitochondria, because the soil often has fewer ions than the cell.

The path of water

1234561Root hair cells take in waterby osmosis2Water crosses the cortexinto the xylem3Transpiration pull draws itup the xylem4Water leaves the xylem andenters the mesophyll cells5Water evaporates from thesurface of the mesophyll cells6Vapour diffuses out throughthe stomata
Follow the numbers from soil to air.
  • Root hair cell → root cortex cells → xylem → mesophyll cells in the leaf.
Memory trickWater's route in four words: Hair → Cortex → Xylem → Mesophyll.
  • Water moves across the cortex from cell to cell by osmosis, then enters the xylem in the centre of the root.
  • In the leaf, water leaves the xylem, moves into the mesophyll cells, and evaporates from their wet surfaces into the air spaces.

Show the path with a stain

stalk in red dyeovernightveins in leavesstain redcross-section of the stalkred dots = stained xylem(this is where the water travels)xylemvessels
Stand a stalk (or a white flower) in coloured water. The dye shows where the water travels.
  • Method: stand a celery stalk (or a white carnation) in water with red food dye for several hours. Cut across the stalk, and also cut a thin slice lengthways. Look at it with a hand lens or microscope.
  • Result: the dye is only in the xylem: red dots in a ring in the cross-section, red lines along the stalk and in the leaf veins.
  • Conclusion: water moves up the xylem, not through the whole stem.
Exam tipUse a fresh stalk and leave it in the light and warmth. More transpiration pulls the dye up faster, so the result shows sooner.

3Transpiration

What is transpiration?

Transpiration — the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of the vapour through the stomata.

lightwaxy cuticleupper epidermispalisade mesophyll(many chloroplasts)spongy mesophyll(air spaces)xylem + phloem(vein)lower epidermisguard cellstomaair spaces let CO₂ diffuse to the cells; stomata control gas exchange
Water evaporates from the wet surfaces of the mesophyll cells into the air spaces, then vapour diffuses out through the stomata.
  • Two steps: (1) evaporation inside the leaf, (2) diffusion out of the stomata. Both words appear in the definition, so write both.
  • Why it matters: the moving water carries mineral ions up to the leaves, supplies water for photosynthesis, keeps cells turgid and cools the leaf as water evaporates.
  • EXT The stomata are surrounded by guard cells that open and close them. They are usually open in the day (so CO₂ can enter) and closed at night.
OPEN (daytime)guard cells TURGID(full of water)bend apart: the pore opensCO₂ diffuses in,water vapour diffuses outCLOSED (night / dry)guard cells FLACCID(lost water)straighten: the pore closesvery little water vapourescapesthick innerwallporechloroplastGuard cells are the only epidermal cells with chloroplasts
EXT Turgid guard cells bend apart and open the stoma; flaccid guard cells straighten and close it.

Transpiration pullEXTENDED

drinking through a strawsuck at the top:liquid rises up the straw≈water moving up the xylemwater evaporatesfrom the leafwater molecules sticktogether: an unbrokencolumn of watertranspiration pull: evaporationat the top pulls the water up
EXT Evaporation at the top acts like suction on a straw. The water column does not break because water molecules stick together.
  • EXT Water evaporating from the mesophyll cells pulls more water out of the xylem. This suction (transpiration pull) draws water up from the roots as a continuous unbroken column, because water molecules stick to each other (cohesion).
  • EXT The plant does not need energy to lift the water. The energy comes from the Sun, which drives evaporation.

What speeds it up?

LIGHT ↑stomata open widerso more water vapourescapesrate of transpiration ↑TEMPERATURE ↑water evaporates fasterand vapour diffusesout fasterrate of transpiration ↑HUMIDITY ↓ (drier air)drier airsteeper concentrationgradient: vapour leavesthe leaf fasterrate of transpiration ↑WIND SPEED ↑blows away the humid airnext to the leaf, keepingthe gradient steeprate of transpiration ↑
Four conditions that increase the rate of transpiration.
ConditionEffect on rateWhy
higher temperaturefasterwater evaporates faster and vapour diffuses faster
higher wind speedfasterwind blows away the humid air at the leaf surface, so the concentration gradient stays steep
EXT brighter lightfasterguard cells open the stomata for photosynthesis, so more vapour escapes
EXT lower humidity (drier air)fastera steeper concentration gradient of water vapour between the air spaces and the air
  • Opposites slow it down: cool, still, humid, dark.
Common mistake"High humidity makes the plant take up more water." Humid air reduces the concentration gradient, so transpiration slows.

4Measuring uptake

The potometer

01020bubble moves towards the shootleafy shoot,cut under waterair-tight sealcapillarytubescale in mmwater reservoirrate of water uptake = distance moved by bubble ÷ time
A potometer measures how fast a cut shoot takes up water. The bubble moves as the shoot takes up water.
  • Set up: cut the shoot under water (so no air enters the xylem), fit it air-tight, and fill the tube with water. Introduce one air bubble and start timing.
  • Measure: the distance the bubble moves in a set time. Rate = distance ÷ time.
  • Test a factor: change one thing (a fan for wind, a lamp for light, a water bath for temperature) and keep the rest the same. Repeat and find a mean.
  • Limitation: it measures water uptake, not transpiration itself. Most of the water taken up is lost by transpiration, so it is a good estimate.
Worked example

In a potometer the bubble moves 24 mm in 6 minutes in still air and 45 mm in 6 minutes with a fan on. Find each rate and the percentage increase.

  1. still air: 24 ÷ 6 = 4.0 mm/min
  2. fan: 45 ÷ 6 = 7.5 mm/min
  3. increase = (7.5 − 4.0) ÷ 4.0 × 100 = 87.5%
  4. the wind removed humid air around the leaf, keeping a steeper concentration gradient

Too much water lost: wiltingEXTENDED

TURGID CELLvacuole full of water,pushing on the cell wallFLACCID CELLlittle water: membrane pullsaway from the wallturgid cells hold leaves and stems up → flaccid cells = wilting(red arrows = turgor pressure; wilting = water lost faster than roots can replace it)
EXT A turgid cell is firm; a flaccid cell is limp. When many cells go flaccid the plant wilts.
  • EXT If the plant loses water faster than the roots can replace it, the cells lose water, their vacuoles shrink and they become flaccid. The plant wilts.
  • EXT Wilting has a benefit: leaves droop, less surface is in the Sun, and the stomata close, so less water is lost. Watering restores turgor.

5Moving sugar around

Translocation

SOURCE (S): where sucrose is made or releasedSINK (K): where sucrose is used or storedSKKKleaves (photosynthesis)are a SOURCEfruit: a SINK(stores sugar)blue arrow = water inthe XYLEM (up only)red arrows = sucrose and aminoacids in the PHLOEM (source tosink: up or down)storage organ (tuber): SINKgrowing roots: SINK
Sucrose and amino acids travel in the phloem from sources to sinks.

Translocation — the movement of sucrose and amino acids in the phloem, from sources to sinks.

Source — a part of the plant that releases sucrose or amino acids, for example leaves (photosynthesis).

Sink — a part of the plant that uses or stores sucrose or amino acids, for example roots, fruits, growing shoot tips and storage organs.

  • EXT A part can be a source at one time and a sink at another. A potato tuber is a sink in summer (it stores starch made from sucrose) but a source in spring (stored starch is changed back to sucrose to feed the new shoots).
Exam tipSource → sink can be up or down. Translocation depends on where sugar is needed, not on gravity.

6Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the tissue that carries water up a plant.
  2. What do phloem tubes carry?
  3. Which cells absorb water from the soil?
  4. How does water get into a root hair cell?
  5. Define transpiration.
  6. Give two conditions that speed up transpiration.
  7. Name the structure that measures water uptake of a shoot.
  8. EXT What substance makes xylem walls strong and waterproof?
  9. EXT Why does a plant wilt?
  10. EXT Name a part of a plant that is a sink.

Exam-style

  1. Describe how you would show that water travels in the xylem. [3]
  2. Explain why transpiration is faster on a windy day. [3]
  3. EXT Explain how the structure of a xylem vessel helps it carry water. [3]
  4. EXT Explain why a potato tuber is a sink in summer and a source in spring. [3]
Show answers
  1. xylem
  2. sucrose and amino acids
  3. root hair cells
  4. by osmosis
  5. loss of water vapour from leaves by evaporation at the mesophyll cell surfaces, then diffusion through the stomata
  6. high temperature, wind (or low humidity, bright light)
  7. potometer
  8. lignin
  9. it loses water faster than it takes it up, so cells lose turgor
  10. roots, fruits, growing shoot tips or storage organs
  11. stand a celery stalk or white flower in coloured water
    leave for some hours
    cut across and see the dye in the ring of xylem
  12. wind blows away humid air around the leaf
    concentration gradient of water vapour stays steep
    so vapour diffuses out faster
  13. hollow with no end walls and no cytoplasm, so water flows freely
    lignin makes the walls strong
    waterproof so water does not leak out
  14. in summer it stores sucrose as starch
    in spring starch is changed to sucrose
    sucrose is sent to the growing shoots
Practise this chapter
Xylem & phloemWater uptake & transpirationTranslocation
Next chapterTransport in animals →
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