6.2 Transport in plants

Why this matters

Plants face a unique challenge: roots are buried in soil collecting water and minerals, leaves are high in the air collecting sunlight, but the chemistry of photosynthesis needs both. The solution is two specialised transport tissues — xylem to lift water + dissolved minerals from roots to leaves, and phloem to carry sugars (made by photosynthesis) from leaves to growing or storage parts. Water itself enters the plant at the root hair cells by osmosis, then is pulled up the xylem by transpiration: evaporation of water from the leaves through stomata. Stomata are the leaf's compromise between needing CO₂ in for photosynthesis and not wanting to lose too much water. Get the structure-fits-function logic of xylem, phloem, root hair cells, and guard cells nailed, and the Edexcel 6.2 questions all fall into place.

How to learn this topic

Build on what you already know

• Edexcel 1BI0 1: cells and movement — osmosis is the movement of water down a water potential gradient through a partially permeable membrane.
• Edexcel 1BI0 5: photosynthesis — leaves make glucose using CO₂ + water + light, and glucose is converted to sucrose for transport.
• KS3: plants have roots, stems and leaves with different jobs; gas exchange happens at the leaf.

1. Recap the two transport tissues — xylem and phloem — and what each carries.
2. Water uptake — root hair cells absorb water from soil by osmosis.
3. Transpiration stream — how evaporation at the leaves pulls water up the xylem.
4. Factors that change transpiration rate (temperature, light, wind, humidity).
5. Translocation — phloem carries sugars from source to sink and requires energy.
6. Stomata + guard cells — turgid/flaccid mechanism for opening and closing.
7. Wilting — what happens when water uptake can't keep up with transpiration.

Key terms

xylem

Plant transport tissue made of dead, hollow, lignified cells joined end-to-end to form continuous tubes. Carries water and dissolved mineral ions from roots to leaves in one direction (up). (Edexcel wants three features named: DEAD, HOLLOW, LIGNIFIED. 'Lignin strengthens walls of xylem cells' is the marking phrase.)

phloem

Plant transport tissue made of living sieve tube cells with companion cells. Carries sucrose and amino acids from source to sink in either direction (translocation). (Phloem is LIVING — that's the contrast with xylem. Companion cells supply the ATP for active loading.)

root hair cell

A specialised epidermal cell on a young root with a long thin extension (the root hair) that increases the surface area for absorption of water and dissolved mineral ions from the soil. (Two Edexcel marking phrases: 'root hair cells absorb water from soil' AND 'root hairs increase surface area for absorption'.)

transpiration

The loss of water from a plant by evaporation, mostly from the leaves through the stomata. (Examiners want 'evaporation' AND 'through stomata'. Just 'water loss' isn't enough.)

transpiration stream

The continuous column of water pulled up through the xylem from roots to leaves, driven by evaporation from the leaves. (Water is PULLED up — not pushed. Cohesion between water molecules (hydrogen bonds) keeps the column unbroken.)

translocation

The movement of sucrose and amino acids through the phloem from source (where they are made or stored) to sink (where they are used or stored). Requires energy. (Edexcel marking chain: 'via the phloem | this process is translocation | from leaves to roots | requires energy'. Hit all four phrases.)

stoma

A small hole / pore in the leaf surface (mostly on the underside) through which gases enter and leave the leaf. Surrounded by two guard cells. ('Stomata are small holes/pores in leaf surface' is the verbatim Edexcel phrase.)

guard cell

One of a pair of cells flanking a stoma. When turgid (water in), they curve outward and the stoma opens; when flaccid (water out), they straighten and the stoma closes. (Edexcel chain: guard cells become turgid (when water enters) so stomata open; guard cells become flaccid (when water leaves) so stomata close.)

turgid

Describes a plant cell that is swollen with water — its vacuole presses the cytoplasm against the cell wall, generating turgor pressure. (Turgid = stoma OPEN (in guard cells). Plant cells in general need turgor to stay rigid.)

flaccid

Describes a plant cell that has lost water — the vacuole shrinks, turgor pressure falls, and the cell becomes floppy. (Flaccid = stoma CLOSED (in guard cells). Flaccid plant cells in general → wilting.)

wilting

The drooping of plant leaves and stems when water loss by transpiration exceeds water uptake by roots and plant cells lose turgor. (Edexcel wants a chain: root hair cells can't absorb water → transpiration continues → guard cells flaccid → plant cells flaccid → leaves droop.)

source and sink

Source = a region where sugars are made or stored (e.g. photosynthesising leaves). Sink = a region where sugars are used or stored (e.g. growing roots, fruits). (Translocation moves sugars from source to sink — phloem can run in either direction depending on which is which.)

Notes

The two transport tissues

Plants have two specialised transport tissues running in vascular bundles through roots, stem and leaves:

• Xylem — DEAD, HOLLOW, LIGNIFIED cells joined end-to-end to form continuous tubes. Carries water + dissolved mineral ions from roots to leaves. Lignin strengthens the walls of xylem cells — it waterproofs them and stops the tubes collapsing as water is pulled upward under tension.
• Phloem — LIVING sieve tube cells with companion cells alongside. Carries sucrose + amino acids between any source and any sink in the plant. Companion cells provide the ATP needed for active loading of sugars into the sieve tubes.

Key contrast: xylem cells are dead because all they need to be is hollow tubes for passive water flow; phloem cells are alive because moving sugars (translocation) requires energy.

Water from soil to leaf — the path

1. Root hair cells absorb water from soil by osmosis. Water moves from the soil (higher water potential) into the root hair cell (lower water potential, because the cell contains dissolved solutes) across a partially permeable membrane.
2. Root hairs increase surface area for absorption — each root hair is a long thin extension of a single epidermal cell, and a root tip has thousands of them. More surface area = faster water uptake.
3. Water moves across the root, cell by cell, until it reaches the xylem.
4. Xylem carries water + dissolved minerals from roots to leaves as a continuous column of water under tension. The water is pulled upward — not pushed.
5. At the leaf, water leaves the xylem and enters the spongy mesophyll cells, where it evaporates into the air spaces and exits through the stomata as water vapour.

Transpiration and the transpiration stream

Transpiration is the loss of water from a plant by evaporation, mostly from the leaves through the stomata.

The transpiration stream is the continuous flow of water up through the xylem from roots to leaves that results from this evaporation. The chain:

1. Water evaporates from the surface of spongy mesophyll cells into the leaf air spaces.
2. Water vapour diffuses out through the stomata down its concentration gradient.
3. As water leaves the mesophyll, more water is pulled in from the xylem.
4. The water column in the xylem is pulled upward — water molecules cohere (hydrogen bonds) so the column doesn't break.
5. Water lost at the leaves is replaced by uptake at the root hair cells.

### Factors that change the rate of transpiration

Think of transpiration as evaporation + diffusion of water vapour. Anything that speeds up evaporation OR steepens the water-vapour concentration gradient out of the leaf will speed it up:

• Higher temperature → water evaporates faster — molecules have more kinetic energy, so more water evaporates from the mesophyll cell surfaces and diffuses out through the stomata.
• Light → light causes stomata to open — guard cells become turgid in light. More open stomata = more water vapour can escape, so transpiration rate increases.
• Wind → wind carries water vapour away from around the stomata, keeping the concentration gradient out of the leaf steep. In still air, water vapour builds up around the leaf and slows diffusion.
• Lower humidity → drier air outside the leaf steepens the gradient, so water vapour diffuses out faster.

So on a hot, sunny, windy day with low humidity, the rate of transpiration increases sharply.

Translocation — sugars in phloem

Translocation is the movement of sucrose and amino acids through the phloem from source (where they're made or stored) to sink (where they're used or stored).

Sugars are made by photosynthesis in the leaves and need to reach non-photosynthetic tissues like growing roots, growing shoot tips, and developing fruits. These tissues get sugars via the phloem — this process is translocation, sugars move from leaves to roots (or to other sinks), and the process requires energy because sucrose is actively loaded into the sieve tubes by the companion cells.

Unlike transpiration (passive, driven by evaporation), translocation depends on living cells using ATP. That's why phloem is alive and xylem is dead.

Source and sink can swap depending on the season. In summer, leaves are source and roots are sink. In spring, roots release stored sugars (now source) and growing buds are sink — so phloem can carry sugars upward as well as downward.

Stomata and guard cells

Stomata are small holes / pores in the leaf surface (mostly on the underside). Each stoma is bounded by two guard cells. The pore opens and closes because of changes in the turgidity of the guard cells:

• Guard cells open and close stomata — they're the active part of the mechanism.
• Guard cells become turgid (when water enters) so stomata open — water enters guard cells by osmosis in the light, the cells swell, the unevenly thickened walls force them to curve outward, and the pore between them opens.
• Guard cells become flaccid (when water leaves) so stomata close — when water leaves guard cells (dark or drought), they lose turgor, straighten, and the pore closes.

Open stomata let CO₂ diffuse in for photosynthesis and let water vapour and O₂ diffuse out. Closed stomata save water but stop photosynthesis. The guard cell mechanism is the plant's compromise between gas exchange and water conservation.

Wilting — when water can't keep up

If the soil is dry, root hair cells cannot absorb water (there's no water potential gradient — or the gradient is reversed). But water continues to be lost by transpiration from the leaves, because stomata don't close instantly and water vapour keeps diffusing out.

The result is a chain of consequences inside the plant:

• Guard cells become flaccid / lose turgidity as they too lose water — this closes the stomata, which slows further water loss but stops CO₂ uptake, so photosynthesis halts.
• More broadly, plant cells lose water and become flaccid / lose turgor. Plant cells normally rely on water-filled vacuoles pressing against rigid cell walls (turgor pressure) to stay rigid. Lose the water, lose the pressure, and the cell goes floppy.
• At the whole-plant level, leaves and stems can no longer support themselves and the plant droops — this is wilting.

Wilting is a recoverable state if water becomes available again. Closed stomata cut further losses, and once the soil is rewatered, root hair cells take up water by osmosis, cells regain turgor, and the plant stiffens up. But prolonged wilting damages tissues permanently.

Exam tips

• Edexcel 6.2 lives inside a small box: xylem, phloem, root hair cells, transpiration, stomata, guard cells. Learn the four marking phrases for each and you've covered most question types.
• For 'how water moves from soil to leaf', hit four phrases: root hair cells absorb water from soil | root hairs increase surface area | xylem carries water + minerals from roots to leaves | lignin strengthens xylem walls.
• For 'why does transpiration speed up on a hot/sunny/windy day', four phrases again: rate of transpiration increases | higher temperature → water evaporates faster | light causes stomata to open | wind carries water vapour away.
• For 'how sugars reach the roots', the marking chain is: via the phloem | this process is translocation | from leaves to roots | requires energy. Don't skip 'requires energy'.
• For guard cells: small holes/pores in leaf surface | guard cells open and close stomata | turgid (water in) → open | flaccid (water out) → closed. The turgid/flaccid pair is the key contrast.
• For wilting, the chain is: dry soil → root hair cells can't absorb water → transpiration continues → guard cells become flaccid → plant cells become flaccid → leaves droop. Don't write 'plant runs out of energy'.
• Xylem is DEAD, HOLLOW, LIGNIFIED. Phloem is LIVING with sieve tubes + companion cells. Don't mix these up — Edexcel asks the contrast almost every year.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in Edexcel GCSE Biology (1BI0)

• 6.1 Photosynthesis
• 6.3 Plant hormones