Transport in Plants
The structure and function of xylem and phloem; the pathway of water from root hair to leaf; transpiration and the factors that affect its rate; Extended cohesion-tension mechanism and wilting; and Extended translocation of sucrose and amino acids between sources and sinks.
Xylem and Phloem
CORE EXTENDEDPlants have two separate vascular tissues running throughout their roots, stems, and leaves. Xylem and phloem together form the vascular bundles — the plant’s transport network. Knowing what each transports, and where it sits in cross-sections, is essential for exam questions at both Core and Extended levels.
Functions
| Tissue | Transports | Direction | Additional role |
|---|---|---|---|
| Xylem | Water and mineral ions | Upward only (roots → stems → leaves) | Structural support for the plant |
| Phloem | Sucrose and amino acids | Both directions (up and down from source to sink) | — |
Position in Cross-sections
| Plant part | Xylem position | Phloem position |
|---|---|---|
| Root | Central star-shaped core (X-shaped in transverse section) | Between the arms of the xylem star |
| Stem | Inner side of vascular bundle (closer to centre) | Outer side of vascular bundle (closer to epidermis) |
| Leaf | Upper side of midrib / vascular bundle | Lower side of midrib / vascular bundle |
Xylem Vessel Structure — Extended
Xylem vessels are dead, hollow tubes — their structure is perfectly suited to transporting water over long distances with minimal resistance.
| Structural feature | How it relates to function |
|---|---|
| Thick walls impregnated with lignin | Lignin is a hard, waterproof material that strengthens the cell wall, preventing vessels from collapsing under the suction of transpiration pull; also provides structural support to the plant |
| No living cell contents | Dead cells with no nucleus, cytoplasm, or organelles — nothing obstructs water flow through the tube |
| Cells joined end to end with no cross walls | Forms one long, uninterrupted tube from roots to leaves — water flows freely without barriers |
Unlike phloem (which is alive and actively moves sugars), xylem vessels are completely dead — this is essential for their function. Living cells would have organelles, membranes and a nucleus that would block water flow. The dead, hollow tubes act like plumbing pipes, offering minimum resistance to upward water movement.
Which row correctly states the substances transported by xylem and phloem?
| Xylem transports | Phloem transports | |
|---|---|---|
| A | Sucrose and amino acids | Water and mineral ions |
| B | Water and glucose | Sucrose and mineral ions |
| C | Water and mineral ions | Sucrose and amino acids |
| D | Amino acids and water | Glucose and mineral ions |
Water Uptake
CORERoot Hair Cells
Water enters the plant through root hair cells — specialised cells on the outer surface of roots.
Root hair cells have a long, thin hair-like extension that projects outward from the root surface into the soil. No chloroplasts (underground — no light).
The hair-like extension greatly increases the surface area of the root for absorbing water (by osmosis) and mineral ions (by active transport). A single root can have billions of root hairs, massively increasing total uptake.
Pathway of Water through the Plant
Root hair cells → root cortex cells → xylem → mesophyll cells (in leaf)
Water enters root hair cells by osmosis (soil water has higher water potential than cell sap). It passes by osmosis through the root cortex cells toward the centre. Once in the xylem, water is drawn upward by transpiration pull. In the leaf, water passes into mesophyll cells and evaporates from their surfaces into the air spaces.
Method: Place a celery stalk (or white carnation) in coloured dye solution (e.g. red eosin or food colouring). Leave for several hours. Cut transverse sections through the stem at intervals. Observe under microscope or with the naked eye.
Result: The coloured dye travels upward through the xylem vessels only. In a cross-section of the stem, only the xylem vessels appear stained. This confirms that xylem is the tissue responsible for water transport.
Conclusion: Water moves through xylem, not phloem. The stain should only be visible in the xylem (inner part of vascular bundles in a dicot stem).
Explain why root hair cells are well adapted for absorbing water from the soil. [3 marks]
- Root hair cells have a long hair-like extension [1 mark]
- This greatly increases the surface area of the root cell in contact with soil water [1 mark]
- The larger surface area allows more water to enter by osmosis (and mineral ions by active transport) per unit time [1 mark]
Transpiration
CORE EXTENDEDTranspiration is the loss of water vapour from leaves (and other aerial parts of the plant).
Mechanism: Water evaporates from the surfaces of mesophyll cells into the interconnecting air spaces → diffuses out through the stomata as water vapour into the surrounding air.
Factors Affecting Transpiration Rate
| Factor | Effect on transpiration rate | Core explanation |
|---|---|---|
| Temperature | Higher → faster | More heat energy → water evaporates faster from mesophyll surfaces |
| Wind speed | Higher → faster | Wind blows away water vapour near the stomata, maintaining a steep concentration gradient between inside the leaf and the air |
| Humidity | Higher humidity → slower | Moist air outside the leaf reduces the water vapour concentration gradient between the leaf’s air spaces and the surrounding air → less diffusion → slower transpiration |
A potometer measures the rate of water uptake by a cut shoot. As transpiration removes water from the leaves, water is drawn up the stem, and a bubble in a capillary tube moves toward the shoot. The rate of bubble movement = a proxy for transpiration rate.
Controlling variables: Keep temperature, light intensity, and humidity constant when testing wind speed (and vice versa). Cut the shoot underwater to prevent air entering xylem. Re-cut the stem at an angle to maximise exposed xylem.
Important limitation: A potometer measures water uptake, not transpiration directly. Not all water taken up is transpired — some is used in photosynthesis. However, in practice the two are closely linked and the potometer is a valid proxy.
Transpiration Pull (Cohesion-Tension) — Extended
Step 1 — Evaporation and pull: Water evaporates from mesophyll cells into air spaces and out through stomata. This creates a water deficit in the mesophyll cells, lowering their water potential.
Step 2 — Cohesion: Water molecules are held together by forces of attraction (hydrogen bonds — cohesion). As molecules at the top of the xylem are pulled into the mesophyll, they pull the molecules below them upward — the entire water column moves as one unit.
Step 3 — Tension: The pulling force is transmitted all the way down to the roots. Water is pulled up from the soil into root hair cells and through the xylem by this tension (transpiration pull).
Result: water moves continuously from roots to leaves without the plant expending any energy directly — the driving force is solar energy evaporating water at the leaf surface.
Wilting — Extended
Wilting occurs when the rate of water loss by transpiration exceeds the rate of water uptake by the roots. When water loss is too high, cells lose turgor pressure → cells become flaccid → the plant droops (wilts).
Hot, dry, or windy conditions that accelerate transpiration; waterlogged soil (displaces air, suffocates roots, reduces active transport of ions); soil too dry (water unavailable); damaged roots.
If water becomes available before permanent damage occurs, cells can regain water by osmosis → turgor pressure is restored → plant recovers. Guard cells close stomata during wilting to conserve water.
Extended: how internal leaf structure affects transpiration rate
The rate of water vapour loss depends on:
- The large internal surface area provided by the interconnecting air spaces — more surface exposed for evaporation
- The size and number of stomata — more/larger stomata = faster diffusion of water vapour out
- Guard cells open and close stomata in response to light, CO₂ concentration, and water availability — regulating transpiration rate
On a hot, dry, windy day, transpiration rate is high. Which combination of conditions explains this?
- A. High temperature reduces evaporation; wind increases water vapour concentration near leaves
- B. High temperature increases evaporation; wind removes water vapour, maintaining a steep diffusion gradient
- C. High temperature closes stomata; wind increases the concentration of water inside the leaf
- D. Dry air increases humidity near stomata; heat has no effect on diffusion
Explain how water moves from the soil to the top of a tall tree, using the terms: osmosis, transpiration pull, cohesion, xylem. [5 marks]
- Water enters root hair cells from the soil by osmosis (soil water has higher water potential) [1 mark]
- Water passes through root cortex cells and enters the xylem vessels [1 mark]
- At the leaf, water evaporates from mesophyll cells and diffuses out through stomata (transpiration) — creating a water deficit that generates transpiration pull [1 mark]
- Water molecules are attracted to each other by forces of attraction (cohesion) / hydrogen bonds, forming a continuous column [1 mark]
- The transpiration pull is transmitted down the continuous water column in the xylem, pulling water upward from roots to leaves without energy expenditure by the plant [1 mark]
Translocation
EXTENDEDTranslocation is the movement of sucrose and amino acids through the phloem, from regions where they are produced or released (sources) to regions where they are used or stored (sinks).
Sources and Sinks
| Term | Definition | Examples |
|---|---|---|
| Source | A part of the plant that releases sucrose or amino acids into the phloem | Mature leaves (producing sucrose by photosynthesis); storage organs releasing stored sucrose during germination |
| Sink | A part of the plant that uses or stores sucrose or amino acids received from the phloem | Growing shoot tips (using sucrose for respiration and cell division); roots (storing sucrose as starch); developing fruits and seeds |
The same plant part can switch roles depending on the plant’s stage of growth:
A potato tuber — acts as a sink during summer when leaves photosynthesise and sucrose is transported down for storage. Acts as a source in spring when the stored starch is converted back to sucrose and transported up to fuel new shoot growth.
A leaf — a young, developing leaf is a sink (importing sucrose from mature leaves for growth). A fully mature leaf is a source (exporting sucrose produced by photosynthesis to the rest of the plant).
Translocation vs Transpiration — Key Differences
| Feature | Transpiration (xylem) | Translocation (phloem) |
|---|---|---|
| Tissue used | Xylem (dead cells) | Phloem (living cells) |
| Substance moved | Water and mineral ions | Sucrose and amino acids |
| Direction | Upward only | Both up and down (source to sink) |
| Energy required? | No (transpiration pull) | Yes (active loading of phloem) |
| Driven by | Evaporation at leaf surface | Metabolic activity of phloem cells |
During spring, a tree uses stored carbohydrates in its roots to produce new leaves. In terms of translocation, how should the roots be described at this time?
- A. As a sink, because they are storing sucrose produced by new leaves
- B. As both source and sink simultaneously
- C. As a source, because they are releasing sucrose into the phloem for transport to growing shoots
- D. As neither source nor sink, because roots do not carry out photosynthesis
Comprehensive Practice Questions
Mixed questions across all of Topic 8.
Which feature of xylem vessels is most directly responsible for allowing water to flow through them without resistance?
- A. Thick walls reinforced with lignin
- B. No living cell contents, and no cross walls between cells
- C. Small diameter to increase water pressure
- D. Thin walls that allow water to pass through easily
A student uses a potometer to investigate the effect of wind speed on transpiration rate in a leafy shoot. The bubble moves 10 mm in 5 minutes in still air, and 28 mm in 5 minutes with a fan blowing.
(a) Calculate the transpiration rate (mm/min) in each condition. [2 marks]
(b) Explain why wind increases transpiration rate. [2 marks]
(c) Identify one variable the student must control in this investigation. [1 mark]
- (a) Still air: 10 ÷ 5 = 2 mm/min [1 mark]; with fan: 28 ÷ 5 = 5.6 mm/min [1 mark]
- (b) Wind blows away water vapour that accumulates near the stomata [1 mark]; this maintains a steep water vapour concentration gradient between the inside of the leaf and the air outside, increasing the rate of diffusion of water vapour out [1 mark]
- (c) Any one of: temperature / light intensity / humidity / leaf surface area [1 mark]
A scientist injects a radioactive sugar into the phloem of a plant stem. An hour later, radioactivity is detected in both the growing root tips below the injection site and in developing fruits above it.
(a) What does this result demonstrate about the direction of flow in phloem? [1 mark]
(b) Using the terms source and sink, explain why sugar is being transported to both root tips and developing fruits. [3 marks]
(c) Would a similar experiment injecting a radioactive mineral ion into the xylem show movement in both directions? Explain your answer. [2 marks]
- (a) Phloem can transport substances in both directions simultaneously (bidirectional flow) [1 mark]
- (b) The leaf (injection site) is a source — it produces sucrose by photosynthesis and loads it into the phloem [1 mark]; the growing root tips and developing fruits are both sinks — they require sucrose for respiration, cell division, and storage [1 mark]; translocation moves sucrose from source to the nearest available sinks, which exist both above and below [1 mark]
- (c) No — xylem only transports in one direction: upward from roots to shoots [1 mark]; mineral ions injected into xylem would only move upward, not downward toward the roots [1 mark]
High-Frequency Mistakes — Topic 8 Overall
- 🌿Saying phloem transports glucose (not sucrose)Phloem transports sucrose and amino acids. Glucose is converted to sucrose before loading into phloem. Writing “glucose” for phloem content loses the mark every time.
- ↕Saying xylem moves substances in both directionsXylem is unidirectional — upward only (root → leaf). Only phloem moves in both directions. The different structures explain this: xylem is pulled upward by transpiration pull; phloem is actively loaded and can move either way.
- 💧Saying transpiration is the same as evaporationTranspiration is specifically the loss of water vapour from plants through stomata (and other surfaces). Evaporation is a general physical process. Transpiration involves living organisms and biological structures (stomata, mesophyll air spaces). Use the correct biological term.
- 📈Saying humidity increases transpirationHigh humidity decreases transpiration — moist air outside reduces the water vapour concentration gradient, slowing diffusion. Dry air (low humidity) increases transpiration by maintaining a steep gradient.
- ⛓Ext: Saying plants use energy to pull water up xylemWater movement in xylem is passive — driven by solar energy evaporating water at the leaf surface (transpiration pull). The plant expends no metabolic energy on this. Only phloem loading (translocation) requires metabolic energy.
- 🌿Ext: Confusing source and sink directionA source is where sucrose is released from (e.g. photosynthesising leaf). A sink is where sucrose is going to (e.g. growing root, fruit). The same organ can switch roles — a tuber is a sink in summer and a source in spring. Think of “source = supplier; sink = receiver”.
- 🪴Ext: Saying xylem cells are aliveXylem vessels are dead cells. This is essential to their function — living contents would block the tube. Only phloem sieve tube elements are living (they require energy for active loading of sucrose).
Highest-yield items: xylem vs phloem functions (Core — what each transports); the water pathway (root hair → cortex → xylem → mesophyll); transpiration factors (temperature and wind speed, Core; add humidity for Extended); and the potometer practical. For Extended: the cohesion-tension mechanism needs the full chain (evaporation → pull → cohesion → continuous column), and translocation source/sink questions almost always appear in Paper 4.