Transport in Plants
Summary: Plants have two transport systems: xylem (transports water and dissolved mineral ions from roots to leaves, driven by transpiration pull) and phloem (transports sucrose and amino acids from sources to sinks by translocation). Transpiration is the loss of water vapour from leaves and is affected by temperature, humidity, light intensity, and wind speed. Tags: igcse biology transport-in-plants xylem phloem transpiration translocation Created: 2026-07-16 Last Updated: 2026-07-16
1. Why Plants Need Transport Systems
Plants need transport systems because:
- They are relatively large organisms — diffusion alone cannot supply all cells
- Photosynthesising cells (in leaves) need a constant supply of water and mineral ions from the roots
- Non-photosynthesising cells (in roots and stems) need sugars produced by the leaves
- The xylem and phloem form the vascular system — found together in vascular bundles running through roots, stems, and leaves
2. Xylem — Water and Mineral Transport
Function: Transports water and dissolved mineral ions from the roots, up through the stem, to the leaves and other aerial parts of the plant.
Direction of flow: Upward only (roots → leaves).
Structure of xylem vessels:
| Feature | How It Relates to Function |
|---|---|
| Dead cells with no cytoplasm or organelles (at maturity) | No obstruction to water flow — creates a hollow, continuous tube |
| No end walls between adjacent xylem vessel elements | Forms a continuous, uninterrupted column of water from roots to leaves |
| Walls thickened with lignin (a strong, waterproof substance) | Provides strength — prevents the vessel from collapsing under the tension (negative pressure) created by transpiration pull; waterproof — keeps water inside the vessel |
| Lignin deposited in spiral, annular (ring), or reticulate (net-like) patterns | Allows flexibility (stretching and bending) while providing structural support |
| Narrow lumen (diameter) | Helps maintain a continuous water column — capillarity contributes to water movement |
Xylem also provides structural support to the plant — lignified vessels act like reinforcing rods.
3. Mechanism of Water Transport — Transpiration Pull
Water moves up the xylem by the transpiration pull (also called the cohesion-tension mechanism):
- Transpiration — Water evaporates from the surfaces of mesophyll cells into air spaces in the leaf, then diffuses out of the leaf through stomata (as water vapour)
- Tension (negative pressure) — The loss of water from mesophyll cells lowers their water potential. Water moves by osmosis from nearby cells, which in turn draw water from xylem vessels in the leaf. This creates a suction (tension) at the top of the xylem column
- Cohesion — Water molecules are strongly attracted to each other by hydrogen bonds (cohesion). The tension at the top pulls the entire water column upwards as a continuous, unbroken chain
- Adhesion — Water molecules are also attracted to the xylem walls (adhesion), helping to pull water up
- Root pressure — Minor contribution: active transport of mineral ions into xylem lowers water potential in root xylem; water enters by osmosis, creating slight upward pressure
4. Transpiration
Definition: Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata in the leaves.
Why transpiration happens:
- Stomata open to allow CO2 to enter for photosynthesis
- Water vapour inevitably diffuses out at the same time (the air spaces inside the leaf are saturated with water vapour; the external air is usually drier)
- Transpiration is a consequence of gas exchange — it is unavoidable
Importance of transpiration:
- Creates the transpiration pull that draws water and dissolved minerals up the xylem
- Cools the plant (evaporative cooling)
Factors Affecting the Rate of Transpiration
| Factor | Effect on Transpiration Rate | Explanation |
|---|---|---|
| Temperature | Higher temperature → faster rate | Water molecules have more kinetic energy → faster evaporation from mesophyll cells; warmer air can hold more water vapour → steeper concentration gradient between leaf and external air |
| Humidity | Higher humidity → slower rate | The air already contains more water vapour → the concentration gradient (water potential gradient) between inside and outside the leaf is less steep → slower diffusion of water vapour out |
| Light intensity | Higher light intensity → faster rate | Light causes stomata to OPEN (guard cells photosynthesise → become turgid and curve). More stomata open → larger pathway for water vapour to escape. Also, light provides energy → higher leaf temperature → faster evaporation |
| Wind speed / air movement | Higher wind speed → faster rate | Moving air carries away water vapour from around the leaf surface → maintains a steep water vapour concentration gradient. In still air, water vapour accumulates around the leaf, reducing the gradient |
Wilting: When the rate of transpiration exceeds water uptake by roots, cells lose turgor pressure and become flaccid → the plant wilts. This reduces water loss because stomata close and leaf surface area exposed to sun is reduced.
5. Potometer — Measuring Transpiration Rate
A potometer is used to measure the rate of water uptake by a leafy shoot, which gives an estimate of the transpiration rate (most water absorbed is lost through transpiration).
Setup:
- A leafy shoot is cut under water (to prevent air entering xylem) and attached to a capillary tube filled with water
- An air bubble is introduced into the capillary tube
- As the plant takes up water, the bubble moves along the capillary tube
- The distance the bubble travels in a given time is used to calculate the rate of water uptake
Precautions:
- Cut the shoot under water — prevents air bubbles from entering and blocking xylem vessels
- Ensure the apparatus is airtight — any leaks invalidate the measurement
- The potometer measures water uptake, not directly transpiration (a small amount of water is used in photosynthesis, but most is lost through transpiration)
- The leafy shoot must be cut at a slant — increases the cut surface area for water uptake
6. Root Hair Cells
Structure and function:
- Root hair cells are specialised epidermal cells in the root with long, thin extensions (root hairs) that penetrate between soil particles
- The long extension greatly increases the surface area for absorption of water and mineral ions
- They have a thin cell wall — reduces the diffusion distance
- They contain many mitochondria — provide ATP for the active transport of mineral ions against their concentration gradient
- They have no waxy cuticle (unlike leaf epidermal cells), so water can pass through freely
Mechanism of absorption:
- Water: Enters root hair cells by osmosis (soil water has a higher water potential than the cell sap in root cells, partly because root cells actively transport mineral ions into their cytoplasm)
- Mineral ions: Enter by active transport against their concentration gradient (concentration in the soil is usually lower than inside the root cells)
- Once inside the root, water and minerals travel through the root cortex (by osmosis/diffusion through cell walls [apoplast pathway] or through cytoplasm [symplast pathway]) to the xylem in the centre
7. Phloem — Sucrose and Amino Acid Transport
Function: Transports sucrose and amino acids from sources (where they are produced or stored) to sinks (where they are used or stored).
Direction of flow: Up AND down (bidirectional).
Translocation is the movement of sucrose and amino acids in the phloem.
Structure of phloem:
| Feature | How It Relates to Function |
|---|---|
| Sieve tube elements (living cells) | Form the main conducting cells of the phloem. They are living but have no nucleus and few organelles — reduces obstruction to flow |
| Sieve plates | Perforated end walls between adjacent sieve tube elements — allow the flow of phloem sap (sucrose solution) from one cell to the next |
| Companion cells | Adjacent to each sieve tube element — contain a nucleus and many mitochondria. They provide energy (ATP) for the active loading of sucrose into the sieve tubes at the source |
Sources and sinks:
| Source (sucrose moves FROM) | Sink (sucrose moves TO) |
|---|---|
| Photosynthesising leaves (produce glucose → converted to sucrose) | Growing tips (meristems) — need energy for cell division |
| Storage organs releasing stored starch (e.g. potato tubers in spring) | Developing fruits and seeds — need energy for development |
| Roots — store starch for winter | |
| Any respiring cell that is not photosynthesising |
Mechanism of translocation:
- At the source, sucrose is actively loaded into the sieve tubes (requires energy/ATP from companion cells)
- This lowers the water potential inside the sieve tubes
- Water enters by osmosis from the adjacent xylem, increasing hydrostatic pressure
- At the sink, sucrose is unloaded (used in respiration or converted to starch for storage)
- This raises the water potential; water leaves by osmosis, decreasing hydrostatic pressure
- The pressure difference drives the bulk flow of phloem sap from source to sink
Sources
- BBC Bitesize GCSE Biology — Transport in plants, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 30 Plant Form and Physiology, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 8: Transport in plants, Cambridge Assessment International Education
- CK-12 Biology for High School — Plant Transport, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Cell Structure and Organisation — Root hair cells, xylem vessels, and palisade mesophyll cells as specialised cells
- Movement Into and Out of Cells — Osmosis (water uptake by roots), active transport (mineral ion uptake, sucrose loading)
- Plant Nutrition — Photosynthesis in leaves produces sucrose; mineral requirements (nitrates, magnesium)
- Biological Molecules — Sucrose (disaccharide), starch (storage polysaccharide)
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Xylem transports food and phloem transports water” | It is the opposite — xylem transports water and minerals; phloem transports sucrose and amino acids |
| ”Xylem cells are living” | Mature xylem vessel elements are dead — they have no cytoplasm, no nucleus, and no organelles |
| ”Transpiration is the same as translocation” | Transpiration = loss of water vapour from leaves. Translocation = movement of sucrose and amino acids in phloem |
| ”The potometer measures transpiration directly” | The potometer measures water uptake — most of this water is lost through transpiration, but a small fraction is used in photosynthesis and metabolic processes |
| ”Root hair cells only absorb water by osmosis” | Root hair cells also actively transport mineral ions — often against a concentration gradient, requiring energy |
| ”Plants only transport upwards” | Xylem transport is unidirectional (upward). Phloem transport is bidirectional (up and down — from source to sink wherever they are) |
| “Transpiration is just a plant sweating” | Transpiration is primarily a consequence of stomata opening for CO2 uptake for photosynthesis — water loss is an inevitable side effect of gas exchange |