Transport in Plants: Xylem Phloem and Translocation
Transport Systems in Plants
Unlike animals, plants do not have a heart or a circulatory system with blood. Instead, they have two specialised transport tissues: xylem (which transports water and dissolved mineral ions) and phloem (which transports organic solutes, mainly sucrose). These tissues run through the plant as continuous vascular bundles.
Water Transport in the Xylem
Uptake of Water by Roots
Water enters root hair cells from the soil by osmosis — the root hair cell has a more negative water potential (lower/more negative ψ) than the soil solution due to the dissolved solutes inside the cell.
Root hair cells are adapted for water absorption:
- Long, thin extensions of epidermal cells that increase surface area enormously
- Thin cell walls for a short diffusion pathway
- Large central vacuole containing solutes that maintain a water potential gradient
Pathways Across the Root
Water moves from the root surface to the xylem via three pathways:
| Pathway | Route | Features |
|---|---|---|
| Apoplast | Through cell walls and intercellular spaces | No membrane crossing; fastest route; carries most water (~90%) |
| Symplast | Through cytoplasm of cells, moving between cells via plasmodesmata (cytoplasmic connections) | Crosses membranes; slower |
| Vacuolar | Through vacuoles of cells in sequence | Variant of symplast; slowest |
The apoplast pathway is blocked at the endodermis by the Casparian strip — a band of suberin (a waxy, waterproof substance) in the cell walls of endodermal cells. This forces water to enter the symplast (through the cell membrane of endodermal cells), giving the plant control over which mineral ions enter the xylem. It also enables the active transport of ions into the xylem, which lowers xylem water potential and draws water in by osmosis.
Xylem Structure
Xylem vessels are dead, hollow, elongated cells arranged end to end in continuous tubes:
- Cell walls are thickened with lignin — a waterproof, rigid substance that provides mechanical support and prevents collapse under negative pressure
- End walls are perforated or absent, creating a continuous lumen for unimpeded water flow
- No cytoplasm, nucleus, or organelles — the entire lumen is available for water transport
- Pits (unlignified areas) in the walls allow lateral movement of water to adjacent cells
The Cohesion-Tension Theory
The accepted mechanism for water transport up the xylem is the cohesion-tension theory:
1. Transpiration — water evaporates from the surfaces of spongy mesophyll cells into the air spaces of the leaf, then diffuses out through stomata. This creates a tension (negative pressure) in the leaf.
2. Tension is transmitted down through the continuous water column in the xylem. Water is essentially pulled upward.
3. Cohesion — water molecules are attracted to each other by hydrogen bonds, creating a continuous, unbroken column of water. The cohesive forces prevent the column from breaking under tension.
4. Adhesion — water molecules are attracted to the lignified walls of xylem vessels, helping to support the water column and resist gravity.
5. At the roots, the tension pulls water from the soil into the root xylem, creating a continuous flow from roots to leaves.
Evidence for the theory:
- Xylem sap is under negative pressure (tension), measurable with a pressure probe
- Transpiration rate correlates with the rate of water uptake
- Tree trunks are measurably narrower during the day (when transpiration is highest) because the xylem is under greater tension
Transpiration
Transpiration is the loss of water vapour from the aerial parts of the plant, mainly through stomata in the leaves.
Factors affecting transpiration rate:
| Factor | Effect | Explanation |
|---|---|---|
| Temperature | Increases rate | Water molecules have more kinetic energy; air can hold more water vapour |
| Humidity | Decreases rate when high | Reduces the water potential gradient between leaf air spaces and external air |
| Wind | Increases rate | Removes humid air from around stomata, maintaining the diffusion gradient |
| Light intensity | Increases rate | Stomata open wider in light (for photosynthesis), allowing more transpiration |
Transpiration can be measured using a potometer, which measures the rate of water uptake (as a proxy for transpiration).
Stomatal Control
Stomata are opened and closed by guard cells:
- Guard cells take up K⁺ ions by active transport → water potential decreases → water enters by osmosis → guard cells swell and become turgid → their inner walls (thicker, less elastic) buckle outward → stoma opens
- When K⁺ is lost, water leaves, guard cells become flaccid, and the stoma closes
- Stomata generally open in light and close in darkness and when water stress triggers abscisic acid (ABA) release
Translocation in the Phloem
Phloem Structure
Sieve tube elements are the main conducting cells of phloem:
- Living cells arranged end to end
- Sieve plates — perforated end walls that allow mass flow of phloem sap
- Very few organelles; no nucleus at maturity — dependent on companion cells
- Thin layer of cytoplasm lining the cell wall; large central lumen
Companion cells:
- Closely associated with sieve tube elements via numerous plasmodesmata
- Have a dense cytoplasm, large nucleus, and many mitochondria
- Carry out metabolic functions for the sieve tube element
- Actively load sucrose into the sieve tube (see below)
The Mass Flow (Pressure Flow) Hypothesis
Translocation moves organic solutes (mainly sucrose) from sources (where sugars are produced or mobilised, e.g. photosynthesising leaves, storage organs releasing reserves) to sinks (where sugars are used or stored, e.g. growing roots, developing fruits, storage organs accumulating reserves).
The mechanism:
1. Loading at the source: Companion cells use active transport (H⁺ ATPase pumps create a proton gradient; sucrose-H⁺ cotransporters use this gradient to move sucrose into the companion cell against its concentration gradient). Sucrose then passes into the sieve tube element through plasmodesmata.
2. This loading of sucrose lowers the water potential inside the sieve tube at the source.
3. Water enters the sieve tube by osmosis from the adjacent xylem, increasing the hydrostatic pressure at the source end.
4. Mass flow of phloem sap occurs from the high-pressure source end to the lower-pressure sink end through the sieve plates.
5. Unloading at the sink: Sucrose is removed from the sieve tube (by active transport or conversion to other molecules, e.g. starch for storage). Water potential rises, water leaves by osmosis, and pressure drops.
Evidence for the mass flow hypothesis:
- Aphid stylets can be used to sample phloem sap — sap exudes under pressure, confirming positive hydrostatic pressure
- Radioactive tracer (¹⁴C-labelled CO₂) fed to a leaf appears in phloem sap and is transported to sinks
- Ringing experiments: removing a ring of bark (including phloem) causes sugars to accumulate above the ring
Limitations:
- Does not fully explain bidirectional flow (different solutes moving in opposite directions in the same sieve tube)
- Sieve plates would seem to impede mass flow (though callose deposits may be artefacts of preparation)
Exam Tips
- AQA commonly asks you to compare and contrast xylem and phloem — use a table covering structure, contents, direction, mechanism, alive/dead
- When explaining the cohesion-tension theory, use the key terms: transpiration, tension, cohesion, adhesion — in that logical order
- For translocation, emphasise that loading is an active process requiring ATP from companion cells — evidence includes the high density of mitochondria and the effect of metabolic inhibitors
- Potometer questions require you to note that it measures water uptake, not transpiration directly (a small amount of water is used in photosynthesis and cell expansion)