Transport in Plants

Spec Mapping — OCR H420 Module 3.1.3 — Transport in plants, content statements covering xylem and phloem tissue structure, water uptake by roots, the apoplast / symplast / vacuolar pathways, the cohesion-tension mechanism of transpiration, translocation of sucrose by mass flow (Münch hypothesis), and the adaptations of xerophytes and hydrophytes (refer to the official OCR H420 specification document for exact wording). This lesson is the plant counterpart to the animal circulation Lessons 6–11 — same Fick's-law and water-potential physics, different tissue architecture.

Plants do not have a muscular pump like the mammalian heart, yet they still move water and dissolved substances over long distances — sometimes tens of metres up the trunk of a tree. They achieve this by using the physical properties of water and specialised tissues: xylem for the upward flow of water and mineral ions, and phloem for the transport of dissolved organic substances (mainly sucrose). This lesson examines the tissues involved, the pathways water takes across the root, how water rises to the top of a tree by the cohesion-tension mechanism, how sucrose moves by mass flow in the phloem, and the adaptations of plants to extreme environments.

The history is long. Stephen Hales (1727 Vegetable Staticks) was the first to measure root pressure and xylem tension quantitatively, inserting glass manometers into vine stems. The cohesion-tension theory in its modern form is associated with Henry Dixon and John Joly (1894), who proposed that water rises in trees by virtue of the cohesion of water molecules under tension — and that the tension itself is generated by transpiration from the leaves, not by any "push" from the roots. The mass-flow translocation theory is Ernst Münch's (1930 Die Stoffbewegungen in der Pflanze) and remains the textbook framework today. Each of these schools of thought (paraphrased) emphasised that plants exploit the physical properties of water — its cohesion via hydrogen bonding, its adhesion to charged surfaces, and its high tensile strength under continuous columns — without any active "pump" comparable to the mammalian heart.

Key Definitions:

• Xylem — plant tissue that transports water and mineral ions from the roots to the leaves; consists of dead, lignified vessels and tracheids.
• Phloem — living plant tissue that transports dissolved organic substances (primarily sucrose) from sources to sinks; consists of sieve tube elements and companion cells.
• Transpiration — the evaporation of water from the leaves of a plant.
• Translocation — the active transport of dissolved sugars through the phloem.
• Xerophyte — a plant adapted to arid conditions (e.g., cactus, marram grass).
• Hydrophyte — a plant adapted to living in water (e.g., water lily).

Xylem Tissue

Xylem tissue forms continuous tubes running from the roots to the leaves. It consists of:

• Xylem vessels — the main conducting elements in angiosperms. Each vessel is formed from a column of dead cells whose end walls have been broken down to produce a long, continuous tube. The walls are thickened and lignified to resist the inward pull of water under tension.
• Tracheids — narrower, spindle-shaped cells present in all vascular plants (and the only conducting element in gymnosperms).
• Pits — unlignified areas of the wall where water can move sideways between adjacent vessels or tracheids.
• Lignin — a rigid polymer that reinforces the cell walls, providing structural support and waterproofing.

Xylem vessels are dead at maturity, and the empty lumen provides an unobstructed path for water flow. The walls cannot be stretched significantly, which is essential for withstanding the negative pressures (tensions) generated during transpiration.

Exam Tip: A common mark-scheme point: xylem vessels are "dead, hollow, lignified, with no end walls". Memorise that exact phrase.

Phloem Tissue

Phloem is living tissue, consisting of:

• Sieve tube elements — elongated cells aligned end-to-end to form sieve tubes. Their end walls are perforated with pores, forming sieve plates that allow cytoplasm to flow between cells. Mature sieve tube elements lose most of their organelles, including the nucleus, tonoplast and ribosomes, leaving only a thin peripheral layer of cytoplasm.
• Companion cells — small, dense cells adjacent to each sieve tube element. They retain their nucleus and all the usual organelles and are connected to the sieve tube via numerous plasmodesmata. Companion cells carry out metabolic functions on behalf of the sieve tube element, including producing ATP and transporting solutes into and out of it.

Water Uptake by the Roots

Water enters the plant through root hair cells, which are long extensions of epidermal cells near the root tip. Their key adaptations are:

• Extended shape — greatly increases surface area for water absorption.
• Thin cell wall — reduces distance for water uptake.
• Large vacuole — helps maintain a low water potential relative to the soil.
• Many mitochondria — produce ATP for active transport of mineral ions (which lowers water potential further and drives more water in by osmosis).
• No cuticle — the waxy waterproof layer is absent here so water can cross freely.

The Apoplast, Symplast and Vacuolar Pathways

Once inside the root, water moves across the cortex towards the xylem in the centre. There are three possible routes:

1. Apoplast pathway — water moves along the cell walls and intercellular spaces without entering the cytoplasm. The porous cellulose wall readily permits water movement, and because it is the fastest route, it carries the largest proportion of the water.
2. Symplast pathway — water moves through the cytoplasm of cells, passing from one cell to the next via plasmodesmata (cytoplasmic bridges through the cell walls).
3. Vacuolar pathway — water passes through the cytoplasm and vacuoles of successive cells, crossing tonoplast membranes at each step.

flowchart LR
    RH[Root hair] --> CORTEX[Cortex]
    CORTEX --> ENDO[Endodermis]
    ENDO --> XY[Xylem]
    XY --> STEM[Stem]
    STEM --> LEAF[Leaves]
    LEAF --> ATM[Atmosphere via stomata]
    CORTEX -.->|apoplast via walls| ENDO
    CORTEX -.->|symplast via plasmodesmata| ENDO
    CORTEX -.->|vacuolar via tonoplast| ENDO

The Casparian Strip

At the endodermis (the innermost layer of the cortex), a band of suberin — a waxy, waterproof substance — is deposited into the radial and transverse walls, forming the Casparian strip. Water flowing along the apoplast route is blocked here and forced to cross the endodermal cell membrane into the symplast. This has two crucial consequences:

1. All water must pass through a living cell membrane at the endodermis, allowing the plant to control which ions enter the xylem (selective transport through membrane proteins).
2. Ions actively transported into the xylem create a low water potential that draws water in by osmosis — this generates root pressure, a small additional upward push of water.

The Cohesion-Tension Mechanism of Transpiration

Transpiration is the evaporation of water from leaves, and it is the main driving force for water movement up a plant. The mechanism is known as cohesion-tension:

Step 1: Evaporation from Spongy Mesophyll

• Water evaporates from the wet surfaces of mesophyll cells inside the leaf.
• Water vapour diffuses through the sub-stomatal air spaces and out of the stomata along its water potential gradient.

Step 2: Water is Drawn from Adjacent Cells

• As each mesophyll cell loses water, its water potential falls.
• Water moves in by osmosis from neighbouring cells, eventually drawing water from the xylem vessels in the leaf veins.

Step 3: Tension in the Xylem

• Water leaving the xylem creates tension (negative pressure) in the water column.
• This tension is transmitted down the entire length of the xylem, right to the roots, because water molecules cling to each other by cohesion (hydrogen bonding).
• They also cling to the walls of the xylem vessels by adhesion (hydrogen bonding to polar cellulose and lignin).

Step 4: Uptake at the Roots

• The tension draws water up from the root xylem, which in turn draws water from the root cortex, and ultimately from the soil.
• The unbroken water column is pulled upward, like a long thin straw.

flowchart TB
    SOIL[Soil water] --> ROOT[Root hair cell]
    ROOT --> CORT[Cortex]
    CORT --> END[Endodermis: Casparian strip]
    END --> XY[Root xylem]
    XY --> STEMX[Stem xylem: cohesion and adhesion]
    STEMX --> LEAFX[Leaf xylem]
    LEAFX --> MES[Mesophyll cells]
    MES --> EVAP[Evaporation into air spaces]
    EVAP --> STOM[Stomata]
    STOM --> ATM[Atmosphere]

Because the column is under tension, any disruption (e.g., an air embolism) can break it and stop flow in that vessel — another reason lignified walls are essential, as they prevent the vessels being squeezed shut.

Transpiration → water vapour out of stomata

flowchart LR
    S[Source: leaf mesophyll] -->|Active loading of sucrose| SE[Sieve tube at source]
    SE -->|Water enters by osmosis| P1[High hydrostatic pressure]
    P1 --> MASS[Mass flow along sieve tube]
    MASS --> P2[Low hydrostatic pressure at sink]
    P2 -->|Sucrose unloaded| SINK[Sink: root or fruit]
    SINK -->|Water leaves| XY[Xylem]