Transport mechanisms

Water enters a plant from the soil through the roots. The journey from soil to xylem involves three possible pathways across the root cortex and a checkpoint at the endodermis.

Root hairs and uptake.

• Root hair cells are extensions of the epidermis with a large surface area for absorption.
• Their cytoplasm contains dissolved solutes (mineral ions, sugars), giving them a lower (more negative) water potential than the surrounding soil water.
• Water therefore moves from the soil into the root hair cell down the water potential gradient by osmosis.

Three pathways across the cortex.

1. Apoplast pathway — water moves through the cell walls and intercellular spaces, without crossing membranes. The cellulose cell walls are hydrophilic and allow water to flow along them, much like water moving through a sponge. This is the fastest pathway and most water travels by it.
2. Symplast pathway — water enters the cytoplasm through the cell-surface membrane (by osmosis) and then moves from cell to cell through plasmodesmata (cytoplasmic strands passing through pores in the cell walls). The water remains in the continuous cytoplasm of all the cells.
3. Vacuolar pathway — water moves from cell to cell through the cytoplasm and vacuoles by osmosis, crossing both the cell-surface membrane and the tonoplast. This is the slowest pathway.

Endodermis and the Casparian strip.

• The endodermis is a single cell layer surrounding the central vascular cylinder of the root.
• Its radial and transverse cell walls contain a band of waterproof suberin — the Casparian strip.
• The Casparian strip blocks the apoplast pathway completely. Water in the apoplast has no choice but to enter the cytoplasm (symplast) of the endodermal cells.
• This forces water across a selectively permeable membrane, giving the plant control over which solutes enter the xylem.

Entry into xylem.

• Beyond the endodermis, water moves into the xylem in the centre of the root.
• Mineral ions are actively pumped into the xylem (using ATP), lowering the water potential there.
• Water follows by osmosis. The slight upward push this creates is called root pressure — it contributes to water movement but is much smaller than transpiration pull.

Once in the xylem, water moves UPWARDS from roots to leaves — a height of up to ~100 m in the tallest trees. The accepted explanation is the cohesion-tension theory.

1. Transpiration creates tension at the leaf.

• Water evaporates from the wet surfaces of mesophyll cell walls into the air spaces of the leaf.
• The water vapour then diffuses out through open stomata into the atmosphere, down the water potential gradient.
• The water lost is replaced by water drawn from the xylem at the end of the leaf vein. This creates a low (very negative) water potential / negative pressure (tension) at the top of the xylem.

2. Water is pulled up the xylem.

• The continuous water column in the xylem transmits the tension from leaf to root.
• Water is pulled upwards through the xylem — this is the transpiration pull.
• The driving energy comes from the SUN, which evaporates water from the leaves; the plant does not need to use ATP to pull water up.

3. Cohesion holds the column together.

• Water molecules are polar and form hydrogen bonds with each other.
• These many H-bonds give water strong cohesion — the molecules stick together.
• Cohesion is strong enough to maintain an unbroken water column even under tension and even in trees taller than 100 m.

4. Adhesion to xylem walls.

• Water also forms H-bonds with the hydrophilic cellulose and lignin of the xylem cell walls — this is adhesion.
• Adhesion helps maintain the continuous column, especially in the very narrow xylem vessels of small twigs and roots.

5. Lignin prevents collapse.

• The xylem is under negative pressure, which would tend to crush the vessels inwards.
• The heavily lignified walls are rigid and prevent the vessels from collapsing.

Evidence supporting cohesion-tension:

• Tree trunk diameter decreases very slightly during the day (when transpiration is high and tension is greatest), as measured by sensitive dendrometers.
• If the xylem column is broken (cavitation — formation of an air bubble), water transport above the break stops.
• Direct measurements with pressure probes confirm negative pressures in xylem during transpiration.

Transpiration is the loss of water vapour from the aerial parts of a plant (mainly through stomata). About 99% of the water absorbed by a plant is lost in transpiration — only ~1% is used in photosynthesis and other metabolism.

The rate of transpiration is affected by four main environmental factors:

1. Temperature.

• Higher temperature → faster transpiration.
• Reasons: (i) water molecules have more kinetic energy, so evaporation from mesophyll cell walls is faster; (ii) the saturation vapour pressure of warm air is higher, so the air can hold more water — creating a steeper water potential gradient between the inside of the leaf (saturated) and the air outside.

2. Humidity.

• Higher humidity → slower transpiration.
• Reason: humid air already contains a lot of water vapour, so the air outside the stoma has a less negative water potential. The water potential gradient between leaf and air is smaller, and water vapour diffuses out more slowly.

3. Wind speed (air movement).

• Higher wind → faster transpiration.
• Reason: wind blows away the layer of humid air at the leaf surface, maintaining a steep water potential gradient between the leaf and the surrounding air. Still air allows a humid 'boundary layer' to build up at the leaf surface, slowing transpiration.

4. Light intensity.

• Higher light → faster transpiration.
• Reason: stomata open in response to light to allow CO₂ uptake for photosynthesis. Open stomata permit more water vapour to diffuse out. Stomata close in the dark, greatly reducing transpiration at night.

Measuring transpiration: the potometer.

A potometer is the standard apparatus for measuring transpiration rate in school labs. It actually measures the rate of water uptake by a cut shoot, which is taken as approximately equal to transpiration rate.

• Cut the shoot under water to avoid air bubbles entering xylem.
• Set up the potometer with the shoot in water, an air bubble in the capillary tube, and a reservoir / syringe for resetting the bubble.
• Note the distance moved by the bubble in a measured time.
• Calculate rate = distance / time (or volume per unit time if the tube diameter is known).
• Change ONE variable at a time (e.g. increase wind speed with a fan) and measure the new rate.

Stomata are pores in the leaf epidermis (mostly on the lower surface) flanked by two specialised guard cells. They open to allow CO₂ in (and inevitably let water vapour out) and close to conserve water.

Mechanism of opening.

1. Light is detected by blue-light receptors in the guard cell membrane. (Low CO₂ inside the leaf has a similar effect.)
2. Proton pumps (H⁺ ATPases) in the guard cell membrane actively transport H⁺ OUT of the cell, using ATP from the guard cells' own chloroplasts and mitochondria.
3. This creates a negative voltage inside the cell, which drives K⁺ ions INTO the guard cell through K⁺ channels (and also Cl⁻ and uptake of malate from starch breakdown).
4. The accumulation of K⁺ (and other solutes) lowers the water potential of the guard cell cytoplasm.
5. Water enters by osmosis down the water potential gradient. The guard cells become turgid.
6. The inner wall of each guard cell (next to the stoma) is thicker and less elastic than the outer wall. As the cell becomes turgid, the thin outer wall stretches more, and the cell bends outwards into a sausage / banana shape, pulling the inner walls apart.
7. The two guard cells bend apart → stoma opens.

Mechanism of closing.

• In response to darkness, water stress (high abscisic acid signalling) or high CO₂:
• K⁺ ions leave the guard cells.
• Water potential rises, water leaves by osmosis.
• Guard cells become flaccid, the walls straighten, and the stoma closes.

Why this matters.

• Open stomata = CO₂ in, water out.
• Closed stomata = water saved, but CO₂ uptake stops → photosynthesis stops.
• The plant must balance gas exchange against water loss — a tradeoff modulated by light, humidity, CO₂ level and water stress.

Translocation is the movement of dissolved organic solutes (mainly sucrose, plus amino acids and hormones) through the phloem from sources (where they are produced or released) to sinks (where they are used or stored).

Sources include:

• Mature, photosynthesising leaves (the main daytime source of sucrose).
• Storage organs being mobilised (e.g. potato tubers in spring, when starch is broken down to sucrose and exported to growing shoots).

Sinks include:

• Growing roots, shoots, fruits and seeds.
• Storage organs being filled (e.g. developing potato tubers in late summer).

The pressure flow hypothesis (also called mass flow) is the accepted mechanism of translocation:

Stage 1 — Active loading at the source.

1. In the companion cell, proton pumps (H⁺ ATPases) in the cell-surface membrane actively pump H⁺ OUT of the cell, using ATP. This creates a steep H⁺ gradient (high outside, low inside).
2. H⁺ then flows back IN down its concentration gradient through H⁺/sucrose co-transporter proteins. These co-transporters simultaneously bring sucrose IN against its concentration gradient — a form of secondary active transport.
3. Sucrose passes from the companion cell into the adjacent sieve tube element through plasmodesmata, raising the sucrose concentration in the sieve tube at the source.

Stage 2 — Water enters by osmosis.

• The high sucrose concentration in the sieve tube lowers its water potential.
• Water enters by osmosis from the surrounding xylem and apoplast, raising the hydrostatic pressure at the source end of the sieve tube.

Stage 3 — Unloading at the sink.

• At the sink, sucrose is removed from the sieve tube — used for respiration, incorporated into cellulose for growth, or converted to starch for storage.
• The sucrose concentration drops, water potential rises, and water leaves the sieve tube back into the surrounding xylem.
• This lowers the pressure at the sink end of the sieve tube.

Stage 4 — Mass flow down the pressure gradient.

• High pressure at source + low pressure at sink → pressure gradient along the sieve tube.
• Phloem sap (sucrose solution) flows by mass flow / pressure flow from source to sink through the sieve plates, carrying dissolved sucrose with it.

Evidence supporting pressure flow:

• Aphid stylet experiments: aphids pierce a single sieve tube with their stylet; the sap continues to flow out under pressure even after the aphid is removed, confirming positive pressure in phloem (~1-2 MPa).
• Ringing experiments: removing a ring of bark (which contains phloem) blocks downward sucrose transport — sugars accumulate above the ring and the tissue below the ring withers.
• ATP-poison effect: inhibitors of respiration in companion cells block loading and stop translocation, confirming ATP-dependence at the source.

Xerophytes are plants adapted to survive in conditions of low water availability — hot deserts, sand dunes, cold deserts (tundra), and salty marshes. Their adaptations all aim to reduce transpiration while still allowing some CO₂ uptake for photosynthesis.

Key adaptations:

1. Thick waxy cuticle. The outer epidermis is covered by a thick layer of waxy cuticle (an extracellular layer of waxes and cutin). It is impermeable to water, preventing loss through the epidermal cells (cuticular transpiration). Water can leave only through the (controlled) stomata.

2. Sunken stomata in pits. Stomata are recessed into deep pits or grooves in the epidermis, often surrounded by epidermal hairs (trichomes). The pits and hairs trap a layer of still, humid air outside each stoma. This reduces the water potential gradient between the inside of the leaf and the air immediately outside the stoma → slower diffusion of water vapour out → reduced transpiration.

3. Rolled leaves. Some xerophytes (e.g. marram grass) have leaves that roll up tightly in dry conditions, with the stomata on the inner concave surface. The rolled leaf encloses a humid microclimate of trapped water vapour, again reducing the water potential gradient.

4. Reduced leaf surface area. Many xerophytes have needle-shaped leaves (pines) or even spines (cacti — modified leaves with photosynthesis carried out by the green succulent stem). This gives a low surface area : volume ratio, reducing the area available for evaporation.

5. CAM photosynthesis (Crassulacean Acid Metabolism). Cacti and other succulents open their stomata only at NIGHT, when air is cooler and more humid (smaller water potential gradient → less water lost). CO₂ taken in at night is fixed and stored as malate / malic acid, then released for photosynthesis in the day when stomata are closed.

6. Deep / extensive root systems. Some xerophytes have very deep tap roots (e.g. mesquite, several metres) to reach deep ground water; others have extensive shallow roots that maximise capture of any rare rainfall before it evaporates.

7. Other adaptations (background): silvery / hairy leaves that reflect light and reduce leaf temperature; CAM and C4 photosynthesis pathways; succulent water storage tissue in stems and leaves.

Hydrophytes (plants of waterlogged habitats) and mesophytes (plants of average conditions) provide useful contrasts in examination questions — they show the OPPOSITE adaptations (thin or absent cuticle, stomata on upper surface, large flat leaves).