Transport in Plants¶

Part of Module 3: Exchange and transport.

Plant transport is best understood as a systems topic. Water and mineral ions must move from the soil to the leaves; assimilates produced by photosynthesis must move to the tissues that need or store them. The topic combines anatomy, water relations, and energy-linked mechanisms, and it ties directly back to the exchange-surface principles established in the preceding topics.

Learning Objectives¶

Why Plants Need Transport Systems¶

Like animals, plants are multicellular organisms with a low surface area to volume ratio. Diffusion alone is too slow to meet the metabolic needs of all their cells, and substances must be moved over long distances — from root tips absorbing water to leaves conducting photosynthesis, or from leaves producing sucrose to roots and meristems consuming it. Plants have evolved two specialised vascular tissues — xylem and phloem — arranged into a connected vascular system that runs throughout the plant.

Xylem: Structure and Function¶

Xylem tissue transports water and mineral ions upward through the plant and also provides mechanical support. It is composed mainly of xylem vessels, which have several key adaptations:

• Elongated hollow tubes with no end walls: xylem vessels lose their end walls at maturity, forming continuous tubes with an unobstructed lumen for water flow.
• Dead at maturity: the cell contents are absent, leaving only the cell wall; this hollow lumen allows water to pass through without cellular resistance.
• Lignified walls: the cell walls are impregnated with lignin, a hard, waterproof polymer that provides tensile strength and resists the negative pressures (tension) generated by the transpiration pull. Lignin is deposited in patterns — annular, spiral, reticulate, or pitted — leaving non-lignified regions.
• Pits: non-lignified areas in the walls allow lateral movement of water and ions between adjacent vessels or into surrounding cells.

Distribution of Xylem in the Plant¶

In herbaceous dicotyledonous plants, xylem and phloem are distributed differently depending on the organ:

• Root: xylem forms a central cylinder (star-shaped in cross-section) surrounded by phloem. This central position provides strength as the root grows through resistant soil.
• Stem: xylem and phloem are in vascular bundles arranged in the outer region of the stem. This peripheral arrangement acts like scaffolding, resisting the bending forces exerted by wind and the weight of leaves.
• Leaf: xylem and phloem form a network of veins that gives structural support to the thin leaf lamina and distributes water and assimilates throughout the photosynthetic tissue.

Phloem: Structure and Function¶

Phloem tissue transports assimilates — mainly sucrose and amino acids — in both directions between sources and sinks. It is composed primarily of sieve tube elements and companion cells.

Sieve tube elements are arranged end to end to form continuous sieve tubes. Their adaptations reflect their transport function:

• Sieve plates with pores at each end allow the flow of sucrose solution between successive elements.
• They lack nuclei and most organelles at maturity, leaving only a thin layer of cytoplasm; this minimises resistance to flow.

Companion cells are intimately associated with sieve tube elements and are essential to their function:

• Connected to sieve tube elements through numerous plasmodesmata (pore connections between cells).
• Retain a large nucleus, many mitochondria (providing ATP for active transport), and many ribosomes (for protein synthesis).
• Because sieve tube elements lack nuclei and cannot control their own protein synthesis, companion cells maintain them metabolically and drive the active loading of assimilates.

Water Transport Through the Plant¶

Entry at Root Hair Cells¶

Water enters the plant via root hair cells, which have a large surface area for absorption. The soil solution has a higher water potential than the cytoplasm of root hair cells (due to the solutes accumulated inside), so water enters by osmosis down a water potential gradient.

Movement Across the Root Cortex: Apoplast and Symplast Pathways¶

Once inside the root, water travels through the cortex toward the xylem by two routes:

Apoplast pathway: water moves through the spaces within cell walls and between cells (the apoplast), without crossing any membranes. This is a passive route driven by the cohesive and adhesive properties of water. It is the faster of the two routes.

Symplast pathway: water moves from cell to cell through the cytoplasm, passing between adjacent cells via plasmodesmata. This pathway is driven by water potential gradients across cell membranes.

The Casparian Strip¶

The apoplast pathway is blocked at the endodermis — a single layer of cells forming the innermost layer of the root cortex — by the Casparian strip. This is a band of the waterproof substance suberin that surrounds each endodermal cell, sealing the cell walls. It forces all water out of the apoplast and into the symplast before it can enter the xylem. This is functionally significant: because water must now pass through the cell membrane of the endodermis, the plant has selective control over which ions are allowed to enter the xylem. The endodermis acts as a checkpoint.

Upward Movement: the Cohesion-Tension Theory¶

The cohesion-tension theory explains how water moves up the xylem against gravity over distances that can exceed 100 metres in tall trees:

1. Transpiration pull: water evaporates from the moist cell walls of mesophyll cells in the leaf into the leaf's air spaces, then diffuses out through open stomata (transpiration). This evaporation lowers the water potential in the leaf cells, creating tension (negative pressure) in the xylem.
2. Cohesion: hydrogen bonds between adjacent water molecules mean that the column of water in the xylem behaves as a continuous unit. The tension created at the top of the column is transmitted all the way down to the roots, pulling more water upward.
3. Adhesion: hydrogen bonding also occurs between polar water molecules and the cellulose and lignin in the xylem vessel walls. This adhesion helps water cling to the vessel walls and supports the water column against the tendency to break under tension.

The driving force for xylem transport is therefore physical and passive — it requires no metabolic energy from the plant. Root pressure (generated by active ion uptake in the endodermis, which draws water in by osmosis) can contribute to upward movement but is insufficient on its own to raise water to the top of tall plants.

Movement Within the Leaf¶

After the xylem transports water to the leaf, water exits the xylem and moves to photosynthesising cells mainly via the apoplast pathway. It evaporates from the moist surfaces of mesophyll cells into the intercellular air spaces and then diffuses out through the stomata.

Transpiration¶

Transpiration is the evaporation and diffusion of water vapour from the aerial parts of a plant, primarily through stomata. It is a direct consequence of the plant's gas exchange system: stomata must open to allow carbon dioxide to diffuse in for photosynthesis, and whenever they are open, water vapour diffuses out down its concentration gradient. Transpiration is therefore a side effect of gas exchange, not a purposeful process — though it does provide the driving force for the transpiration stream.

Factors Affecting Transpiration Rate¶

Four main environmental variables control the rate:

• Light intensity: at high light intensities, stomata open fully to maximise CO₂ uptake for photosynthesis, increasing water vapour loss.
• Temperature: at higher temperatures, water molecules have greater kinetic energy and evaporate more rapidly from mesophyll cell surfaces, increasing transpiration rate.
• Humidity: low ambient humidity maintains a steep water potential gradient between the leaf air spaces and the atmosphere, increasing the rate of diffusion of water vapour outward. High humidity reduces this gradient and slows transpiration.
• Wind speed: moving air removes the layer of humid air that accumulates around stomatal openings, steepening the water potential gradient and increasing transpiration.

Measuring Transpiration: The Potometer¶

A potometer measures the rate of water uptake by a cut plant shoot. Because nearly all the water a plant takes up is lost by transpiration, water uptake is used as a proxy for transpiration rate. The correct procedure for setting up a potometer is important:

1. Cut the shoot underwater (at a slant to maximise surface area) to prevent air entering the xylem.
2. Assemble the potometer with the shoot submerged in water.
3. Keep the capillary tube end submerged throughout.
4. Ensure the apparatus is airtight.
5. Dry the leaves and allow the shoot to acclimatise.
6. Close the tap, introduce an air bubble, and record its initial position.
7. Measure the distance the bubble moves over a set time to calculate the volume of water taken up.

The rate of water uptake can be calculated as:

Rate = (π r² × d) ÷ t

where r = radius of the capillary tube, d = distance moved by the bubble, and t = time taken.

Leaf Structure and Gas Exchange¶

The internal structure of a leaf is highly adapted for gas exchange:

• Upper epidermis with waxy cuticle: the cuticle is impermeable to water vapour, preventing water loss from the upper leaf surface.
• Palisade mesophyll cells: tightly packed, elongated cells beneath the upper epidermis, densely packed with chloroplasts for photosynthesis.
• Spongy mesophyll cells: loosely arranged cells with large intercellular air spaces that form an interconnected network allowing gases to diffuse rapidly throughout the leaf interior.
• Stomata: pores on the lower epidermis (mainly) surrounded by guard cells. They open to allow CO₂ in and O₂ out during photosynthesis, and close in dry or dark conditions to limit water loss.
• Vascular tissue (xylem and phloem): the leaf veins deliver water (xylem) and carry away assimilates (phloem).

Adaptations to Water Availability¶

Xerophytes¶

Xerophytes are plants adapted to environments with limited water availability. Without specialised adaptations they would quickly desiccate. Their structural modifications all reduce the rate of transpiration:

• Thick waxy cuticle: reduces water loss from the leaf surface between stomata.
• Rolled or folded leaves: in plants like marram grass, the leaf rolls inward in dry conditions, enclosing the stomata on the lower surface and trapping humid air around them, reducing the diffusion gradient for water vapour.
• Sunken stomata in pits: sheltered stomata are less exposed to moving air, reducing the diffusion gradient and slowing water loss.
• Leaf hairs (trichomes): trap a layer of humid air close to the leaf surface, reducing the water potential gradient between the leaf and the atmosphere.
• Reduced leaf surface area: needle-like or small leaves (as in many cacti) reduce the total area across which transpiration can occur.
• Water storage organs: succulent tissues in stems or leaves store water as a reserve during dry periods.

Hydrophytes¶

Hydrophytes are plants adapted to aquatic or very wet environments. They typically have:

• Reduced or absent cuticle (water loss is not a concern).
• Large air spaces (aerenchyma) in stems and roots to support buoyancy and provide oxygen to submerged tissues.
• Stomata on the upper leaf surface (facing the air) rather than the lower surface.
• Reduced root systems (water is readily available).

Translocation and the Mass Flow Hypothesis¶

What is Translocation?¶

Translocation is the movement of assimilates — primarily sucrose and amino acids — in the phloem from sources to sinks. Unlike xylem transport, which is predominantly passive, translocation requires metabolic energy. Sources are regions where assimilates are produced or released (green leaves, storage organs releasing stored material). Sinks are regions where assimilates are actively consumed or stored (growing roots, meristems, developing fruits and seeds, storage organs loading material).

The Mass Flow Hypothesis¶

The mass flow hypothesis proposes that translocation is driven by a pressure gradient between source and sink:

1. Active loading at the source: sucrose is actively loaded from companion cells into sieve tube elements. This uses a co-transport mechanism: H⁺ ions are actively pumped out of companion cells into the surrounding cells, creating a concentration gradient. H⁺ ions then re-enter companion cells via carrier proteins, co-transporting sucrose against its concentration gradient. Sucrose then moves from companion cells through plasmodesmata into the sieve tube elements.
2. Water enters sieve tubes by osmosis: the high concentration of sucrose in the sieve tube elements lowers their water potential. Water moves in from the adjacent xylem and companion cells by osmosis, increasing hydrostatic pressure in the sieve tubes at the source.
3. Pressure flow toward the sink: the high hydrostatic pressure at the source pushes the sucrose solution along the phloem toward the sink, where pressure is lower.
4. Unloading at the sink: sucrose is actively unloaded from sieve tube elements into companion cells and then into sink cells, where it is used in respiration, growth, or converted to storage compounds (e.g. starch). As sucrose is removed, water potential in the sieve tubes at the sink rises, and water leaves the phloem by osmosis (some returning to the xylem).

This creates the pressure gradient that sustains flow: high hydrostatic pressure at the source (loading), low hydrostatic pressure at the sink (unloading). The system is maintained by continuous active loading at the source and active unloading at the sink, both requiring ATP supplied by companion cells.

Evidence for the mass flow hypothesis includes the observation that metabolic inhibitors (which stop ATP production) halt translocation, and that the contents of phloem sieve tubes are under positive pressure (confirmed by aphid stylet experiments where severing the stylet causes phloem sap to exude).

Common Confusions¶

Xylem transport and phloem translocation are fundamentally different systems. Xylem carries water and mineral ions upward only, passively, driven by transpiration pull. Phloem carries sucrose and amino acids in both directions, actively, driven by loading and unloading.

A potometer measures water uptake, not transpiration. These are closely related but not identical — a small amount of water is used in photosynthesis and for cell expansion rather than being transpired. Exam questions about potometers often ask students to give the limitation of this method.

Transpiration is not a waste process. Although it has a cost in terms of water loss, it provides the motive force for the transpiration stream, which delivers mineral ions to all parts of the plant. The stomatal opening that enables it is also necessary for CO₂ uptake.

Key Terms¶

• Xylem: vascular tissue transporting water and mineral ions upward through the plant; provides support through lignified cell walls.
• Xylem vessel: an elongated, dead, hollow xylem cell with lignified, non-end-walled structure forming a continuous tube.
• Lignin: a hard, waterproof polymer deposited in xylem cell walls, providing strength and waterproofing.
• Pit: a non-lignified area in the xylem wall through which water and ions can move laterally.
• Phloem: vascular tissue transporting assimilates (sucrose, amino acids) in both directions between sources and sinks.
• Sieve tube element: a phloem cell connected end to end with others via perforated sieve plates; lacks nucleus and most organelles at maturity.
• Sieve plate: a perforated end wall between sieve tube elements allowing flow of assimilates.
• Companion cell: a metabolically active cell linked by plasmodesmata to a sieve tube element; provides energy and control for loading/unloading.
• Plasmodesmata: cytoplasmic channels through plant cell walls connecting adjacent cells; key to the symplast pathway and to phloem loading.
• Apoplast pathway: movement of water through cell walls and intercellular spaces, without crossing cell membranes.
• Symplast pathway: movement of water through cytoplasm from cell to cell via plasmodesmata.
• Casparian strip: a band of suberin in the walls of endodermal cells that blocks the apoplast pathway, forcing water into the symplast before it enters the xylem.
• Suberin: the waterproof substance forming the Casparian strip.
• Endodermis: the innermost layer of the root cortex, containing the Casparian strip.
• Transpiration: evaporation and diffusion of water vapour from the aerial parts of a plant, mainly through stomata.
• Transpiration stream: the continuous column of water pulled upward through the xylem by transpiration.
• Cohesion: attraction between water molecules through hydrogen bonds; holds the water column together in xylem.
• Adhesion: attraction between water molecules and the cellulose/lignin of xylem walls; helps maintain the water column.
• Cohesion-tension theory: the explanation for upward xylem transport: transpiration creates tension which, transmitted through the cohesive water column, pulls water up from the roots.
• Stomata: pores in the leaf epidermis, surrounded by guard cells, through which gas exchange and transpiration occur.
• Guard cell: a cell that changes shape (swells or shrinks) to open or close the stoma, controlling gas exchange and water loss.
• Potometer: apparatus for measuring water uptake by a cut plant shoot, used as a proxy for transpiration rate.
• Translocation: the movement of assimilates in the phloem from source to sink.
• Source: a region of a plant that produces or releases assimilates (e.g. photosynthesising leaf).
• Sink: a region that consumes or stores assimilates (e.g. growing root, meristem, fruit).
• Mass flow hypothesis: the explanation for translocation: active loading at the source creates high hydrostatic pressure in phloem; active unloading at the sink creates low pressure; the resulting gradient drives the flow of solution from source to sink.
• Xerophyte: a plant adapted to dry environments, with structural features that reduce the rate of transpiration.
• Hydrophyte: a plant adapted to aquatic or waterlogged environments.

Connected Pages¶

• 3.1.1 Exchange surfaces
• 3.1.2 Transport in animals
• 4.2.1 Biodiversity