Introduction to Transport in Plants
Welcome to your study notes for Transport in Plants and Transpiration! If you have ever wondered how a towering oak tree pulls hundreds of litres of water from the soil up to its highest leaves without a mechanical pump or heart, you are in the right place. Plants have evolved an elegant, efficient transport network consisting of two main vascular tissues: xylem (which carries water and dissolved mineral ions upward) and phloem (which moves sugars and amino acids wherever they are needed).
Don't worry if plant anatomy feels a bit overwhelming at first. We will break down every structure, pathway, and mechanism step-by-step so you can master the CCEA AS 2 specification with confidence!
1. Plant Anatomy & Transport Tissues
To understand how substances move through a plant, we first need to look at the internal architecture of the root and stem in dicotyledonous (dicot) plants.
Dicotyledonous Root (Transverse Section)
From the outside to the centre, a dicot root contains the following layers:
• Epidermis: A single outer layer of cells. Many of these cells extend into long, thin extensions called root hairs, which dramatically increase the surface area for the uptake of water and mineral ions.
• Cortex: A thick region made of unspecialised parenchyma cells. These cells store starch and provide a pathway with plenty of air spaces for water movement.
• Endodermis: A single-cell thick ring surrounding the central vascular tissue. Its cell walls contain a waterproof, waxy band called the Casparian strip (made of suberin). Older endodermal cells may become heavily suberised, leaving special passage cells to allow water through.
• Pericycle: A layer of meristematic cells just inside the endodermis. This layer is responsible for giving rise to lateral (side) roots.
• Vascular Arrangement: At the very centre of the root, xylem forms a distinct 'X' or star-shaped central core. Phloem is located in clusters between the arms of the xylem star.
Dicotyledonous Stem (Transverse Section)
In the stem, the arrangement of tissues is quite different from the root:
• Vascular Bundles: Instead of a single central star, transport tissues are grouped into separate vascular bundles arranged in a neat ring near the outer edge of the stem.
• Xylem: Positioned on the inside of each vascular bundle.
• Phloem: Positioned on the outside of each vascular bundle.
• Cambium: A layer of meristematic cells situated between the xylem and phloem that can divide to produce new vascular tissue.
• Sclerenchyma / Collenchyma Fibres: Tough supporting tissue forming a protective cap on the outer edge of each bundle, providing mechanical support to keep the stem upright.
Memory Trick for Tissue Placement:
In the stem: Xylem is on the INside (think: eXit is out, but Xylem is in!), while Phloem is on the Periphery (outside).
Xylem: Structure and Function
Xylem tissue is responsible for transporting water and dissolved mineral ions upwards from the roots to the leaves. It also provides structural support.
• Xylem Vessel Elements: These are elongated, dead cells arranged end-to-end. Their end walls break down completely to form continuous, hollow, non-living tubes (like unbroken pipes).
• Lignification: Cell walls are heavily thickened and reinforced with lignin, deposited in spiral, annular (ring), or reticulate (net-like) patterns. Lignin makes the walls waterproof and provides immense mechanical strength, preventing the vessels from collapsing inwards under high negative pressure (tension).
• Pits: Small, unlignified thin areas in the cell walls that allow lateral (sideways) movement of water between adjacent xylem vessels, bypassing any air locks or blockages.
Phloem: Structure and Function
Phloem tissue transports organic solutes (assimilates), primarily sucrose and amino acids, throughout the plant.
• Sieve Tube Elements: Living cells arranged end-to-end to form long tubes. To allow easy fluid flow, they have very few organelles (no nucleus, no vacuole, and only a thin rim of cytoplasm). The end walls are perforated, forming sieve plates with open pores.
• Companion Cells: Structurally and metabolically connected to sieve tube elements via microscopic cytoplasmic channels called plasmodesmata. Companion cells have dense cytoplasm packed with mitochondria and ribosomes to generate the \( \text{ATP} \) needed for active phloem loading.
Key Takeaway: Xylem consists of dead, lignified, hollow tubes for water transport under tension. Phloem consists of living sieve tube elements supported by metabolically active companion cells for organic solute transport.
2. Water & Mineral Uptake Across the Root
Before water can travel up the stem, it must first be absorbed from the soil and cross the root cortex to reach the xylem.
Step 1: Uptake into Root Hair Cells
1. Plant roots actively pump mineral ions from the soil into root hair cells against their concentration gradient using \( \text{ATP} \).
2. The accumulation of mineral ions significantly lowers the water potential (\( \Psi \)) inside the root hair cell cytoplasm and vacuole.
3. Because the water potential of the soil water is higher than that of the root cell sap (\( \Psi_{\text{soil}} > \Psi_{\text{root}} \)), water moves into the root hair cell down a water potential gradient by osmosis.
Step 2: Pathways Across the Root Cortex
Once inside the root hair cell, water moves across the parenchyma cells of the cortex toward the central stele via three distinct routes:
• 1. The Apoplast Pathway: Water moves exclusively through the non-living cell walls and intercellular spaces by diffusion and mass flow. This route offers the lowest resistance to water flow.
• 2. The Symplast Pathway: Water moves through the continuous network of living cytoplasm from cell to cell, passing through connecting channels called plasmodesmata by osmosis and diffusion.
• 3. The Vacuolar Pathway: Water moves from vacuole to vacuole across cell membranes and tonoplasts (vacuolar membranes) by osmosis. This route encounters multiple membranes and therefore offers the highest resistance.
Step 3: The Endodermis and the Casparian Strip
The vast majority of water travels rapidly through the cortex via the apoplast pathway. However, when water reaches the endodermis, it hits an impassable barrier: the Casparian strip.
• The Casparian strip is a band of waterproof, waxy suberin embedded in the radial and transverse cell walls of endodermal cells.
• It completely blocks the apoplast pathway.
• Water and dissolved minerals are forced out of the cell walls and across the selectively permeable plasma membrane into the symplast pathway.
Why is the Casparian Strip so important?
• Selectivity & Protection: By forcing all water across a living plasma membrane, the plant can control exactly which minerals enter the xylem and block potentially toxic substances.
• Root Pressure Generation: Endodermal cells actively pump mineral ions into the central xylem vessels. This lowers the water potential in the xylem, causing water to follow by osmosis. This influx builds up positive hydrostatic pressure (root pressure), pushing water upwards.
Common Examiner Trap: Never say the Casparian strip blocks water entry into the xylem! It only blocks the apoplast pathway, redirecting water into the symplast pathway so the plant can regulate it.
Key Takeaway: Water enters root hairs by osmosis following active mineral uptake, moves across the cortex mainly via the apoplast, and is diverted into the symplast by the suberised Casparian strip at the endodermis.
3. Ascent of Sap: Water Transport Up the Stem
How does water travel dozens of metres upward against gravity? The primary mechanism is explained by the Cohesion-Tension Theory.
The Cohesion-Tension Theory (Step-by-Step)
• 1. Transpiration Pull: Solar heat causes water to evaporate from the surfaces of mesophyll cells into the sub-stomatal air spaces, and diffuse out through stomata into the atmosphere. This loss of water lowers the water potential (\( \Psi \)) of mesophyll cells.
• 2. Tension Generation: Water is drawn out of xylem vessels in the leaf veins by osmosis down a water potential gradient to replace the lost water. This creates negative hydrostatic pressure (tension) at the top of the xylem vessels.
• 3. Cohesion: Water molecules are polar and form hydrogen bonds with one another. This strong attraction between water molecules is called cohesion. Because of cohesion, water forms an unbroken, continuous column throughout the entire xylem tube.
• 4. Mass Flow Upward: The tension created in the leaves pulls the entire continuous column of water upwards from the roots to the leaves—just like sucking liquid up through a drinking straw!
• 5. Adhesion: Water molecules also form hydrogen bonds with the hydrophilic components (such as cellulose and lignin) of the xylem vessel walls. This attraction is called adhesion. Adhesion helps support the weight of the water column and prevents it from breaking.
Supplementary Mechanisms
While the cohesion-tension mechanism is the main driver of water movement, two minor forces contribute:
• Root Pressure: The active secretion of mineral ions into the root xylem lowers \( \Psi \), drawing water in by osmosis and generating positive hydrostatic pressure that pushes sap up. This can cause guttation (the exudation of water droplets on leaf margins) in low-light, high-humidity conditions.
• Capillarity: In very narrow xylem vessels, adhesive forces between water molecules and the vessel walls pull water upwards a short distance.
Quick Summary Table: Cohesion vs. Adhesion
• Cohesion: Water molecule attracted to water molecule (via hydrogen bonds) \( \rightarrow \) maintains continuous column.
• Adhesion: Water molecule attracted to xylem vessel wall \( \rightarrow \) supports column against gravity.
Key Takeaway: The Cohesion-Tension Theory relies on evaporation from leaves (transpiration pull) creating tension, which draws up a continuous column of water held together by cohesion and supported by adhesion.
4. Transpiration & Measuring Water Loss
Definition of Transpiration
Transpiration is the loss of water vapour from the aerial parts of a plant (mainly the leaves) by evaporation from mesophyll cell surfaces into sub-stomatal air spaces, followed by diffusion through open stomata down a water vapour concentration gradient.
Environmental Factors Affecting Transpiration Rate
• Light Intensity: Increasing light intensity increases the rate of transpiration. Stomata open in the light to allow \( \text{CO}_2 \) uptake for photosynthesis, providing more pathways for water vapour to diffuse out.
• Temperature: Higher temperatures increase transpiration. Heat gives water molecules more kinetic energy, increasing the rate of evaporation from mesophyll surfaces, while also lowering the relative humidity of external air, steepening the diffusion gradient.
• Humidity: Higher external humidity decreases transpiration. Humid air contains a high concentration of water vapour, which reduces (flattens) the water vapour concentration gradient between the sub-stomatal air spaces and the surrounding air.
• Air Movement (Wind): Increased air movement increases transpiration. Wind blows away the layer of humid, still air (the boundary layer) sitting directly outside open stomata, maintaining a steep concentration gradient.
Investigating Water Uptake: The Potometer
A potometer is an apparatus used in the laboratory to investigate water movement in a cut leafy shoot.
Crucial CCEA Exam Point: A potometer measures the rate of water uptake, NOT directly the rate of transpiration! Water uptake is only an estimate of transpiration because a small amount of absorbed water is consumed in photosynthesis or retained to maintain cell turgidity.
Essential Precautions When Setting Up a Potometer:
• Cut the shoot at a slant under water: Cutting under water prevents air bubbles from being sucked into the xylem vessels (which would break the continuous water column). Cutting at a slant increases the surface area available for water uptake.
• Assemble the apparatus under water: Ensures all tubing is completely filled with water and free of unwanted air locks.
• Ensure all joints are airtight and watertight: Smear joints with petroleum jelly (Vaseline) to prevent air entering or water leaking.
• Dry the leaves before starting: Surface water on the leaves blocks stomata and artificially reduces transpiration.
• Introduce a single air bubble: Carefully introduce one air bubble into the capillary tube to track the movement of the meniscus along the scale over time.
Calculating the Rate of Water Uptake:
The volume of water absorbed over time is calculated using the volume of a cylinder:
\( \text{Volume of water taken up } (V) = \pi r^2 \times d \)
Where:
• \( r \) = internal radius of the capillary tube
• \( d \) = distance moved by the air bubble
The rate is then:
\( \text{Rate of water uptake} = \frac{\pi r^2 \times d}{t} \)
Where \( t \) = time taken for the bubble to travel distance \( d \).
Key Takeaway: Transpiration is the passive diffusion of water vapour from leaves. A potometer measures the rate of water uptake, which closely approximates transpiration under controlled conditions.
5. Plant Adaptations to Water Availability
Plants are classified into ecological groups based on the water availability of their natural habitat:
1. Mesophytes
Plants adapted to moderate, adequate water supplies (e.g., common garden plants, deciduous trees). They easily replace water lost by transpiration under normal conditions.
2. Xerophytes
Plants adapted to dry, arid conditions where water is scarce. They possess specialised adaptations to minimise water loss:
• Thick Waxy Cuticle: Provides an impermeable, waterproof barrier over the epidermis to reduce cuticular evaporation.
• Sunken Stomata in Pits or Grooves: Traps a layer of moist, humid air outside the stomata, reducing the water vapour concentration gradient.
• Rolled or Curled Leaves (e.g., Marram grass Ammophila): Encloses the lower epidermis (where stomata are located) to create a humid microclimate protected from wind.
• Epidermal Hairs / Trichomes: Trap a boundary layer of still, moist air next to the leaf surface.
• Reduced Leaf Surface Area: Leaves modified into spines or needles (e.g., cacti, pine trees) reduce the surface-area-to-volume ratio for evaporation.
• Extensive Root Systems: Either very deep taproots to reach low water tables or extensive, shallow root systems to rapidly absorb rainfall.
3. Hydrophytes
Plants adapted to living partially or completely submerged in water, or floating on water (e.g., the water lily Nymphaea):
• Stomata Restricted to the Upper Epidermis: Floating leaves have stomata only on their upper surface so gas exchange can occur directly with the atmosphere rather than the water below.
• Reduced Cuticle and Minimal Xylem: Water loss is not a hazard, so thick cuticles and extensive water-conducting tissues are unnecessary.
• Aerenchyma Tissue: Large, spongy internal air spaces throughout the leaf and stem that provide buoyancy to keep leaves afloat and facilitate rapid internal diffusion of oxygen and carbon dioxide.
Key Takeaway: Xerophytes feature structural adaptations (thick cuticles, sunken stomata, rolled leaves) to conserve water, while hydrophytes feature aerenchyma and upper stomata to cope with an aquatic environment.
6. Translocation in Phloem
Definition of Translocation
Translocation is the mass transport of soluble organic assimilates (mainly sucrose and amino acids) through the phloem sieve tubes from sources to sinks.
• Source: A plant organ that produces or releases assimilates (e.g., mature photosynthesising leaves, or storage organs such as root tubers in early spring).
• Sink: A plant organ that consumes or stores assimilates (e.g., growing root tips, developing flower buds, fruits, seeds, and new shoots).
The Mass Flow Hypothesis (Münch Mechanism)
The widely accepted model for translocation involves mass flow driven by hydrostatic pressure gradients:
Step 1: Active Phloem Loading at the Source
• Companion cells use \( \text{ATP} \) to actively pump hydrogen ions (\( \text{H}^+ \)) out of their cytoplasm into the surrounding cell walls using a \( \text{H}^+ \)-ATPase proton pump.
• A high concentration gradient of \( \text{H}^+ \) is created outside the cell.
• Hydrogen ions diffuse back into the companion cell through specialised \( \text{H}^+ \)/sucrose co-transporter proteins, bringing sucrose molecules with them against the sucrose concentration gradient.
• Sucrose then diffuses into the adjacent sieve tube elements via plasmodesmata.
Step 2: Generation of High Hydrostatic Pressure at the Source
• The accumulation of sucrose significantly lowers the water potential (\( \Psi \)) inside the sieve tube element.
• Water enters the sieve tube from the adjacent xylem by osmosis.
• This influx of water increases the liquid volume, generating a high hydrostatic pressure inside the sieve tube at the source.
Step 3: Mass Flow to the Sink
• The phloem sap is pushed down the sieve tube along a hydrostatic pressure gradient (from the high-pressure source to the low-pressure sink).
Step 4: Phloem Unloading at the Sink
• Sucrose is unloaded from the sieve tube into sink cells (either actively or passively), where it is rapidly utilised for respiration or converted into insoluble storage compounds like starch.
• The removal of sucrose increases (raises) the water potential (\( \Psi \)) inside the sieve tube at the sink.
• Water exits the sieve tube by osmosis and moves back into the xylem vessels.
• This loss of water creates a low hydrostatic pressure at the sink, maintaining the pressure gradient from source to sink.
Experimental Evidence for Translocation
• Ringing Experiments: Removing a complete outer ring of bark (which contains the phloem, leaving the inner xylem intact) causes the tissue immediately above the ring to swell. Chemical analysis shows this swelling is packed with accumulated sucrose. Over time, tissues below the ring (roots) starve and die because sugars cannot travel downward.
• Aphid Stylet Experiments: Aphids insert needle-like mouthparts (stylets) directly into single phloem sieve tubes. When the aphid's body is severed from its stylet, phloem sap continues to exude under positive hydrostatic pressure from the cut stump, showing that phloem contents are under pressure.
• Radioactive Tracers (\( ^{14}\text{CO}_2 \)): Exposing leaves to radioactive carbon dioxide (\( ^{14}\text{CO}_2 \)) results in the synthesis of \( ^{14}\text{C} \)-labelled sucrose. Placing photographic film over plant sections (autoradiography) reveals that radioactivity is confined strictly to the phloem and demonstrates bidirectional movement (upward to growing shoots and downward to roots).
Limitations of the Mass Flow Hypothesis:
While mass flow accounts for overall transport, it does not fully explain why different solutes move at different velocities, or how bidirectional movement occurs simultaneously in the same sieve tube element (though bidirectional transport easily occurs in separate, adjacent sieve tubes).
Key Takeaway: Translocation is the active loading of sucrose at sources, creating high hydrostatic pressure that drives mass flow of sap along phloem sieve tubes to low-pressure sinks.
Quick Review: Common Pitfalls to Avoid
• Xylem vs. Phloem Energy: Transpiration pull in xylem is a passive physical process powered by solar heat. Translocation in phloem requires active transport (\( \text{ATP} \)) for loading and unloading sucrose.
• Potometer Precision: Always state that a potometer measures the rate of water uptake, which serves as an estimate of transpiration.
• Root vs. Stem Diagrams: In roots, xylem is the central 'X' shape. In stems, xylem is on the inner portion of vascular bundles arranged in an outer ring.
• Casparian Strip Target: Remember that the Casparian strip blocks the apoplast pathway only, forcing water into the symplast pathway.