Welcome to Plant Transport and Transpiration
Have you ever wondered how a towering oak tree or a giant redwood manages to lift hundreds of litres of water from the soil up to leaves tens of metres in the air without a mechanical pump or a beating heart? Or how sugars produced during photosynthesis in a sunlit leaf reach underground storage roots?
In this chapter of AS 2: Organisms and Biodiversity, we will explore the internal plumbing of flowering plants. We will break down the structures of xylem and phloem, follow the journey of water from the soil to the atmosphere, examine how to measure this in the lab, and discover how organic nutrients are moved to where they are needed most.
1. Vascular Tissue Distribution: Root, Stem, and Leaf
Plants have two primary transport tissues: xylem (which transports water and dissolved mineral ions upwards) and phloem (which transports dissolved organic solutes like sucrose and amino acids). How these tissues are arranged depends on the plant organ.
A. The Root (Cross-Section)
In the root, the vascular tissue is grouped in the centre within the stele (vascular cylinder). This central arrangement helps the root resist pulling strains as the plant is buffeted by wind.
• Xylem: Forms a distinct central cross or star shape (\(X\)-shape).
• Phloem: Found in separate clusters nestled between the "arms" of the xylem star.
• Endodermis: A specialized single layer of cells surrounding the stele, containing a waxy waterproof strip.
B. The Stem (Cross-Section)
In the stem, vascular tissues are organised into separate vascular bundles arranged in an outer ring near the periphery. This provides structural support and flexibility, resisting bending forces.
• Phloem: Located toward the outside (exterior) of each bundle.
• Xylem: Located toward the inside (centre) of each bundle.
• Vascular Cambium: A layer of meristematic (dividing) cells situated directly between the xylem and phloem, producing new vascular tissue.
C. The Leaf (Cross-Section of Midrib/Vein)
In the leaves, vascular bundles run through veins and the central midrib:
• Xylem: Located on the upper (adaxial) side of the bundle, closest to the upper epidermis.
• Phloem: Located on the lower (abaxial) side of the bundle, closest to the lower epidermis.
Memory Trick: To remember stem vs. leaf arrangements, remember "X is In" for the stem (xylem on the inside) and "Sky-lem" for the leaf (xylem faces up toward the sky!).
Key Takeaway: Xylem forms a central star in roots, sits on the inside of stem bundles, and lies on the upper side of leaf veins. Phloem sits between root xylem arms, on the outside of stem bundles, and on the lower side of leaf veins.
2. Structure and Function of Vascular Tissues
A. Xylem Vessels
Xylem vessels are specially adapted to transport water and dissolved mineral ions under tension (negative pressure) while providing mechanical support to the plant.
• Dead and Hollow: During development, xylem vessel elements die, lose their cell contents (protoplasm), and their end walls break down, leaving a continuous, empty, open-ended tube with minimal resistance to water flow.
• Lignified Walls: The secondary cell walls are thickened with lignin (laid down in spiral, annular/ring, or reticulate/net-like patterns). Lignin makes the walls rigid and waterproof, preventing vessels from collapsing inward under extreme negative pressure (tension) and giving structural support to the whole plant.
• Pits: These are unlignified, thin areas in the cell walls that allow lateral (sideways) movement of water between adjacent vessels, preventing blockages if an air bubble forms in one vessel.
B. Phloem Tissue
Phloem is a living tissue responsible for translocation—the transport of soluble organic assimilates (sucrose and amino acids).
• Sieve Tube Elements: Living, elongated cells joined end-to-end. Their end walls are perforated like a sieve, forming sieve plates that allow sap to flow easily from cell to cell. To create an open channel for mass flow, they lose their nucleus, vacuole, and most organelles, retaining only a thin layer of peripheral cytoplasm.
• Companion Cells: Because sieve tube elements have almost no organelles, each is connected to a metabolically active companion cell via cytoplasmic channels called plasmodesmata. Companion cells contain a large nucleus, abundant ribosomes, and dense clusters of mitochondria to generate the \(ATP\) required for the active transport of sucrose into the phloem.
Key Takeaway: Xylem consists of dead, lignified, continuous tubes for water transport. Phloem consists of living sieve tube elements supported metabolically by companion cells packed with mitochondria.
3. Water and Mineral Uptake: From Soil to Xylem
Water enters root hair cells from the soil by osmosis down a water potential (\(\Psi\)) gradient, as the cytoplasm and vacuolar sap of root hairs have a more negative water potential than the dilute soil solution.
Pathways Across the Root Cortex
Once inside the root epidermis, water and dissolved mineral ions travel across the root cortex toward the central stele via three distinct routes:
1. The Apoplast Pathway: Water and dissolved minerals travel through the porous, interconnected cellulose cell walls and intercellular spaces. Movement occurs by diffusion and mass flow without crossing any selectively permeable plasma membranes. This offers the least resistance to water movement.
2. The Symplast Pathway: Water moves through the continuous living cytoplasm of adjacent plant cells, which are interconnected by microscopic cytoplasmic channels called plasmodesmata. Water moves down a water potential gradient by osmosis.
3. The Vacuolar Pathway: Water moves directly through the cell walls, plasma membranes, cytoplasm, and vacuoles of cortical cells down a water potential gradient via osmosis. This route encounters the highest resistance.
The Casparian Strip and the Endodermis
When water moving via the apoplast pathway reaches the endodermis (the inner boundary of the cortex), it hits an impermeable barrier: the Casparian strip.
• The Casparian strip is a waxy band of suberin embedded in the radial and transverse walls of endodermal cells.
• Because suberin is completely waterproof, it blocks the apoplast pathway.
• All water and dissolved minerals are forced out of the cell walls and into the symplast pathway (crossing the selectively permeable plasma membrane of the endodermal cells).
Why is this essential for the plant?
• Quality Control: Forcing water across a living, selectively permeable membrane allows the plant to regulate which mineral ions enter the xylem and prevents toxic substances from entering.
• Generating Root Pressure: Endodermal cells actively pump mineral ions into the xylem vessels. This lowers the water potential (\(\Psi\)) inside the xylem, causing water to follow by osmosis. This builds up a positive hydrostatic pressure called root pressure, which helps push water up the stem over short distances.
Key Takeaway: The apoplast pathway travels through cell walls; the symplast pathway travels through cytoplasm via plasmodesmata. The waxy Casparian strip blocks the apoplast, forcing all water into the symplast to allow selective uptake and create root pressure.
4. Transpiration and the Cohesion-Tension Theory
What is Transpiration?
Transpiration is defined as the loss of water vapour from the upper parts of a plant, primarily through the stomata of the leaves.
The Mechanism: Cohesion-Tension Theory
While root pressure provides a small upward push, the primary force pulling water to the top of tall plants is generated at the leaves. This is explained by the Cohesion-Tension Theory:
1. Evaporation: Solar heat energy evaporates water from the wet cell walls of spongy mesophyll cells into the leaf's sub-stomatal air spaces.
2. Diffusion: Water vapour diffuses out of the leaf into the surrounding drier atmosphere through open stomata, moving down a water vapour potential gradient.
3. Tension (Transpiration Pull): The loss of water from mesophyll cells lowers their water potential (\(\Psi\)). Water is drawn from neighboring cells and eventually pulled out of the xylem vessels in the leaf veins by osmosis. This creates a negative hydrostatic pressure (tension) at the top of the xylem column.
4. Cohesion: Water molecules are polar and form strong hydrogen bonds with one another. This attractive force between like molecules is called cohesion. Because of cohesion, water forms a continuous, unbroken column from the roots all the way up to the leaves.
5. Adhesion: Water molecules also form hydrogen bonds with the hydrophilic cellulose and lignin in the xylem vessel walls. This attraction between unlike substances is called adhesion, which prevents the water column from breaking or pulling away from the vessel walls.
As water evaporates at the leaf surface, the tension pulls the entire cohesive column of water upward—much like sucking a drink through a straw!
Everyday Analogy: Think of the water column in xylem like a tug-of-war rope. Cohesion is the fibers of the rope holding tightly to each other so the rope doesn't snap. Adhesion is the friction of the rope against your hands. Tension is you pulling the rope upward from the top.
Environmental Factors Affecting Transpiration Rate
• Temperature: Increases rate. Warmer temperatures give water molecules more kinetic energy, accelerating evaporation from mesophyll cells. It also lowers the relative humidity of the air outside the leaf, steepening the water vapour potential gradient.
• Humidity: Decreases rate. High humidity in the external air decreases the water vapour potential gradient between the leaf air spaces and the atmosphere, slowing diffusion.
• Wind Speed / Air Movement: Increases rate. Air currents blow away the humid "boundary layer" of still air directly outside the stomata, maintaining a steep concentration gradient.
• Light Intensity: Increases rate. Light stimulates stomata to open wide to allow carbon dioxide in for photosynthesis, providing an exit route for water vapour.
Key Takeaway: Transpiration creates tension (negative pressure) in the leaf. Water's cohesive properties maintain an unbroken column, pulling water upward through the xylem.
5. Measuring Water Uptake: The Potometer
A potometer is an apparatus used in the laboratory to measure the rate of water uptake by a leafy shoot. Under steady conditions, the rate of water uptake is assumed to be approximately equal to the rate of transpiration.
Essential Practical Precautions
Examiners frequently test the setup of a potometer. Make sure you know these four essential rules:
1. Cut the leafy shoot underwater and at an angle: Cutting underwater prevents air bubbles from entering the xylem vessels and breaking the cohesive water column. Cutting at an angle provides a larger surface area for water uptake.
2. Assemble the entire apparatus underwater: Ensures that no stray air bubbles are trapped within the capillary tubing.
3. Seal all joints with waterproof petroleum jelly (Vaseline): Ensures the entire system is completely airtight and watertight so water movement is due solely to plant uptake.
4. Dry the leaves before taking readings: Any moisture left on the leaves will artificially reduce transpiration by flattening the water vapour gradient.
Calculating Transpiration Rate
As the plant takes up water, an air bubble inside a calibrated capillary tube moves along a ruler scale.
The volume of water taken up (\(V\)) is calculated as the volume of a cylinder:
\(V = \pi r^2 d\)
Where:
• \(r\) = internal radius of the capillary tube
• \(d\) = distance moved by the air bubble
To calculate the rate of water uptake (rate of transpiration):
\(\text{Rate} = \frac{\text{Volume of water taken up}}{\text{Time taken}} = \frac{\pi r^2 d}{t}\)
Where \(t\) is the time taken for the bubble to travel distance \(d\).
Examiner Warning: Never say a potometer directly measures "transpiration rate." It measures water uptake. A small fraction (typically \(< 1\text{--}2\%\)) of absorbed water is retained by the plant for photosynthesis and maintaining cell turgidity.
Key Takeaway: Potometers measure the rate of water uptake. Precise experimental setup (cutting underwater, airtight seals) is vital to keep the water column continuous.
6. Xerophytic Adaptations
Xerophytes are plants adapted to live in dry, arid habitats where liquid water is scarce (such as sand dunes, deserts, or frozen soils). They possess specialized structural adaptations to minimise water loss by transpiration:
• Thick Waxy Cuticle: Increases the diffusion distance and provides a waterproof barrier to reduce cuticular transpiration (e.g., holly, marram grass).
• Sunken Stomata (in Pits or Grooves): Traps moist, still air outside the pore, reducing the water vapour potential gradient between the leaf interior and the pit.
• Epidermal Hairs (Trichomes): Trap a layer of humid air next to the leaf surface, reducing the concentration gradient and slowing diffusion.
• Curled / Rolled Leaves (e.g., Ammophila / Marram Grass): Encloses the lower epidermis (where most stomata are found) in an internal chamber, trapping humid air and shielding stomata from wind.
• Reduced Leaf Surface Area: Leaves modified into spines or needles reduce the surface-area-to-volume ratio, leaving fewer stomata available for water loss (e.g., cacti, pine trees).
Key Takeaway: Xerophytes reduce transpiration by increasing diffusion distance, trapping a humid boundary layer, and reducing exposed surface area.
7. Translocation of Organic Solutes
Translocation is the transport of soluble organic products of photosynthesis (mainly sucrose and amino acids) through the phloem from sources to sinks.
• Source: Plant organ that produces more assimilates than it requires (e.g., mature green leaves photosynthesising in summer, or storage roots/tubers releasing stored sugars in spring).
• Sink: Plant organ that consumes or stores assimilates (e.g., growing root tips, shoot apical meristems, developing flowers, seeds, and fruits).
Direction of Flow: Translocation in phloem is bidirectional. Unlike xylem (which only transports water upward), phloem sap can travel upward or downward depending on the relative positions of the source and sink tissues.
The Mass Flow Hypothesis: Step-by-Step
Step 1: Phloem Loading at the Source
• Companion cells actively pump hydrogen ions (\(\text{H}^+\)) out into the surrounding cell walls using \(ATP\).
• This creates a high concentration of \(\text{H}^+\) ions in the cell wall.
• \(\text{H}^+\) ions diffuse back into the companion cell through specialized co-transporter carrier proteins, bringing sucrose with them against its concentration gradient.
• Sucrose then diffuses down a concentration gradient from companion cells into the sieve tube elements through connecting plasmodesmata.
Step 2: Generation of High Hydrostatic Pressure
• The accumulation of sucrose in the sieve tube element significantly lowers its water potential (\(\Psi\)).
• Water moves from the adjacent xylem vessel into the sieve tube by osmosis down the water potential gradient.
• The 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
• Sap moves through the sieve tubes from the region of high hydrostatic pressure (source) to the region of low hydrostatic pressure (sink) by mass flow.
Step 4: Phloem Unloading at the Sink
• At the sink, sucrose is actively or passively removed from sieve tube elements into surrounding cells to be used in cellular respiration or converted into insoluble storage compounds like starch.
• The removal of sucrose increases the water potential (\(\Psi\)) inside the sieve tube.
• Water moves out of the sieve tube into the adjacent xylem vessels by osmosis.
• This loss of water lowers the hydrostatic pressure inside the sieve tube at the sink, maintaining the pressure gradient that drives continuous mass flow.
Key Takeaway: Translocation operates by mass flow driven by hydrostatic pressure differences between the source (high pressure due to active loading of sucrose) and the sink (low pressure due to unloading of sucrose).
8. Summary of Common Pitfalls and Examiner Tips
• Xylem Pull vs. Push: Transpiration provides a pulling force (tension/negative pressure) from the top of the plant, not a pushing force from below (except for minor root pressure).
• Potometer Function: Always state that a potometer measures the rate of water uptake, not directly the rate of transpiration.
• The Casparian Strip Transition: Remember that water travelling through the apoplast pathway must cross the endodermal cell membrane and enter the symplast pathway because the Casparian strip is made of impermeable suberin.
• Phloem Directionality: Phloem transport is bidirectional (up or down to different sinks), whereas xylem transport is strictly unidirectional (roots to leaves).
• Stem vs. Root Anatomy: Be precise when identifying vascular tissue under a microscope: root xylem is the central \(X\)-star; stem xylem is on the inner side of the outer ring of bundles.