Plant Transport — A-Level Biology Revision Notes

Root and Stem Histology

Understanding the internal structure of roots and stems is essential for explaining how water and dissolved substances move through plants. Both structures are organised into distinct tissue layers, each with specific functions in transport and support.

Structure of a Dicotyledonous Root

A cross-section through a dicotyledonous root reveals several concentric tissue layers, from the outside inwards:

The epidermis is the outermost tissue layer, in direct contact with the soil. Some epidermal cells extend outwards to form root hair cells, which dramatically increase the surface area available for water and mineral ion absorption.

The cortex is the region between the epidermis and the endodermis, packed with parenchyma cells — non-specialised living cells that provide support and create the pathway for water movement across the root.

The endodermis is the outermost layer of the vascular bundle. Endodermal cells have a band of suberin deposited in their cell walls, forming the Casparian strip. In young plants, the Casparian strip is narrow, but as the plant ages, more suberin is deposited. Specialised passage cells with narrow Casparian strips allow some water movement through the endodermis even in older plants.

The pericycle lies between the endodermis and the xylem/phloem. It is a living tissue that retains meristematic activity — the pericycle is the origin of the lateral root system.

Xylem Tissue — Structure and Cell Types

The xylem is composed of several cell types working together for water transport and structural support:

Tracheids are dead, elongated, lignified cells with empty lumens and tapered end walls that overlap with adjacent tracheids. Regions of the cell wall that are not lignified form pits — clusters of plasmodesmata that allow lateral water movement between adjacent cells. Tracheids provide mechanical strength and give rise to xylem vessels and fibres.

Xylem vessels are formed when neighbouring vessel elements (derived from tracheids) fuse together as their end walls break down, creating continuous hollow tubes. The first xylem vessels to form are protoxylem — these have annular and spiral lignin thickening patterns, allowing them to elongate as the plant grows. Mature xylem vessels are called metaxylem — these are fully lignified with reticulate thickening, making them dead, rigid, and unable to stretch.

Xylem fibres are shorter and narrower than tracheids with much thicker walls. They have pits but do not conduct water — their primary function is providing additional mechanical strength.

Structure of a Dicotyledonous Stem

The stem shares some tissue types with the root but has a different arrangement. From the outside inwards, you can identify: the cuticle (a waxy outer layer), the epidermis, the cortex containing parenchyma cells, the vascular bundles arranged in a ring (each containing xylem, phloem, and cambium between them), sclerenchyma cells of the pericycle forming a supportive cap over each vascular bundle, and the central pith containing parenchyma cells.

Water Transport Across the Root

Water Uptake by Root Hair Cells

Water enters the root by a two-step process. First, potassium ions dissolved in the soil water are actively transported into the root hair cell. This lowers the water potential of the root hair cytoplasm below that of the water surrounding the soil particles. Water then enters the root hair cell by osmosis, moving down a water potential gradient.

The Apoplast and Symplast Pathways

Once water has entered the root hair cells, it moves across the cortex towards the xylem via two main pathways:

The apoplast pathway accounts for approximately 90% of water movement across the root. Water moves through the cell walls and intercellular spaces of the parenchyma cells — the non-living parts of the cells. This pathway is fast because water does not need to cross any cell membranes.

The symplast pathway moves water through the cytoplasm of the parenchyma cells, passing from cell to cell via plasmodesmata (tiny cytoplasmic channels connecting adjacent cells). This represents the living part of the cell.

Creating and Maintaining the Water Potential Gradient

For water to move continuously across the root by osmosis, a water potential gradient must be maintained from the epidermis to the pericycle. This gradient is created by active transport:

Water Transport Up the Xylem

Three mechanisms contribute to moving water upwards through the xylem from roots to leaves. Two have minor effects, while the third is the primary driving force.

Root Pressure (Minor Mechanism)

Root pressure is a pushing force generated when water enters the xylem vessel from the pericycle. Experimental evidence shows that when all leaves are removed from a plant and the stem is cut to a stump, water can be seen exuding from the cut surface — pushed up by root pressure alone. When cyanide (a respiratory inhibitor) is added to the roots, water exudation stops, demonstrating that root pressure depends on aerobic respiration and the active transport of ions it powers. A manometer attached to the cut stem can measure this pressure directly.

Capillarity (Minor Mechanism)

Capillarity is a pulling force that depends on two types of intermolecular interaction. Cohesive forces (hydrogen bonds between water molecules) hold the water column together. Adhesive forces (attractions between water molecules and the lignified xylem wall) pull water up the vessel surface, creating the curved meniscus visible at the water surface.

The distance water can travel by capillarity alone is inversely proportional to the vessel radius. The typical radius of a xylem vessel is approximately 75 µm, which limits capillary rise to just 0.02 m — far too little to explain water reaching the canopy of a tree. However, adhesive forces do play an important role in preventing the water column from being pulled down by gravity.

The Cohesion-Tension Theory (Primary Mechanism)

The cohesion-tension theory is the main explanation for how water moves up the xylem in tall plants. It depends on three key properties of water and one driving force:

Evidence for the cohesion-tension theory includes the measurable reduction in tree trunk diameter during the day (when transpiration is highest, the tension in the water column is strong enough to pull the tree tissues inwards) and the direct measurement of high negative pressures within the xylem using pressure probes.

Transpiration and Factors Affecting Transpiration Rate

Before water vapour can leave the leaf, water must first evaporate from the surface of the spongy mesophyll cells. This water vapour accumulates in the sub-stomatal air space created by the irregular shapes of the spongy mesophyll cells. The water vapour then diffuses out through the stomatal pore, down a water vapour concentration gradient.

As water vapour exits the stomata, it enters a still layer of water vapour surrounding the leaf called the diffusion shell. The diffusion shell acts as a barrier to transpiration — its thickness directly influences how quickly water vapour can diffuse away from the leaf surface.

The Four Factors Affecting Transpiration Rate

1. Atmospheric Humidity

Low humidity means the atmosphere contains relatively little water vapour, so the diffusion shell remains thin. The concentration of water vapour inside the leaf is much higher than outside, creating a steep diffusion gradient and a high transpiration rate. High humidity produces a thicker diffusion shell and a shallower gradient, reducing transpiration.

2. Wind Speed (Air Movement)

At low wind speeds, the diffusion shell remains intact around the leaf, creating a shallow gradient. As wind speed increases, the diffusion shell is blown away, exposing the stomata to drier air and creating a much steeper diffusion gradient — this increases the transpiration rate.

3. Light Intensity

Light intensity controls the degree to which stomata open. During the day when light intensity is high, stomata open wide to allow carbon dioxide in for photosynthesis — this also increases water loss by transpiration. At night, low light intensity causes stomata to close, significantly reducing transpiration rate.

4. Temperature

Temperature determines the amount of heat energy available for evaporation. Higher temperatures provide more kinetic energy to water molecules on the mesophyll cell surfaces, increasing the rate of evaporation. How this translates to overall transpiration rate depends on the interaction with humidity, wind speed and light intensity.

Transpiration vs Evaporation — A Classic Comparison

A porous pot experiment demonstrates the difference between transpiration (a biological process regulated by stomata) and simple evaporation (a purely physical process). A porous pot has small holes of fixed diameter that cannot change. A leaf has stomata whose diameter changes in response to light intensity.

Over a 24-hour period: during early morning and evening (when light intensity is low and stomata are closed), the porous pot loses more water because its pores remain the same size while the stomata are narrow or closed. During the middle of the day (high light intensity), the leaf loses more water because the stomata are wide open. This demonstrates that transpiration is a regulated biological process, not just passive evaporation.

Measuring Transpiration — The Potometer

A potometer is the standard apparatus for investigating transpiration rate in the laboratory. It measures the rate of water uptake by a plant shoot — which closely correlates with transpiration rate, though it is not identical.

Setting Up a Potometer

The potometer and plant shoot must be assembled under water to prevent air bubbles entering the xylem (which would break the water column). The shoot is cut under water, inserted into the potometer, and all joints are sealed with grease. An air bubble is introduced into the capillary tube, and the apparatus is removed from the water and dried.

Calculating Water Uptake

Two key equations are used with potometer data:

Volume of water uptake: V = πr²h, where r is the radius of the capillary tube and h is the distance the air bubble has moved.

Rate of water uptake: R = V ÷ t, where V is the volume calculated above and t is the time taken for the bubble to move that distance. Units are typically cm³ min⁻¹ or mm³ s⁻¹.

Investigating the Effect of Wind Speed on Transpiration

A typical potometer experiment tests the effect of wind speed (the independent variable) on the distance the air bubble moves in one minute (the dependent variable). Different wind speeds can be achieved using a variable speed fan or by positioning the fan at different distances from the shoot. Controlled variables include: the same species, number of leaves, leaf surface area, humidity, temperature, and light intensity. Each condition should be repeated at least three times to calculate a reliable mean.

Plant Adaptations — Xerophytes, Hydrophytes and Mesophytes

Plants are classified by how they are adapted to manage water availability in their environment. Understanding these adaptations requires you to connect leaf structure directly to function — examiners expect this link in every answer.

Xerophytes — Adaptations to Dry Environments

Xerophytic adaptations reduce water loss through three general mechanisms: reducing transpiration rate, storing water, and resisting desiccation.

Hydrophytes — Adaptations to Aquatic Environments

The water lily demonstrates several key hydrophytic adaptations. In the leaf, specialised parenchyma cells called aerenchyma form large air spaces that provide buoyancy so the leaf can float on the water surface. Stomata are found on the upper epidermis only (since the lower surface is in contact with water), and there is little or no waxy cuticle (no need to reduce water loss when surrounded by water).

In the root and stem, aerenchyma air spaces allow gas diffusion and provide buoyancy. There is a lack of structural support tissue and lignification (xylem fibres and tracheids are reduced) because the surrounding water provides physical support. The air spaces also act as reservoirs for storing oxygen and carbon dioxide.

Mesophytes — Adaptations to Moderate Water Availability

Mesophytes can lose significant amounts of water during the day but have stomata that close to regulate loss. Water lost during daytime is replaced by absorption through the night. To cope with winter conditions (when water may be frozen in the soil), many mesophytes shed their leaves in autumn — a process called abscission. Some mesophytes survive winter as underground organs such as bulbs and rhizomes, losing their entire aerial system.

Phloem Transport — Translocation and the Mass Flow Hypothesis

Phloem Tissue Structure

The phloem is composed of three cell types, each with a specific role in translocation:

Sieve tube elements are the cells that join together to form the phloem vessel. Where neighbouring sieve tube elements meet, their end walls form sieve plates with pores that allow the flow of phloem sap. Sieve tube elements have a lumen containing only a thin layer of cytoplasm with primitive, non-functional organelles. They are living cells but lack a nucleus and most organelles.

Companion cells are attached to the sieve tube elements and connected to them via plasmodesmata. Unlike sieve tube elements, companion cells are packed with organelles — mitochondria, rough and smooth endoplasmic reticulum, and Golgi bodies. They provide the metabolic energy (ATP) required for sucrose loading and support the function of the sieve tube elements.

Phloem parenchyma cells are similar to companion cells but contain fewer organelles. They provide structural support and may serve as storage for starch and other metabolites.

The Mass Flow Hypothesis

A source is any region of the plant that produces or releases sugars — typically the photosynthesising leaves. A sink is any region that stores sugars or uses them in respiration for growth — examples include roots, fruits, seeds, and the meristem (growing points).

Sucrose Loading — How Sugar Actually Enters the Phloem

The simple mass flow hypothesis does not explain how the high pressure is generated within the phloem vessel itself (rather than in the leaf cells). The process of sucrose loading explains this, and demonstrates why companion cells and ATP are essential:

Limitations of the Mass Flow Hypothesis

The mass flow hypothesis, as originally proposed, does not account for several observations:

• The role of companion cells — the hypothesis does not explain why sieve tube elements need companion cells.
• The function of sieve plates — sieve plates would seem to impede flow, so why do they exist?
• Bidirectional movement — substances can move in both directions in the phloem simultaneously, which simple mass flow cannot explain.
• The requirement for ATP — if transport were purely passive mass flow, metabolic energy would not be needed.
• Variable speed of translocation — the rate of phloem transport is not constant.
• Pressure generation — the high pressure is created in the phloem itself (via sucrose loading), not in the source cells as the simple hypothesis suggests.

Experimental Evidence for Phloem Transport

Several classic experiments have confirmed that the phloem is responsible for sugar transport and have revealed the mechanism of translocation. These experiments are commonly tested in exams.

1. Radioactive Carbon Dioxide (¹⁴CO₂) and Autoradiography

Radioactive carbon dioxide is fed to a plant over 24 hours. The ¹⁴C is incorporated into sugars during photosynthesis, making the sugars radioactive. A cross-section of the stem is placed on photographic film — the phloem region fogs the film, confirming that sugars are transported in the phloem vessels specifically, not the xylem.

2. The Ringing Experiment

All tissue above the xylem (cortex, epidermis and phloem) is removed in a ring around the circumference of a tree trunk. When this is done during summer, sugar from the leaves accumulates above the ring, causing the bark to swell — demonstrating that sugars travel downwards through the phloem. The trees do not wilt (showing the xylem is functioning normally for water transport), but eventually the roots die because they no longer receive sugars via the phloem.

3. Aphid Stylet Experiments

Aphids (greenfly) feed by inserting a needle-like stylet directly into a phloem sieve tube element. Scientists anaesthetise the aphid and carefully remove its body, leaving the stylet in place. Phloem sap exudes from the cut end of the stylet under pressure, which can then be collected and analysed. This technique confirms that the phloem transports sugars and amino acids, and that the phloem sap is under positive pressure.

4. Evidence for Bidirectional Movement

When all leaves are removed from a plant except one in the middle of the stem, and ¹⁴CO₂ is supplied to that remaining leaf, radioactive sucrose is detected both above and below the leaf — demonstrating bidirectional movement in the phloem. Protein filaments passing through sieve plate pores may allow different substances to move in different directions simultaneously. Cytoplasmic streaming within individual sieve tube elements may also contribute to bidirectional transport.

5. Evidence for Variable Translocation Speed

Two aphids feeding on the same phloem vessel at different positions on a stem can be used to measure translocation speed. ¹⁴CO₂ is supplied to a leaf, and the time taken for radioactivity to appear at each aphid is measured. Dividing the distance between the two aphids by the time difference gives the speed of translocation. Using this method, the speed of translocation has been shown to vary — further evidence that phloem transport is not a simple passive process.

Where Plant Transport Appears on Your Specification

Plant transport is a core topic on every A-Level Biology specification. The content covered on this page applies across all UK exam boards, though the topic name and unit placement vary:

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