Topic 7: Transport In Plants
7.1 Structure of transport tissues
Students should be able to:

1) draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs

2) describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants

3) draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs

4)relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions

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1. Plan Diagrams and General Distribution of Tissues

In herbaceous dicotyledonous plants, the vascular tissues — xylem and phloem — are arranged in characteristic patterns in the root, stem, and leaf, which can be observed in transverse sections under the microscope.

• In a root, the xylem forms a central star-shaped structure with the phloem located between the arms of the xylem. This arrangement provides strength to resist pulling forces as roots anchor the plant in the soil. The vascular tissue is surrounded by the pericycle, endodermis, cortex, and epidermis.

• In a stem, the xylem and phloem occur together in vascular bundles arranged in a ring around the periphery, giving strength and flexibility to resist bending. In each bundle, xylem lies towards the centre and phloem towards the outside, with cambium between them for secondary growth.

• In a leaf, the vascular tissue forms veins (vascular bundles). The xylem lies in the upper part of each vein to supply water to the mesophyll for photosynthesis, while the phloem lies in the lower part to transport assimilates away from the leaf.

Plan diagrams of these sections show the relative positions of the main tissues rather than detailed cell structures.

 

2. Xylem and Phloem Distribution

The distribution of vascular tissues is closely related to their mechanical and transport functions.

• In the root, the central position of xylem and phloem ensures maximum strength and efficient water transport from soil to shoot.

• In the stem, peripheral vascular bundles provide both transport and mechanical support against bending stresses caused by wind.

• In the leaf, closely spaced veins ensure every photosynthetic cell is near a water supply (xylem) and a transport route for sugars (phloem).

This organised arrangement enables the plant to maintain structural integrity and transport efficiency throughout its body.

 

3. Structure of Xylem, Phloem, and Companion Cells

Xylem vessel elements are elongated dead cells aligned end to end to form long continuous tubes for water transport. Their end walls break down, forming a hollow lumen with no cytoplasm, allowing free flow of water. The walls are thickened with lignin, which strengthens the vessels, prevents collapse under tension, and provides waterproofing. Lignin may form spiral, annular, or reticulate patterns, allowing flexibility and some lateral movement of water through pit pairs where lignin is absent.

Phloem sieve tube elements are living, elongated cells joined end to end, with sieve plates — perforated end walls that allow flow of cytoplasm between cells. They have very little cytoplasm, no nucleus, and few organelles, leaving room for the transport of dissolved organic substances. Companion cells are closely associated with sieve tubes via plasmodesmata. They are metabolically active, containing many mitochondria to provide ATP needed for active transport during translocation.

 

4. Relating Structure to Function

The structure of xylem is perfectly adapted for its function in water transport and support. The absence of cytoplasm and the presence of continuous hollow tubes enable an uninterrupted water column, while lignin provides both mechanical strength and resistance to negative pressure during transpiration. Pits in the vessel walls allow lateral movement of water between adjacent vessels and surrounding cells.

The phloem sieve tube elements and companion cells are adapted for translocation of assimilates. Sieve plates permit flow of sap between cells, while companion cells carry out active transport processes to load and unload sucrose, maintaining concentration gradients required for mass flow. Their close connection via plasmodesmata allows efficient transfer of substances and signalling between cells.

7.2 Transport mechanisms

Students should be able to:

1) state that some mineral ions and organic compounds can be transported within plants dissolved in water
2) describe the transport of water from the soil to the xylem through the:
• apoplast pathway, including reference to lignin and cellulose
• symplast pathway, including reference to the endodermis, Casparian strip and suberin
3) explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
4) explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
5) make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
6) state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
7) explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
8) explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

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1. Transport in Solution

Plants transport water, mineral ions, and organic compounds (assimilates) in solution through vascular tissues. Mineral ions such as nitrates, phosphates, and potassium dissolve in water and move via the xylem, while assimilates like sucrose and amino acids are transported in the phloem dissolved in water. The presence of water as a solvent enables all biochemical transport processes to occur efficiently.

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2. Pathways of Water Movement: Apoplast and Symplast

Water is absorbed by root hair cells from the soil via osmosis and travels towards the xylem through two main pathways:

• Apoplast pathway: Water moves through the cell walls and intercellular spaces without crossing cell membranes. The walls of root cells are made of cellulose, which is fully permeable to water, allowing rapid flow. The apoplast pathway continues until it reaches the endodermis, where the Casparian strip — a band of suberin (a waterproof substance) in the cell walls — blocks the pathway. This forces water to enter the symplast, ensuring selective uptake of ions by the cell membrane.

• Symplast pathway: Water moves through the cytoplasm of cells via plasmodesmata, fine strands of cytoplasm connecting adjacent cells. The water potential gradient drives water from cell to cell until it reaches the xylem. The endodermis actively transports mineral ions into the xylem, lowering its water potential and drawing water in by osmosis.

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This combination of pathways ensures both rapid water movement and selective uptake of essential ions.

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3. Transpiration and Water Movement in Xylem

Transpiration is the evaporation of water from the internal surfaces of leaves followed by the diffusion of water vapour through the stomata to the atmosphere. Water evaporates from moist cell walls of mesophyll cells, diffuses through air spaces, and exits via stomata. As water evaporates, it creates a negative pressure in the leaf, pulling more water upward through the xylem to replace it.

This process is part of the cohesion–tension mechanism, driven by the physical properties of water molecules.

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4. Cohesion, Adhesion, and the Cohesion–Tension Mechanism

Water molecules exhibit hydrogen bonding, which gives rise to cohesion — the attraction between water molecules — and adhesion, the attraction between water molecules and hydrophilic surfaces such as cellulose in xylem walls. Cohesion maintains a continuous column of water in the xylem, while adhesion helps counteract gravity by allowing water to “stick” to vessel walls.

When water evaporates from leaves during transpiration, a transpiration pull is generated. This creates tension (negative pressure) in the xylem, drawing water upward from the roots to the leaves. This passive process is called the cohesion–tension theory and is the main force responsible for long-distance water transport in plants.

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5. Xerophytic Adaptations

Xerophytes are plants adapted to survive in dry conditions by reducing water loss through transpiration. Transverse sections of xerophytic leaves (e.g., Nerium or Marram grass) reveal adaptations such as:

• Thick cuticle to reduce evaporation;

• Sunken stomata in pits that trap moist air;

• Hairy surfaces that reduce air movement and water potential gradient;

• Rolled leaves to trap humid air;

• Reduced number of stomata and small leaf surface area;

• Large water storage tissues (succulence) in some species.

These features collectively minimise transpiration and maintain water balance.

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6. Phloem Transport: Sources and Sinks

Translocation is the movement of assimilates (such as sucrose and amino acids) through the phloem from sources — regions of production (e.g., photosynthesising leaves) — to sinks, where they are used or stored (e.g., roots, fruits, and seeds). The assimilates are transported dissolved in water within the sieve tube elements as phloem sap.

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7. Loading of Assimilates into the Phloem

Companion cells actively load sucrose into the phloem sieve tubes using proton pumps and cotransporter proteins. Proton pumps in the plasma membrane of companion cells use ATP to actively transport H⁺ ions out into the cell wall space. This creates a proton gradient. H⁺ ions then re-enter the companion cells via cotransporter proteins, carrying sucrose molecules along with them by secondary active transport. Sucrose then moves into sieve tubes through plasmodesmata or by diffusion, increasing the solute concentration inside sieve tubes and lowering their water potential.

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8. Mass Flow in Phloem

Water enters sieve tubes by osmosis from surrounding xylem, generating a region of high hydrostatic pressure at the source end. At the sink, sucrose is removed for respiration or storage, increasing water potential and causing water to leave by osmosis, lowering hydrostatic pressure. The resulting pressure gradient drives a mass flow of phloem sap from source to sink. This process is a bulk flow mechanism, transporting assimilates rapidly throughout the plant.