CIE AS/A Level Biology -7.1 Structure of transport tissues- Study Notes- New Syllabus
CIE AS/A Level Biology -7.1 Structure of transport tissues- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -7.1 Structure of transport tissues- Study Notes- New Syllabus
Key Concepts:
- draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs
- describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
- draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs
- relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions
Plan Diagrams of Transverse Sections – Stems, Roots, and Leaves of Herbaceous Dicotyledonous Plants
Distribution of Xylem & Phloem in Herbaceous Dicotyledonous Plants
🌱 Overview
- In herbaceous dicotyledonous plants, xylem and phloem are arranged in characteristic patterns within the vascular bundles of stems, roots, and leaves.
- These arrangements are important for support and transport.
🌿 1. Stem (Transverse Section)
- Arrangement:
- Vascular bundles arranged in a ring around the periphery.
- Each bundle: Xylem towards the centre, Phloem towards the outside.
- Cambium between xylem and phloem (allows secondary growth).
- Pith (parenchyma) in the centre, cortex outside the bundles.
- 📌 Function: Ring arrangement provides strength and resists bending.
🌱 2. Root (Transverse Section)
- Arrangement:
- Xylem in the centre, often in an X or star-shaped pattern.
- Phloem located between the arms of the xylem.
- No cambium at this stage in young roots.
- Large cortex surrounding the stele for storage and transport.
- 📌 Function: Central xylem resists pulling forces; root structure suited for anchorage.
🍃 3. Leaf (Transverse Section / Midrib)
- Arrangement:
- Vascular bundles form the veins of the leaf.
- In each bundle: Xylem is upper (closer to upper epidermis), Phloem is lower (closer to lower epidermis).
- Surrounded by bundle sheath cells (support + regulation).
- 📌 Function: Correct positioning ensures efficient water transport to mesophyll and sugar transport away from photosynthetic cells.
📊 Comparison Table
| Organ | Xylem Position | Phloem Position | Arrangement of Vascular Bundles |
|---|---|---|---|
| Stem | Inner side (towards pith) | Outer side (towards cortex) | Bundles in a ring with cambium between |
| Root | Central (X/star shape) | Between arms of xylem | Single central stele, surrounded by cortex |
| Leaf | Upper side of bundle | Lower side of bundle | Bundles within veins, surrounded by bundle sheath |
Summary:
– Stem: Vascular bundles in a ring; xylem inside, phloem outside.
– Root: Xylem in centre (X-shaped), phloem between arms.
– Leaf: Xylem upper, phloem lower in vascular bundles within veins.
– Stem: Vascular bundles in a ring; xylem inside, phloem outside.
– Root: Xylem in centre (X-shaped), phloem between arms.
– Leaf: Xylem upper, phloem lower in vascular bundles within veins.
Xylem and Phloem Cell Structures – Microscope & EM Views
Structure-Function Relationship of Xylem, Phloem, and Companion Cells
🚰 1. Xylem Vessel Elements
- Structure:
- Long, hollow, dead cells joined end-to-end.
- End walls absent or highly perforated → continuous tube.
- Lignin in thickened cell walls → spiral, annular, or pitted patterns for strength.
- Pits (unlignified areas) in walls → allow sideways water movement between vessels.
- Function Link:
- Water & mineral transport from roots to leaves (unidirectional).
- Hollow tube + no cytoplasm → reduces resistance to flow.
- Lignin → prevents collapse under water tension, gives structural support.
- Pits → provide lateral water movement to bypass blockages.
🍬 2. Phloem Sieve Tube Elements
- Structure:
- Living, elongated cells joined end-to-end.
- End walls have sieve plates (pores) → allow flow of phloem sap.
- Thin cytoplasm and no nucleus → more space for translocation.
- Cell walls contain cellulose but no lignin.
- Function Link:
- Translocation of sugars (mainly sucrose), amino acids, and signaling molecules in both directions.
- Sieve plates → facilitate mass flow of phloem sap.
- Lack of nucleus → space for sap but requires metabolic support from companion cells.
⚡ 3. Companion Cells
- Structure:
- Small, nucleated living cells closely associated with sieve tube elements.
- Many mitochondria → high ATP production for active transport.
- Plasmodesmata connect them to sieve tube elements → direct cytoplasmic connection.
- Function Link:
- Provide metabolic support to sieve tube elements (which lack a nucleus).
- Load and unload sucrose into sieve tubes via active transport.
- Maintain pressure gradient for phloem sap movement.
📊 Summary Table
| Cell Type | Key Structural Features | Functional Link |
|---|---|---|
| Xylem Vessel Element | Dead, hollow, lignified walls, pits | Long-distance water & mineral transport; strength |
| Phloem Sieve Tube Element | Living, no nucleus, sieve plates, thin cytoplasm | Transport of organic nutrients bidirectionally |
| Companion Cell | Living, nucleus, many mitochondria, plasmodesmata | Energy supply & metabolic control for sieve tubes |
Summary:
– Xylem: Dead, lignified tubes → water transport & support.
– Phloem sieve tubes: Living but no nucleus → sugar transport.
– Companion cells: Nucleated, energy-rich → control & supply sieve tubes.
– Xylem: Dead, lignified tubes → water transport & support.
– Phloem sieve tubes: Living but no nucleus → sugar transport.
– Companion cells: Nucleated, energy-rich → control & supply sieve tubes.