7.17 · Transport in Plants
Structure of Transport Tissues — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 15 1 quiz
Lesson summary
7.1 Structure of Transport Tissues — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the distribution of xylem and phloem in dicotyledonous roots, stems and leaves, relate xylem and phloem cell structure to function and describe plan diagrams.
- Vascular tissue
- Dicotyledonous plants
- Herbaceous
- Dead, with no contents
- No end walls
- Lignified walls
Section 1 of 10Why plants need transport systems
Why plants need transport systems
Size
- Size plants have many layers of cells and a small SA:V ratio, so diffusion is too slow.
Movement
- Movement water and mineral ions go up from the roots, and sugars go from leaves to where they are used or stored.
Vascular tissue
- Vascular tissue xylem and phloem lie together in vascular bundles in the root, stem and leaf.
Dicotyledonous plants
- Dicotyledonous plants they have seeds with two cotyledons, leaves with a branching network of veins, a broad blade with a stalk (petiole) and a tap root with side branches.
Herbaceous
- Herbaceous herbaceous dicots are non-woody and often live for one growing season.
Section 2 of 10Position of xylem and phloem
| Organ (dicot) | Xylem | Phloem | Arrangement |
|---|---|---|---|
| Root | Central, star or X-shaped | Between the arms of the xylem | Central vascular cylinder surrounded by pericycle and endodermis; central position resists pulling forces |
| Stem | Inner side of bundles near the centre | Outer side of bundles, with cambium between | Ring of bundles near the outside, which resists bending |
| Leaf | Upper (adaxial) side of the midrib and veins | Lower side of the midrib and veins | Midrib bundle with veins branching through the mesophyll |
Section 3 of 10Xylem vessel elements
Dead, with no contents
- Dead, with no contents nothing impedes the flow of water.
No end walls
- No end walls they form a continuous column of water.
Lignified walls
- Lignified walls lignin is strong and waterproof, preventing collapse under negative pressure.
Pits
- Pits they allow lateral movement and bypass of air bubbles.
Narrow lumen
- Narrow lumen it increases adhesion of water to the walls and helps capillarity.
Other xylem cells
- Other xylem cells tracheids, fibres (support) and parenchyma.
Transport
- Transport water moves by mass flow under tension, down a water potential gradient.
- (cohesion–tension)
Section 4 of 10Phloem sieve tube elements and companion cells
Sieve tube elements
- Sieve tube elements they have little cytoplasm, no nucleus and few organelles, so sap can flow.
- Sieve plates with pores let sap pass and can be sealed by callose if damaged.
Companion cells
- Companion cells they have a nucleus, many mitochondria and ribosomes.
- They supply ATP and proteins to the sieve elements through plasmodesmata and actively load sucrose by proton–sucrose co-transport.
Transport
- Transport sucrose moves by mass flow from source to sink (low pressure).
- (high hydrostatic pressure)
Section 5 of 10Xylem and phloem compared
| Feature | Xylem vessel element | Phloem sieve tube element |
|---|---|---|
| Living? | Dead | Living |
| Cell contents | None, hollow lumen | Thin layer of cytoplasm; no nucleus |
| End walls | Absent, so a continuous tube | Sieve plates |
| Walls | Lignified, with pits | Cellulose, not lignified |
| Transports | Water and mineral ions, upwards only | Sucrose and amino acids, up or down |
| Associated cell | None | Companion cell |
- Size
- xylem vessels are typically 20 to 200 µm in diameter and sieve tubes 10 to 40 µm.
Section 6 of 10Xerophyte adaptations
Xerophyte adaptations
Aim
- Aim they reduce the water vapour potential gradient or evaporation.
Features
- Features thick waxy cuticle, rolled leaves, sunken stomata in pits, hairs that trap moist air, fewer stomata and reduced leaf area (spines).
Marram grass
- Marram grass leaves roll with stomata on the inner surface, with hinge cells, hairs and sunken stomata.
Cacti
- Cacti stems store water and leaves are reduced to spines.
Section 7 of 10Plan diagrams and sections
1
Cut a thin transverse section of stem or root with a sharp blade, or use a prepared slide.
2
Mount it in water, optionally stained with toluidine blue (lignin stains blue-green), and add a coverslip.
3
View under low power to find the vascular bundles and their arrangement.
4
Draw a plan diagram showing tissue outlines only, with no individual cells, using sharp continuous lines and correct proportions.
5
Switch to high power and draw a few cells, such as a xylem vessel, sieve tube and companion cell, including cell walls.
6
Label with ruled lines that touch the structure and do not cross, and add the magnification or scale.
Magnification
Magnification image size ÷ actual size, so actual size = image size ÷ magnification, with both in the same units (1 mm = 1000 µm; 1 µm = 1000 nm).
Graticule
- Graticule the value of one eyepiece division = stage micrometer length ÷ number of eyepiece divisions.
Section 8 of 10More from the specification
More from the specification
Phloem structure and translocation of assimilates
- Phloem sieve tube elements transport organic assimilates, predominantly sucrose synthesised from glucose,
- as dissolved solutes to growing and storage parts of the plant.
Xylem tissue and structural features
- Mature xylem vessel elements are dead, hollow tubes with lignified walls that provide mechanical support and conduct water and dissolved mineral ions upwards from roots.
Micrograph interpretation and biological drawing
- In examination drawing tasks, students must identify tissue types from a light micrograph or plan diagram, using sharp, unbroken lines without shading to represent vascular bundles.
Transverse sections of plant transport organs
- Plant stems and roots contain distinct vascular bundles arranged with xylem and phloem, specialised to transport organic compounds and inorganic substances efficiently.
Assimilates
- Organic substances such as sucrose and amino acids transported in the phloem.
Micrograph
- A photograph taken through a microscope.
Compound
- A substance of two or more kinds of atoms chemically combined
Glucose
- A simple sugar; the unit that starch, glycogen and cellulose are built from.
Organic compound
- Compound containing carbon and hydrogen.
Transport system
- A system, such as the circulatory system, that moves substances around the body.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Xylem vessel element | A dead, lignified cell with no end walls, joined end to end to form a hollow vessel. |
| Lignin | A strong, waterproof polymer deposited in xylem cell walls. |
| Sieve plate | A perforated end wall between sieve tube elements. |
| Companion cell | A cell next to a sieve tube element that supports it and actively loads sucrose. |
| Plasmodesmata | Channels through plant cell walls linking the cytoplasm of adjacent cells. |
| Vascular bundle | A strand of transport tissue containing xylem and phloem. |
| Translocation | Movement of assimilates in the phloem from sources to sinks. |
| Xerophyte | A plant adapted to dry conditions by reducing water loss. |
Section 10 of 10Exam technique
1
Putting the xylem on the outside of the phloem in a stem bundle — xylem is on the inner side.
2
Saying phloem sieve tubes are dead — they are living cells.
3
Drawing individual cells in a plan diagram — it shows tissue outlines only.
4
Saying the root has pith — a dicot root has a solid central vascular cylinder.
Marked practiceQuizzes for this lesson
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