In simple terms
A friendly intro before the formal notes — no formulas yet.
Plant's Plumbing System
Just like humans need blood vessels, plants need specialised tissues to move vital substances around. These are the xylem, which carries water and minerals from roots to leaves, and the phloem, which transports sugars (food) from leaves to where they're needed.
Imagine a tall building. The xylem is like the water pipes bringing water from the ground up to all floors, while the phloem is like the food delivery service, taking meals from the kitchen to hungry residents wherever they are.
- 1
Visualise xylem as continuous, hollow tubes.
- 2
Understand why lignin is crucial for xylem function.
- 3
Recall the key components of phloem: sieve tubes and companion cells.
- 4
Link each structural feature to its specific role in transport.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 7.1.1
Draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs
- 7.1.2
Describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
- 7.1.3
Draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs
- 7.1.4
Relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions
Explore the concept
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Full topic notes
Formal explanation with the rigour you need for the exam.
Xylem: The Water Highway
The xylem is primarily responsible for the transport of water and dissolved mineral ions from the roots, through the stem, and up to the leaves. It also provides mechanical support to the plant. Its structure is highly adapted for this role:
- Dead cells: Mature xylem vessels are composed of dead cells, forming hollow, continuous tubes. This lack of cytoplasm and organelles reduces resistance to water flow.
- Lignified walls: The cell walls are heavily thickened and impregnated with lignin, a strong, waterproof polymer. Lignin provides structural strength, preventing the vessels from collapsing under tension (negative pressure) during transpiration. It also makes the cells impermeable to water, forcing water to move through the lumen.
- Continuous tube: Individual xylem vessel elements are joined end-to-end, with their end walls largely broken down or perforated, creating a continuous, open pipeline for efficient water flow.
- Pits: Small, unlignified areas called pits (or bordered pits) are present in the cell walls. These allow lateral movement of water between adjacent vessels and into surrounding parenchyma cells, ensuring water can bypass blockages and reach all parts of the plant.
Phloem: The Sugar Superhighway
The phloem is responsible for translocating sugars (primarily sucrose, the end product of photosynthesis) and amino acids from sources (e.g., leaves) to sinks (e.g., roots, growing points, storage organs). It comprises two main cell types working in close concert:
- Sieve tube elements: These are living cells, but highly modified. They are elongated and arranged end-to-end to form sieve tubes. Their most distinctive feature is the sieve plates at their end walls, which are perforated with pores, allowing the flow of sap (water and dissolved solutes) between elements. Sieve tube elements have a reduced amount of cytoplasm, lack a nucleus, ribosomes, and a vacuole at maturity, making space for the efficient passage of sap.
- Companion cells: Each sieve tube element is associated with one or more companion cells. These are fully metabolically active cells with a prominent nucleus, dense cytoplasm, and numerous mitochondria. They are connected to the sieve tube elements by many plasmodesmata, which are microscopic channels through the cell walls, allowing the transfer of substances like ATP and proteins. Companion cells perform the metabolic functions necessary for the sieve tube elements, including active loading and unloading of sucrose into and out of the sieve tube elements, driven by their abundant mitochondria providing ATP.
epidermis – the outermost layer; made up of a single layer of parenchyma cells and its outer wall is covered with a cuticle - cuticle prevents infection of the plant by bacteria or fungi - also aids in reducing water loss.
cortex – divided into three regions: - hypodermis provides mechanical support - middle cortex is involved in photosynthesis - inner cortex helps in gaseous exchange and stores food materials.
endodermis – the innermost layer of the cortex, consists of a single layer of cells that contain starch grains.
pith – large, central, parenchymatous zone with intracellular spaces; helps in storage of food materials Structure of leaves (dicot) Transverse section Image: https://www.brainkart.com/.
upper epidermis – the outermost layer made up of a single layer of parenchyma cells without intracellular spaces; outer walls have cuticles, stomata are less in number.
lower epidermis – single layer of parenchymatous cells with a thin cuticle - contains numerous stomata - chloroplasts are only present in guard cells - helps in exchange of gases - loss of water vapour is facilitated through this chamber.
transports water and mineral ions via mass flow (passive).
unidirectional movement (from roots à rest of the plant).
transports sucrose and amino acids via mass flow (active process) from the source to the sink.
bidirectional movement (translocation).
piliferous layer (also called epiblema, rhizodermis) – the outermost layer; unicellular root hairs present, cuticle and stomata absent.
cortex – it is a multi-layered large zone made of parenchymatous cells with intracellular spaces and stores food and water.
endodermis – the innermost layer of the cortex; the cells closely packed and have Casparian strips within their walls (water-impermeable deposits of suberin) which regulate water and mineral uptake by the roots.
pericycle – produces lateral roots when cells here divide.
stele – all tissues inner to endodermis constitute stele; here it includes pericycle and vascular bundle.
vascular bundle – xylem and phloem.
Distribution in stems, roots and leaves
In a transverse section of a dicot stem, vascular bundles are arranged in a ring: xylem lies towards the centre and phloem towards the outside, with cambium between them in many species. In roots, a central stele contains xylem in a star-shaped pattern with phloem between the arms. In leaves, veins branch through mesophyll with xylem on the upper side and phloem on the lower side in most dicots — exam questions often ask you to label these from micrographs.
Worked examples
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Explain how the structure of xylem vessels is adapted for the efficient transport of water from the roots to the leaves. (4 marks)
- 1
Continuous lumen: Xylem vessels are hollow, continuous tubes with no end walls, allowing for an uninterrupted column of water and reducing resistance to flow.
A xylem vessel appears wide in a micrograph taken at magnification. Calculate the actual diameter in . [3 marks]
- 1
Actual size .
How it all connects
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Glossary
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Quick check
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Revision flashcards
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Main function of xylem?
Transport of water and mineral ions from roots to leaves; also provides structural support via lignin.
Key takeaways
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- ✓
epidermis – the outermost layer; made up of a single layer of parenchyma cells and its outer wall is covered with a cuticle - cuticle prevents infection of the plant by bacteria or fungi - also aids in reducing water loss.
- ✓
cortex – divided into three regions: - hypodermis provides mechanical support - middle cortex is involved in photosynthesis - inner cortex helps in gaseous exchange and stores food materials.
- ✓
endodermis – the innermost layer of the cortex, consists of a single layer of cells that contain starch grains.
- ✓
pith – large, central, parenchymatous zone with intracellular spaces; helps in storage of food materials Structure of leaves (dicot) Transverse section Image: https://www.brainkart.com/.
- ✓
upper epidermis – the outermost layer made up of a single layer of parenchyma cells without intracellular spaces; outer walls have cuticles, stomata are less in number.
- ✓
lower epidermis – single layer of parenchymatous cells with a thin cuticle - contains numerous stomata - chloroplasts are only present in guard cells - helps in exchange of gases - loss of water vapour is facilitated through this chamber.
- ✓
transports water and mineral ions via mass flow (passive).
- ✓
unidirectional movement (from roots à rest of the plant).
- ✓
transports sucrose and amino acids via mass flow (active process) from the source to the sink.
- ✓
bidirectional movement (translocation).
- ✓
piliferous layer (also called epiblema, rhizodermis) – the outermost layer; unicellular root hairs present, cuticle and stomata absent.
- ✓
cortex – it is a multi-layered large zone made of parenchymatous cells with intracellular spaces and stores food and water.
- ✓
endodermis – the innermost layer of the cortex; the cells closely packed and have Casparian strips within their walls (water-impermeable deposits of suberin) which regulate water and mineral uptake by the roots.
- ✓
pericycle – produces lateral roots when cells here divide.
- ✓
stele – all tissues inner to endodermis constitute stele; here it includes pericycle and vascular bundle.
- ✓
vascular bundle – xylem and phloem.
Practice — then mark it
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