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.
Xylem moves water and mineral ions by mass flow — a passive process driven by transpiration — in one direction, from the roots to the rest of the plant.
Phloem moves assimilates (sucrose and amino acids) by mass flow from source to sink. Loading the sieve tubes is an active process, and translocation can run in either direction.
When comparing xylem and phloem, remember that xylem transports water and minerals upwards, while phloem transports sugars (sucrose) both up and down. Crucially, xylem cells are dead and lignified at maturity, whereas phloem sieve tube elements are living but highly modified, and are supported by living companion cells. Examiners love questions that require you to explicitly link a structural feature to its functional advantage.
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.
Dicot stem, outside in: epidermis — a single layer of cells whose outer wall carries a cuticle that reduces water loss and keeps out bacteria and fungi; cortex — hypodermis for mechanical support, a middle region with chloroplasts, and an inner region for gas exchange and food storage; endodermis — the innermost layer of the cortex, a single layer of cells containing starch grains; pith — a large central zone of parenchyma with intercellular spaces that stores food.
Dicot leaf: upper epidermis — a single layer of parenchyma cells without intercellular spaces, a thick cuticle and few stomata; lower epidermis — a thin cuticle and numerous stomata, with chloroplasts only in the guard cells, allowing gas exchange and the loss of water vapour from the air spaces beneath.
Dicot root, outside in: piliferous layer (epiblema) — the outermost layer, with unicellular root hairs and no cuticle or stomata; cortex — a wide, many-layered zone of parenchyma with intercellular spaces that stores food and water; endodermis — closely packed cells whose walls carry the Casparian strip, a water-impermeable band of suberin that regulates water and mineral uptake; pericycle — produces lateral roots when its cells divide; stele — everything inside the endodermis, here the pericycle and the vascular bundle of xylem and phloem.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
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
Key terms for this topic — skim now; the Check step will test them.
- xylem
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.
- Dead cells
Mature xylem vessels are composed of dead cells, forming hollow, continuous tubes.
- Lignified walls
The cell walls are heavily thickened and impregnated with lignin, a strong, waterproof polymer.
- 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.
- phloem
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).
- Sieve tube elements
These are living cells, but highly modified. They are elongated and arranged end-to-end to form sieve tubes.
- Companion cells
Each sieve tube element is associated with one or more companion cells.
- plasmodesmata
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.
- Dicot leaf
Upper epidermis — a single layer of parenchyma cells without intercellular spaces, a thick cuticle and few stomata; lower epidermis — a thin cuticle and numerous stomata, with chloroplasts only in the guard cells, allowing gas exchange and the loss of water vapour from the air spaces beneath.
- Sieve plates
Perforated end walls between sieve tube elements allowing flow of phloem sap.
- Translocation
Mass flow of organic solutes (mainly sucrose) through phloem from source to sink.
- Xylem vs phloem transport direction
Xylem: unidirectional (upward). Phloem: bidirectional depending on source and sink.
Quick check
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Revision flashcards
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Key takeaways
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Xylem moves water and mineral ions by mass flow — a passive process driven by transpiration — in one direction, from the roots to the rest of the plant.
Phloem moves assimilates (sucrose and amino acids) by mass flow from source to sink. Loading the sieve tubes is an active process, and translocation can run in either direction.
Practice — then mark it
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