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The key difference
Xylem transports water and dissolved mineral ions upward from roots to leaves in dead, hollow vessels. Phloem transports dissolved sugars (sucrose) both up and down in living sieve tube elements with companion cells.
Plants have two specialised transport tissues that run alongside each other in vascular bundles. Although xylem and phloem are physically close together, they transport different substances in different directions by different mechanisms. Examiners frequently ask you to compare their structure and function.
Feature
Xylem
Phloem
Substance transported
Water and dissolved mineral ions
Dissolved sugars (mainly sucrose) and amino acids
Direction of transport
Upward only (roots → leaves)
Both directions (up and down): from source to sink
Cell status
Dead cells at maturity
Living cells
Cell structure
Hollow tubes: no end walls, no cytoplasm, no nucleus. Walls reinforced with lignin
Sieve tube elements with sieve plates (perforated end walls). Accompanied by companion cells
Mechanism
Transpiration pull: passive (no energy required)
Translocation: active process requiring energy from respiration
Wall thickening
Lignin: provides strength and waterproofing
No lignin: thin cellulose walls
Cross-section appearance
Larger, often angular, thick-walled, empty lumen
Smaller, thin-walled, may show sieve plates
Why Xylem Cells Are Dead
Xylem vessels are dead at maturity because their function requires an uninterrupted hollow tube. The end walls between cells break down, the cytoplasm and nucleus disintegrate, and the walls become thickened with lignin: a waterproof, rigid polymer. This creates a continuous column of water from root to leaf with minimal resistance. The lignin also provides structural support, which is why wood is largely made of xylem tissue.
Translocation in Phloem
Phloem transports dissolved sugars by a process called translocation. Sugars are loaded into sieve tube elements at a source (e.g. A photosynthesising leaf) and transported to a sink (e.g. A growing root tip, developing fruit, or storage organ). This process requires energy, which is why phloem cells must be alive. The companion cells have many mitochondria to supply the ATP needed for loading sugars into the sieve tubes.
How to Identify Them in a Cross-Section
In a cross-section of a stem or root:
Xylem: larger cells, often towards the inside (centre) of the vascular bundle, thick walls, empty lumen, often stained red/pink
Phloem: smaller cells, towards the outside of the vascular bundle, thin walls, may appear as clusters of cells with sieve plates visible in longitudinal sections
Try a comparison question (4 marks)
Explain why phloem cells must be alive to carry out their function, whereas xylem cells are dead. [4]
Show the answer and marking guidance
Phloem transports sugars by translocation, which is an active process requiring energy from respiration
Companion cells provide this energy (ATP): they must be alive to respire
Xylem transports water by transpiration pull, which is a passive process: no energy input required
Xylem cells being dead and hollow (no cytoplasm, no end walls) creates a continuous uninterrupted tube for efficient water flow
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Saying phloem only moves sugars downward: translocation goes both up and down, from source to sink
Forgetting that xylem transport is passive (driven by transpiration pull) while phloem transport is active
Saying xylem is "made of lignin": lignin thickens and strengthens the cell walls but is not the only component
Not mentioning companion cells when describing phloem: they are essential for providing energy for translocation