Structure of transport tissues — common mistakes

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.

Xylem vessels are composed of dead, lignified cells forming hollow tubes for water and mineral transport, providing structural support. Phloem consists of living sieve tube elements and metabolically active companion cells, specialised for transporting sugars. For more details and practice questions, check out our free Cambridge 9700 Biology course and past papers.

Xylem vessels are adapted with lignified walls for strength against negative pressure, they are hollow and continuous for efficient flow, and their narrow lumen aids capillary action. These features are key for efficient water and mineral transport. You can find more structured explanations and worked examples in our premium revision lessons.

Companion cells are essential for phloem function. They are metabolically active, containing a nucleus, dense cytoplasm, and numerous mitochondria, supplying ATP and performing metabolic functions for the sieve tube elements. They actively load and unload sucrose into/out of the sieve tubes. Master these concepts with our comprehensive revision notes and past papers available on the platform.