Explain how the structures of the xylem and phloem are adapted to their…

The idea

Xylem and phloem are transport tissues with structures matched to their roles. Xylem tissue contains dead, hollow cells whose walls are strengthened with lignin. Being dead and hollow means there is no cytoplasm to obstruct flow, so water and dissolved mineral ions move through the plant in a continuous column, and lignin keeps the vessels rigid and prevents collapse. Phloem tissue contains living cells that use energy from respiration to load and move sucrose around the plant to growing regions and storage organs. A useful method is to compare the two tissues feature by feature: dead versus living, lignified versus not, water and minerals versus sucrose, passive flow versus energy-requiring transport.

What earns the marks

• Xylem cells are dead and hollow, so water and dissolved mineral ions can flow through them without obstruction.
• Lignin strengthens and waterproofs xylem cell walls, keeping the vessels rigid and preventing them from collapsing.
• Xylem transports water and mineral ions through the plant, for example from the roots to the leaves.
• Phloem cells are living and use energy from respiration to transport sucrose around the plant.
• Phloem carries sucrose to growing regions and storage organs, moving it in either direction as the plant requires.

Common mistakes and how to put them right

• Saying xylem cells are living.

  Put it right by stating that xylem cells are dead and hollow, which allows a continuous flow of water and minerals.

• Confusing the direction of phloem transport with xylem transport.

  Put it right by noting that phloem moves sucrose around the plant, including to growing and storage regions, whereas xylem mainly carries water and minerals upward.

• Claiming phloem transport needs no energy.

  Put it right by explaining that living phloem cells use energy from respiration to move sucrose.

Exam tips

• Compare xylem and phloem directly, pairing each structural feature with the function it enables.
• State that lignin strengthens xylem walls and that dead hollow cells allow flow, rather than just naming lignin.
• For phloem, link living cells and energy use to the transport of sucrose around the plant.