OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What material strengthens the walls of xylem tubes?

1. Cellulose

2. Chitin

3. Lignin

4. Starch

The correct answer is: Lignin

Xylem tubes are specialized structures in plants responsible for the transport of water and dissolved minerals from the roots to the rest of the plant. The strength of the walls of these tubes is critical for withstanding the negative pressure created during water transport and to prevent collapse under tension. Lignin is the material that fortifies the walls of xylem tubes, providing rigidity and structural support. It is a complex organic polymer that is deposited in the cell walls, particularly in the xylem vessel elements and fibers, allowing these structures to resist the physical stresses they encounter. This lignification process is crucial as it enables plants to grow tall and transport water efficiently without the risk of the xylem collapsing. The other materials listed have different roles; cellulose provides mechanical support in various plant structures, chitin is a component of fungal cell walls and exoskeletons of arthropods, and starch serves as an energy reserve in plants. Hence, they do not contribute to the strengthening of xylem tubes in the same way that lignin does.