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

What are xylem tubes primarily composed of?

1. Living cells with end walls

2. Dead cells joined end to end

3. Thin-walled cells

4. Root hairs

The correct answer is: Dead cells joined end to end

Xylem tubes are primarily composed of dead cells that are elongated and joined end to end. This structure is essential for their function in plants. The dead cells form hollow tubes that allow for efficient transport of water and essential minerals from the roots to the leaves. The composition of the xylem is designed to withstand the negative pressure generated during water transport, and the presence of lignin in the walls of these cells provides structural support, helping to maintain the integrity of the plant as it draws water upward. In contrast, living cells with end walls are characteristic of phloem, which is responsible for transporting nutrients throughout the plant. Thin-walled cells may not provide the necessary structural support for water transport at high pressures. Root hairs, while important for absorbing water from the soil, are not part of the xylem structure and serve a different function in the plant's overall system.