A disease causes newly formed xylem vessels in sunflower plants to contain much less lignin. Scientists measured xylem vessels from healthy and diseased plants. They measured the internal diameter of the vessels when a weak pull and a stronger pull were applied to the water.
| Plant | Internal diameter with a weak pull (µm) | Internal diameter with a stronger pull (µm) | Rate of water transport with a stronger pull (cm³ per hour) |
|---|---|---|---|
| Healthy | 42 | 39 | 5.8 |
| Diseased | 41 | 18 | 2.4 |
Explain how the structure of a healthy xylem vessel allows it to transport water, and use the data to explain why the disease reduces water transport when the pull becomes stronger.
114 exam-style questions on OCR GCSE Biology 2.2 The challenges of size, covering 2.2.1 Exchange surfaces and surface area : volume ratio, 2.2.2 Substances transported in organisms, 2.2.3 The human circulatory system, 2.2.4 The heart and blood vessels, 2.2.5 Red blood cells and plasma, 2.2.6 Uptake of water and mineral ions by plants, 2.2.7 Transpiration and translocation, 2.2.8 Xylem and phloem, 2.2.9 Environmental factors and water uptake, and 2.2.10 Using a potometer. Each one has a worked solution and a mark scheme showing where the marks go.