Transport in plantsEdexcel A-Level Biology B: Mind map

Exam questions on Transport in plants

1. A botanist stains transverse and longitudinal sections of a sunflower stem and examines them with a light microscope. The stain turns lignin red. In the vascular bundles, some tubes have thick red walls with rings and small gaps, and contain no cell contents. Other tubes have thin walls, are separated by perforated end plates and lie beside small cells with dense cytoplasm.
   Explain how the lignified walls and the small gaps seen in the xylem tubes help xylem to transport water.2 marks
2. Researchers place the cut end of a root in a solution containing a dye whose molecules are too large to cross cell surface membranes but pass freely through cell walls. The root cortex cells are joined by cytoplasmic strands. The endodermis, the layer around the central vascular tissue, has a waterproof band of suberin in its cell walls.
   Suggest why the dye is seen throughout the cortex but not beyond the endodermis.2 marks
3. A research team studies a coast redwood tree 90 m tall. When they drill a small hole through the bark into a mature xylem vessel, no sap spurts out. Instead, a little air is drawn into the hole. Over a hot afternoon the trunk diameter is measured with sensitive instruments and is found to shrink slightly.
   Explain how these observations support the cohesion-tension model of water transport.3 marks