Revision: Longer answers

Root pressure is a force that is partly responsible for the movement of water through xylem in stems. Explain how the active transport of mineral ions into xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue (5)

• Water potential in xylem reduced (by entry of ions);

• Water potential gradient established between xylem and surrounding cells;

• Plasma membranes of surrounding cells are partially permeable;

• Water enters xylem by osmosis;

• Volume of water in xylem increases;

• Cannot move back due to gradient;

• Pressure in xylem increases (and forces water upwards);

Explain how water enters a plant root from the soil and travels through to the endodermis. (5)

• water enters root hair cells;

by osmosis;

• because active uptake of mineral ions has created a WP gradient;

• water moves through the cortex;

• (by osmosis) down a WP gradient;

• through cell vacuoles and cytoplasms / symplastic pathway;

• through cell walls / apoplastic pathway;

Change in the circumference of tree trunks can be related to change in the diameter of xylem vessels. The graph shows changes in circumference of three different species of trees measured on a hot dry day in June.

Use your knowledge of the cohesion-tension theory to explain the changes in the circumference of tree A (5)