Review What two forces are responsible for 90% of the upward flow of water through a plant Review What is the hypothesis that explains the movement of fluid through phloem of a plant Infer Explain how movement of sugars in phloem contributes to homeostasis in a plant.

Ch 23 Plant Structure and Function 23.5 Transport in Plants

Transpiration As water evaporates through open stomata, the cell walls within the leaf begin to dry out Dry cell walls draw water from cells deeper inside the leaf’s vascular tissue so that water is pulled up through xylem The hotter and windier the day, the greater amount of water pulled.

Cell Walls Pull Water Upward Cohesion Water molecules are attracted to one another Adhesion Water molecules form hydrogen bonds with other substances.

Capillary Action Tendency of water to rise in a thin tube Water is attracted to walls tube and water molecules are attracted to one another Thinner the tube, the higher the water will rise.

Putting it all together Tubes of xylem are lined with cellulose cell walls Water adheres very strongly Transpiration pull from leaves Concentration gradient forces water in roots to a much smaller amount.

Nutrient Transport Pressure-flow hypothesis 1. Membranes of sieve tube cells use active transport to move sugars from their cytoplasm into the sieve tube itself.

2. Water then follows by osmosis, creating pressure in the tube at the source of the sugars.

3. Sugars are actively pumped out of the tube and into the surrounding tissue where it is needed. Water then leaves the tube via osmosis.

During growing season As growing season ends As spring approaches Sugars are directed into ripening fruits or into roots for storage As growing season ends Plant drops its fruits and stores nutrients in the roots As spring approaches Phloem cells in roots pump sugars back into phloem sap Pressure-flow system raises these sugars into stems and leaves to support rapid growth.