1. Review What two forces are responsible for 90% of the upward flow of water through a plant

2. Review What is the hypothesis that explains the movement of fluid through phloem of a plant

3. Infer Explain how movement of sugars in phloem contributes to homeostasis in a plant.

CH 23 PLANT STRUCTURE AND FUNCTION23.5 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

hypothesis1. 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 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.