Biology 13.8 Movement of Water Up Stems

Transpiration = main force that pulls water up stem of a plant = evaporation of water from the leaves.

Movement of water out through stomata

Humidity of atomsphere = less than that of air spaces next to stomata usually.

When stomata are open water vapour molecules diffuse into surrounding air.

Water lost form air spaces = replaced by water evapourating from cell walls of surrounding mesophyll cells.

Plants can control rate of transpiration by changing the size of stomatal pores.

Movement of water across cells of a leaf

Water = lost from mesophyll cells by evaporation from their surfaces to air spaces next to the leaf.

Replaced by water reaching mesophyll cells from xylem - either by symplastic or apoplastic pathways.

Water movement in case of symplastic pathway occurs because:

• Mesophyll cells lost water to air spaces.
• Cells have lower w.p. than neighbouring cells - water enters by osmosis.
• Neighbouring cells now have lower w.p. than their neighbours - osmosis continues in turn.

Water potential gradient is established - pulls water from xylem, across leaf mesophyll and into the atmosphere.

Movement of…