9.2 Water Transport In Multicellular Plants

Question 1

What is the symplast?

Answer 1

The movement of water through the continuous cytoplasm of living plant cells that are connected via the plasmodesmata
-the root hair cell has a higher water potential than the next cell along
-this is the result of water diffusing in from the soil, which has made the cytoplasm more dilute
-So water moves from the root hair cell into the neighbouring cell by osmosis
The process continues from cell to cell across the root until the xylem is reached

Question 2

What is the apoplast pathway?

Answer 2

The movement of water through the cells walls and intercellular spaces

• Water fills the spaces between the loose open network of fibres in the cellulose cell wall
• As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules
• resulting in a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance

Question 3

How does water enter the xylem?

Answer 3

Water reaches the layer of cells (the endodermis) surrounding the xylem vessels
An impermeable band (the Casparian strip) around endodermal cells forces water in the apoplast pathway back into the cytoplasm

Endodermal cells. Love mineral ions into the xylem via active transport
As a consequence, water potential in endodermal cells>xylem
Water diffuses into xylem by osmosis
Initial flow into vascular tissue helps to force water up a stem (root pressure)

Question 4

What is the casparian strip?

Answer 4

An impermeable band
It is impermeable because it is largely composed of suberin (a waxy substance)
The strip prevents toxic solutes from continuing to move into the plant and it stops water from returning to the root cortex from xylem vessels