9.2 Water Transport in Multicellular Plants Flashcards
How does water enter the roots of a plant?
Soil water = low concentration of dissolved minerals = high water potential
Cytoplasm and vacuolar sap of root hair cells -> many different solvents (e.g sugars, mineral ions, amino acids ) = low water potential
Therefore, water moves into the root hair cells by osmosis
How are root hairs well adapted as exchange surfaces?
Microscopic size = penetrate easily between soil particles
Each microscopic hair has a large SA:V ratio and there are thousands on each growing root tip
Thin surface layer = short diffusion distance and fast osmosis
Concentration of splits in cytoplasm of root hair cells = maintains a water potential gradient between soil water and cell
What are the two ways water can move across the root to the xylem?
The symplast pathway
The apoplast pathway
Describe the symplast pathway
Symplast = continuous cytoplasm of living plant cells connected via plasmodesmata
Root hair cells = higher water potential than the next cell ( due to water diffusing in from the soil, which makes the cytoplasm more dilute )
Therefore, water moves from cell to cell by osmosis until xylem is reached
Describe the apoplast pathway
Apoplast = cell walls and the 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 behind them due to cohesion.
It moves from an area of high hydrostatic pressure to low hydrostatic pressure
How does water enter the xylem?
Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis, where it is blocked by the Casparian strip.
Casparian strip = band of waxy material called Suberin that runs around each of the endodermal cells forming a waterproof layer.
As water can’t move past this point, it is forced to join the cytoplasm of the cell, joining the water in the symplast pathway.
This is useful as it means the water has to pass through the selectively permeable cell surface membranes and so potentially toxic solutes in the soil water are prevented from reaching living tissues ( as there are no carrier proteins ).
It then moves into the xylem .
How is the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway increased?
Solute concentration in cytoplasm of endodermal cells is relatively dilute compared to cells in xylem.
Also, endodermal cells move mineral ions into xylem by active transport.
Therefore , the water potential of xylem cells is much lower than water potential of the endodermal cells.
What happens after water has entered the vascular bundle?
Water returns to the apoplast pathway to enter the xylem itself and move up the plant.
Active pumping of minerals into xylem to produce movement of water by osmosis results in root pressure.
Root pressure gives water a push up the xylem ( but is not the main reason for water moving from roots to leaves )
What is the evidence for the role of active transport in root pressure?
Not in the spec or revision guide?
