9.2: Water Transport In Multicellular Plants Flashcards
What is plasmodesmata?
A microscopic canal that passes through plant cell walls
What is a symplast?
The continuous cytoplasm of the living plant cells that is connected through the plasmodesmata
What is an apoplast?
Cell walls and intercellular spaces
What is the endodermis?
A layer of cells surrounding the vascular tissue of the roots
What is the casparian strip?
A band of waxy material (suberin) that runs around each of the endodermal cells, forming a waterproof layer.
What is the casparian strip made of?
Suberin
What is guttation?
Exudation of drops of xylem and phloem sap on the tips or edges of leaves
State a few key functions of water in plants.
-Loss of water by evaporation helps keep plants cool
-Hydrostatic pressure from osmosis provides a hydrostatic skeleton to support stems and leaves
-Hydrostatic pressure drives cell expansion, which is what enables plant roots to force their way through tarmac
-Mineral ions and the products of photosynthesis are transported in water
-Water is a raw material for photosynthesis
What are the features of root hairs that make it more efficient at taking up water?
-Microscopic
-Large SA:V ratio
-Thousands growing on each root hair cell tip
-Thin surface layer for osmosis and diffusion
-The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
What is the purpose of the symplast and apoplast pathways?
Moving the water from the root to the xylem
How does water travel through the symplast pathways?
By osmosis
How does water travel through the apoplast pathways?
By passive diffusion
How is water able to travel continuously through the symplast pathways?
-The root hair cell with water has a higher water potential than the next cell along
-As the water leaves the root hair cell by osmosis, the water potential of the cell falls again, maintaining the steep water potential gradient
In an apoplast pathway, what does the water fill?
The spaces between the loose, open network fibres in the cellulose cell wall
How does water continue to move in the apoplast pathway?
-As more water molecules move into the xylem, more water molecules are pulled through the apoplast behind them (because of the cohesive forces between the water molecules)
-The cohesive forces create tension, which creates a continuous flow of water
Which is faster, apoplastic or symplastic movement?
Apoplastic
When does water stop travelling through the apoplast pathways?
When it reaches the endodermis, containing the casparian strip
What happens to water in the apoplast pathway when it reaches the casparian strip?
It cannot pass through, so it is forced into the cytoplasm of the cell, joining the water in the symplast pathway.
How does water from the apoplast pathway enter the symplast pathway?
It passes through the selectively permeable cell-surface membrane
How does water enter the xylem from the endodermis through the symplast pathway? (Discuss osmosis/diffusion/active transport)
-The solute concentration in the cytoplasm of the endodermal cells is dilute compared to the cells in the xylem (so the water potential of the xylem is lower)
-Endodermal cells move mineral ions into the xylem by active transport
-So overall, the water potential of xylem cells is lower than the water potential of the endodermal cells
-Therefore, the water moves into the xylem, and the rate is increased because it is pushed by the mineral ions moving in
What happens to the water that has left the apoplast pathway once it is in the vascular bundle?
Returns to the apoplast pathway to enter the xylem and move up the plant
Why are minerals actively pumped into the xylem?
To transport them, and also to produce movement of water by osmosis
What does the movement of water caused by the active pumping of minerals into the xylem result in?
Root pressure
What does root pressure do?
Gives water a push up the xylem
How is guttation evidence for the role of active transport in root pressure?
Guttation is the exudation of xylem sap, and it occurs when TRANSPIRATION IS LOW, and as a result of root pressure (which suggests that root pressure is not a linked to transpiration)
