Water transport in multicellular plants Flashcards
(12 cards)
What are the key roles of water in the structure and metabolism of plants?
- Turgor pressure (or hydrostatic pressure) as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems around the leaves
- Turgor also drives expansion => it is is the force that enables plant roots to force their way through tarmac and concrete
- The loss of water by evaporatio helps to keep plants cool
- Mineral ions and the products of photosynthesis are transported in aqueous solutions
- Water is a raw material for photosynthesis
How are the root hairs of root hair cells adapted to their function?
- Their microscopic size means they can penetrate easily between soil particles
- Each microscopic hair has a large SA : V ratio and there are thousands on each growing root tip
- Each hair has a thin surface layer (just the cell wall and
cell-surface membrane) through which diffusion and osmosis can take place quickly - The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and
the cell
Explain why water moves from the soil into the root hair cells.
- Soil water has a very low concentration of dissolved minerals so it has a very high water potential
- The cytoplasm and vacuolar sap of the root hair cell (and the other root cells) contain many different solvents
including sugars, mineral ions, and amino acids so the water potential in the cell is lower - As a result water moves into the root hair cells by osmosis
Name the 2 pathways by which water travels across the root to the xylem.
- Symplast pathway
- Apoplast pathway
Describe the symplast pathway of water movement
- Water moves through the symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata - by osmosis
- 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 next door cell by osmosis - This process continues
from cell to cell across the root until the xylem is reached
Water travels down its ψ gradient:
soil => epidermis => cortex => endodermis => pericycle => xylem
How is a steep concentration gradient maintained in the symplast pathway?
As water leaves the root hair cells by osmosis, the water potential of the cytoplasm falls again, and this maintains a steep ψ gradient to ensure that as much water as possible continues to move into the cell from the soil
Describe the apoplast pathway of water movement.
- This is the movement of water through the apoplast - the cell wallsand 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 the apoplast behind them due to the cohesive forces between the water
molecules - The pull from water moving into the xylem and up the
plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water
through the open structure of the cellulose wall, which offers little or no resistance
What happens to the water in the apoplast pathway when it reaches the endodermis?
Due to the prescence of the casparin strip in the endodermis the water in the apoplast pathway can go no further and is forced into the cytoplasm of the cell, joining the symplast pathway
What is the casparin strip? And what is its function?
- It is a band of waxy cell wall material called suberin that runs around each of the endodermal cells forming a waterproof layer
- This blocks the apoplast pathway ensuring that water crossing the endodermis has to pass through the selectively permeable cell surface membranes
- This excludes any potentially toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them
Explain the role of active transport by endodermal cells for the movement of water
- The solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem
- In addition, it
appears that the endodermal cells move mineral ions into the xylem by active transport - As a result the water potential of the xylem cells
is much lower than the water potential of the endodermal cells - This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the
symplast pathway
What is root pressure?
- The active pumping of minerals into the xylem by root cells that produces a movement of water into the xylem by osmosis
- Root pressure gives water a push up the xylem
Describe the evidence for the role of active transport in moving water from root
endodermis into the xylem.
- Some poisons, such as cyanide, affect the mitochondria and prevent the production of ATP
- If cyanide is applied to root cells so there is no energy supply, the root pressure disappears
- Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved
- If levels of oxygen or respiratory substrates fall, root pressure falls
- Xylem sap may exude from the cut end of stems at certain times
- In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions - for example overnight, when
transpiration is low. This is known as guttation
