Transport in Plants - Water Transport Flashcards
define water potential
the tendency of water molecules to leave a solution
what is the function of root hairs
increase the root’s surface area and absorb water and mineral ions
how are mineral ions absorbed and why
- absorbed by active transport
- these minerals together with sugars and amino acids lower the water potential of the cytoplasm and vacuolar sap
- water then moves in by osmosis
does soil water have a low or high water potential
high so it has a very low concentration of dissolved minerals
does the cytoplasm and vacuolar sap of the root hair cell have a low or high water potential
low as they contain many solutes
how does water enter the root and what happens as a result
- water moves into the root cell by osmosis down the water potential gradient
- the cell becomes turgid
- the adjacent cell will have a lower water potential than the root hair cell
- water will continue moving from cell to cell by osmosis
how can water move across the root
- symplast pathway
- apoplast pathway
describe the symplast pathway
- water travels through the cytoplasm of cells
- the cytoplasm’s of the cells in the root are connected by plasmodesmata through holes in the cell walls
what are plasmodesmata
composed of thin strands of cytoplasm
describe the apoplast pathway
- water moves through the gaps in the cellulose cell walls (adhesion)
- dissolved mineral ions are carried with the water
- it does not pass through the cell membranes
- the water moving up the xylem together with the cohesion between water molecules means there is continuous flow
is the cellulose cell wall permeable
it is fully permeable
the open structure of cellulose offers little resistance to this continuous flow
where does the apoplast pathway stop
the endodermis
What is the casparian strip and where is it found
- band of water proof material
- found in cells in the endodermis
What is the casparian strip made from
Waxy suberin
Which pathway does the casparian strip block
The apoplast pathway, so water is forced into cells via the symplast pathway
How does the casparian strip cause water to move into the xylem
- water has to pass through the cell surface membrane which is selectively permeable
- transport proteins in the cell surface membrane carry out active transport to move mineral ions into the xylem
- lowering water potential means water will enter the xylem by osmosis
Can water pass back into the cortex once it has entered the xylem
No, as the apoplast pathway is blocked
what are the three ways water can move up the stem
- root pressure
- transpiration pull
- capillary action
how does root pressure move water up the stem
- mineral ions are actively transported into the xylem vessels at the roots
- this lowers the water potential, causing water to enter by osmosis and forces water up the stem
what is root pressure affected by
metabolic poisons, temperature and oxygen concentrations (all affect active transport)
how does transpiration pull move water up the stem
- water molecules are attracted to each other by cohesive forces
- this forms a long continuous column of water in the xylem
- as water is lost at the top via transpiration, the column is pulled up through the xylem
what does the pull of water caused by transpiration create
this creates tension in the column. the lignin prevents the xylem collapsing inwards with this tension
what happens if the column is broken
water can flow into adjacent xylem vessels via the pits
how does capillary action move water up the stem
adhesive forces between water molecules and lignin in the narrow walls of the xylem vessels help pull the water up the xylem vessels
why does capillary action only work in a narrow container
greater proportion of water in contact with the walls = greater adhesive forces
