Efficiency of water uptake by a root hair cell: (structure and function)
Plants constantly lose water by transpiration, so it has to be absorbed (replaced) through root hair cell. Water is absorbed from the soil by osmosis, because it is the transportation of water. It moves down a concentration gradient. Water only enters the root near the root tip; where there are root hairs which increase the S.A for osmosis. Water is higher in the epidermal cells than the central cells (as the vacuole's water potential is low-due to containing minerals and ions). Water moves across the cortex down the water potential gradient to the xylem vessels.
Water can move via: symplast or apoplast routes.
- large S.A (thousands of v.long extensions on each root branch)
- short pathway (thin cell-surface membrane/cellulose cell wall)
- Water potential gradient (difference in water potential in and out of cell)
Plants constantly lose water by transpiration, so it has to be absorbed (replaced) through root hair cell. Water is absorbed from the soil by osmosis, because it is the transportation of water. It moves down a concentration gradient. Water only enters the root near the root tip; where there are root hairs which increase the S.A for osmosis. Water is higher in the epidermal cells than the central cells (as the vacuole's water potential is low-due to containing minerals and ions). Water moves across the cortex down the water potential gradient to the xylem vessels.
Water can move via: symplast or apoplast routes.
Apoplastic route
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Symplastic route
|
Passage of water into xylem:
· Endodermal cells actively transport salts into the xylem
· This requires E, an only occur in living tissue
· Endodermal cells have carrier proteins in its cell surface membrane
· For water to enter the xylem, it must enter the cytoplasm of the endodermal cells.
· This is how water from apoplastic pathway are forced into cytoplasm of endodermal cells by casparian strip
· Root Pa is created which helps move water up the plant
2 Pa:
Evidence for root Pa:
· Pa increases with an increase of Temp
· Metabolic inhibitors prevent most E release by respiration and stops root Pa
· When there is a lack of O2, or respiratory substances, it causes root Pa to decrease
Movement of water up through STEMS:
· Transpiration-evaporation of water from leaves-main force that pulls water up stems
· When stomata open, water vapour molecules diffuse out of air spaces into surrounding air
· Water loss from air spaces replaced by water evaporating from the cell walls of the surrounding mesophyll cells
· Plants control rate of transpiration by changing the size of the stomata openings
Movement of water across LEAVES:
· Water lost from mesophyll cells by evaporation from surfaces to air spaces of leaves
· Replaced by water from xylem via apoplastic or symplastic pathways
Cohesion- tension theory:
Mcgraw hill video- water uptake: http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_water_uptake.html
Casparian strip:
· Insoluble
· Waterproof band that stops the water from the A route when it reaches the endodermis
· Water is forced into the cytoplasm of the endodermis cell, where it joins water from the symplastic route
The following are passive:
· Uptake of water by root hairs- passive
· Movement of water from the cortex into an endodermal cell
· Movement of water from an endodermal cell into the xylem
When seedlings are transplanted they sometimes wilt and die because:
· Root hair damaged
· Air pockets instead of water
· Endodermal cells actively transport salts into the xylem
· This requires E, an only occur in living tissue
· Endodermal cells have carrier proteins in its cell surface membrane
· For water to enter the xylem, it must enter the cytoplasm of the endodermal cells.
· This is how water from apoplastic pathway are forced into cytoplasm of endodermal cells by casparian strip
· Root Pa is created which helps move water up the plant
2 Pa:
- Root Pa
- Transpiration pull
- Hg manometer can be used to test for root Pa, can be used as evidence.
Evidence for root Pa:
· Pa increases with an increase of Temp
· Metabolic inhibitors prevent most E release by respiration and stops root Pa
· When there is a lack of O2, or respiratory substances, it causes root Pa to decrease
Movement of water up through STEMS:
· Transpiration-evaporation of water from leaves-main force that pulls water up stems
· When stomata open, water vapour molecules diffuse out of air spaces into surrounding air
· Water loss from air spaces replaced by water evaporating from the cell walls of the surrounding mesophyll cells
· Plants control rate of transpiration by changing the size of the stomata openings
Movement of water across LEAVES:
· Water lost from mesophyll cells by evaporation from surfaces to air spaces of leaves
· Replaced by water from xylem via apoplastic or symplastic pathways
Cohesion- tension theory:
- Transpiration pull- water pulled up due to transpiration
- Tension on the xylem- due to transpiration, negative Pa within xylem
- This causes the water to move up the plant in a chain/continuous pathway (due to cohesion-sticking together)
- Passive- no E required
- Water molecules form H bonds between one another and causes them to stick.
- Evidence to support- tree trunk diameter throughout the day changes
- In the daytime transpiration rate is at its highest and branch diameter decreases, as evaporation is occurring from the leaves and through the stem, so the plant draws more water up by the roots.
Evaporation from the leaves draws water from the xylem by osmosis, water is pulled up the xylem creating a tension. The tension pulls the xylem vessel walls in, so the trunk diameter gets smaller. The trunk has a larger diameter when there is less transpiration. This supports the cohesion tension theory but not root Pa.
Mcgraw hill video- water uptake: http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_water_uptake.html
Casparian strip:
· Insoluble
· Waterproof band that stops the water from the A route when it reaches the endodermis
· Water is forced into the cytoplasm of the endodermis cell, where it joins water from the symplastic route
The following are passive:
· Uptake of water by root hairs- passive
· Movement of water from the cortex into an endodermal cell
· Movement of water from an endodermal cell into the xylem
When seedlings are transplanted they sometimes wilt and die because:
· Root hair damaged
· Air pockets instead of water