Lesson 12 - Water transport in plants Flashcards
Why is water transport in plants significant?
do not have a muscular heart beating to move fluids through the vessels of the xylem and phloem.
Turgor pressure?
Hydrostatic pressure.
What causes turgor pressure?
Osmosis in plant cell provides a hydrostatic skeleton to support the stems and leaves.
What does turgor pressure cause?
Cell expansion. It is the force that enables the plant roots to force their way through tarmac and concrete.
What is the purpose of water loss?
Keeps plants cool.
How are mineral ions and the products of photosynthesis transported?
In aqueous solution.
How is water used in photosynthesis?
Reactant.
Where is water taken into the roots from the soil?
Root hair cells
What is a root hair?
A long , thin extension from a root hair cell. Specialised epidermal cell found near growing root tip.
What is a root hair cell?
Specialised cells responsible for the uptake of water and minerals from the soil. They have long hair-like extensions known as root hairs, which are adapted as exchange surfaces.
Adaptations of root hairs?
- Microscopic size means that they can penetrate easily between soil particles.
- Each microscopic hair has a large SA:V and there are millions growing on each root tip.
- Thin surface layer through which diffusion and osmosis can take place quickly.
- Concentration of solutes in the cytoplasm of root hair cells maintains water potential gradient between the soil water and the cell.
Soil water:
has a very low concentration of dissolved minerals so it has a very high water potential.
Cytoplasm and vacuolar sap:
contains many different solvents, including sugars, mineral ions and amino acids so the water potential in the cell is lower.
Transport to the xylem:
- apoplast pathway
- symplast pathway
Symplast?
Continuous cytoplasm of the living plant cells that is connected through the plasmodesmata. Moves water through the cortex.
How does water move through the symplast?
Osmosis
Explain the movement of water through the symplast?
Root hair cell has a higher water potential than the next cell along. Water moves through a partially permeable membrane, down the water potential gradient, in a net movement, via osmosis.
How is the water potential gradient maintained?
As water leaves the root hair cell by osmosis, the water potential in the root hair cell cytoplasm decreases. Water continues to move from the soil into the root hair cell.
Why is the symplast pathway relatively slow?
Pathway for water in the cytoplasm is obstructed by cell organelles.
Where does water move in the apoplast pathway?
Through cell walls and intercellular spaces.
How does water move through the cell wall in the apoplast pathway?
Water fills spaces between loose, open network of fibres in cellulose cell wall.
Movement of water through the apoplast pathway?
As water molecules are pulled into the xylem, more cell molecules are pulled through the apoplast pathway due to cohesive forces between water molecules.
Resistance in cell wall? Why?
Little to no resistance.
This is due to cohesive forces between molecules that create tension causing continuous flow through open structure.
Features of the endodermis?
- Layer of cells surrounding the vascular tissue (xylem and phloem) of the roots.
- Casparian strip
Casparian strip:
Band of waxy material called suberin. Runs around each of the endodermal cells creating a waterproof layer.
Result of the casparian strip?
- Water in the apoplast pathway is forced into the cytoplasm of the cell, joining water in the symplast pathway.
Movement from apoplast to symplast pathway?
Water passes through selectively permeable cell surface membrane. This prevents any potentially-toxic material from reaching living tissues (membranes have no carrier proteins to admit them).
Movement of water from symplast to xylem?
Solute concentration in the cytoplasm of endodermal cells is relatively dilute in comparison to the cells in the xylem.
Endodermal cells move mineral ions into the xylem by active transport.
Result of mineral ions transported into the xylem?
Water potential in xylem cells is much lower than in endodermal cells.
This increases the rate of diffusion.
What happens when water enters the vascular bundle?
Water returns to apoplast pathway until it reaches the xylem.
What is the system that water enters xylem with?
Root pressure
(Not transpiration)
Why is root pressure an active process?
Active transport used to transport mineral ions from the endodermis cells to xylem.
Evidence for role of active transport in root pressure?
- Poisons that effect the rate of respiration
Cyanide - affect mitochondria and prevent production of ATP.
- When cyanide is applied to root hair cells then root pressure disappears. No energy supply.
Evidence for role of active transport in root pressure?
- temperature
Root pressure increases when temperature increases.
Root pressure decreases when temperature decreases.
Suggests that chemical reactions are involved.
Evidence for role of active transport in root pressure?
- Oxygen supply
- When levels of oxygen / respiratory substrates fall, root pressure decreases.
Evidence for role of active transport in root pressure?
- Guttation
Xylem sap may exude from cut end of stems at certain times. In the natural world, sap is forced out of special pores at ends of leaves in some conditions (e.g. overnight when transpiration is low).
