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Water transport in plants; - Coggle Diagram
Water transport in plants;
The use of water in plants;
Raw material for photosynthesis.
Mineral ions and products of photosynthesis are transported in aqueous solutions.
Evaporation cools leaves.
Turgor drives cell expansion.
Definitions;
Turgor -
the pressure exerted by the cell surface membrane against the cell wall in a plant cell.
Hydro -
water.
-static -
unmoving.
Hydrostatic pressure -
the pressure created by water in an enclosed system.
Cohesion -
Attraction of water molecules to one another.
Tension -
the pressure created up the xylem due to higher pressure in the roots and lower pressure in the leaves.
Adhesion -
attraction of water molecules to the walls of the xylem.
Turgor and water potential;
How do plants manage to move substances over vast distances? = Water is always under pressure (+/-), pressure is measured in mmHg (mm of mercury) or PA, eg; in a leaf ~1.5 Mpa => 11251 mmHg. Turgor pressure is otherwise known as hydrostatic pressure.
Root structure;
Epidermis;
A single layer of cells often with long extensions called root hairs, which increase the surface area enormously.
Endondermis;
A single layer of tightly packed cells containing a waterproof layer called the Casparian strip. This prevents the movement of water between the cells.
Vascular tissue;
This contains xylem and phloem cells, which are continuous with the stem vascular bundles. The arrangement is different, and the xylem usually forms a star shape with 2-6 arms.
Cortex;
A thick layer of packing cells often containing stored starch.
Pericycle;
A layer of undifferentiated meristematic (growing) cells.
Root hair cells (specialised epidermal cells) - microscopic to penetrate between soil particles. Large SA:V of each one and thousands on growing root tips. Thin surface layer through which exchange can rapidly occur. Concentration of solutes in the cytoplasm maintains a water potential gradient between the soil water and the cell.
Water enters the root by osmosis;
Soil water has a very low concentration of dissolved minerals so a high water potential.
Cells in the epidermis absorb minerals through active transport.
The cytoplasm and vascular sap contain many solutes (sugars, mineral ions, amino acids) so the water potential is lower.
Result = water moves into the root hair cells by osmosis.
Movement of water into the xylem;
There is three routes water can take;
1 - The apoplast pathway through the cell walls and inter-cellular spaces.
2 - The symplast pathway through the symplast, the continuous cytoplasm as connected by plasmodesmata.
3 - The vacuolar pathway through the vacuoles! More complex route due to movement across membranes.
Apoplast pathway;
Water moves through the fluid filled space of the cellulose cell wall. Dissolved minerals are carried with the water.
The cell walls are quite thick with an open network of fibres, so water can easily diffuse through cell walls without having to cross cell membranes by osmosis.
However the apoplast pathway stops at the endoderms because of the waterproof Casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast pathway.
The membrane then can filter out toxins and other minerals not needed by the plant before loading the xylem.
This allows the plant to have some control over the uptake of water into the xylem.
As water molecules move into the xylem, more water molecules are pulled behind them due to the cohesive forces of water that create tension, thus a continuous flow.
Symplast pathway;
Water travels through the cytoplasm of cells.
The cytoplasm of all the living cells in the root are connected by the plasmodesmata through holes in the cell wall.
The plasmodesmata are composed of a thin layer of cytoplasm.
As water moves into the cytoplasm of one cell it dilutes the cytoplasm and thus increases he hydrostatic pressure of the adjacent cell and thus water moves from cell to cell by osmosis.
Vacuolar pathway;
The pathway which water takes is not confined to the cytoplasm.
Water can teavel through the vacuole.
More complex as it involves several movements across several membranes/structures such as tonoplastal membrane of vacuole and cell membrane/cell wall.
The Casparian strip in the endodermis (layer surrounding the vascular tissue) results in water only moving by the symplast pathway.
The solute concentrayion of the cytoplasm of the endodermal cells is relatively dilute to that of the xylem.
Endodermal cells also actively transport mineral ions into the xylem.
-So water potential of xylem cells is lower than that of the endodermal cells.
Increasing the rate of water movement into the xylem down the water potential gradient by osmosis.
Once inside the vascular bundle, water return to the apoplast pathway to enter the xylem.
Endodermis cells actively pump mineral ions eg; nitrates into the xylem, to produce movement of water by osmosis, this results in root pressure.
Root pressure results in a 'push' of water up the xylem but this is not a major factor in movement of water up the xylem.
Evidence of this;
Poisons such as Cyanide affect the mitochondria of roots so ATP energy cannot be created. When cyanide is applied, root pressure disappears (no active transport, no movement of water).
Root pressure increases with a rise in temperature, suggesting chemical reactions are involved.
If O2 levels drop, root pressure falls (no transpiration, no ATP, no active transport).
Guttation occurs overnight when transpiration is low - xylem sap seeps out of leaves proving maintenance of pressure even when there is little transpiration.