Means of Transport 3
Active Transport: Uses energy to pump molecules against a concentration gradient. Casparian strip presents in the endodermis (the inner boundary of cortex) impervious to water.
- Penetration of water molecules are unable, so they are directed to wall regions that are not suberised into the cells.
- Then, water movements is symplastic and again cross a membrane to reach the cells of xylem.
- Water and other solutes enters in vascular cylinder in this way only.
- The water is full to move between cells as well as through them, once inside the xylem.
In young roots, water enters directly into xylem vessels or tracheids. Which are part of apoplast.
- Some associated structure also help in absorption like in mycorrhiza is symbiotic association of fungus with root system.
- The fungal filaments penetrate root also. The hyphal absorb mineral ions and water from soil because of their large surface area.
- The root provide sugar and N-containing compounds to fungus. In return, fungus provides minerals and water to roots.
- In plants like Pinus, seeds can’t germinate without presence of mycorrhizae.
Movement of water up a Plant:
- Water inside root follows and increases pressure inside xylem as various ions from soil are activity transported into vascular tissues of roots. This pressure is called root pressure.
- This pressure is responsible for pushing water up to short distance.
Water absorption of plants:-
- Water absorbs from soil by root hairs which are present at the tips of root.
- These root hairs increase the area for absorption and absorbs water and minerals by the process of diffusion.
- Then further movement of water in roots takes place by two different methods.
- Apoplast pathway:- It consists of interconnecting protoplasts.
- Plasmodesmata: Neighboring cells are connected through cytoplasmic strands that extend through plasmodesmata.
- Water moves through plasmodesmata (intracellular movement)
- Movement of water is slower as water has to enter into the cell through down a potential gradient.
- Aided by cytoplasmic streaming (e.g. in cells of hydrilla leaves)
- Apoplast Pathway:- It consist of adjacent cell walls (i.e. continuous throughout the plant except at casparian strips of endodermis in roots).
- Movement of water is through intercellular spaces and walls of cells.
- Water movement is through mass flow and dependent on the gradient.
Adhesive and cohesive properties of water is responsible for mass flow as water evaporates, tension develop in continuous stream of water in apoplast.
The cortical cells are loosely packed so no resistance to water movement, so most of the water movement is through apoplast in roots.
Significance of Imbibition:
- First step of water absorption.
- First phenomenon in seeds germination.
Characteristics of Imbibition:
- Pressure is developed.
- Heat of wetting is developed.
- Increase in volume of Imbibant.
Long Distance transport of water:
In plants, special long distance transport is necessary as sites of production or absorption and sites for storage too far and at much faster rate. This is generally done by mass or bulk flow system.
As a result of pressure differences between two points causes movement of substances in bulk or enbulk.
Bulk flow depends upon +ve hydrostatic pressure gradient or –ve hydrostatic pressure gradient +ve in garden hose and –ve in suction through straw.
Translocation:- Through conducting or vascular tissues of plant, bulk movement of substances takes place. i.e. translocation.
Specialized Vascular Tissues:-
Translocation of mainly water, mineral, some organic N2 and hormones (From root to aerial parts of plant)
Translocation of variety of organic and inorganic solutes (leaves to other parts of plant)
- Three types of solution:-
- Isotonic solution:- When conc. of cell sap is equal to conc. of solution in which cell is dept.
- Hypertonic solution:- When cell sap conc. is less than conc. of solution in which cell is kept.
- Hypotonic solution:- When cell sap conc. is more than conc. of solution is which cell is kept.
- In hypertonic solution, water moves out from cell and cell membrane of plant cell shrinks away from cell wall. This process is called plasmolysis. Water moves out from area of high potential to low potential of water.
- Cells are said to be flaccid if water moves in and out from the cell is in equilibrium.
- Turgor pressure: It is pressure built against the cell wall when water diffuses into the cell. This is called pressure potential (Ѱp). This pressure is responsible for enlargements and extension growth of cells.
- A special type of diffusion when water is absorbed by solids:- colloids is called imbibitions that results in increase in volume.
e.g. Absorption of water by wood
- Pressure produced during imbibitions is called Imbibitional pressure.
- Imbibition involves adsorption i.e. partial absorption and is property of colloids.
Solute Potential: The amount by which water potential is reduced as a result of presence of solute is called solute potential. Also known as osmotic potential.
- Represented in bars.
- Osmotic potential and osmotic pressure are numerically equal but osmotic potential is –ve.
- Indicated by ѰS.
It operates in plant cells as wall pressure and Turgor pressure.
- Usually has +ve value.
- Indicated by Ѱp.
Ѱw = Ѱs + Ѱp + Ѱg Here Ѱg = gravitational potential
Ѱw = Ѱs + Ѱp + Ѱm Here Ѱm = Matrix potential
So, Ѱw = Ѱs + Ѱp
Osmoses:- The process of diffusion though semi-permeable membrane is called osmosis.
- Discovered by Pfeiffer
- Solvent moves from its high conc. to lower concentration.
- The net direction and rate of osmosis depends upon pressure gradient and conc. gradient.
Osmotic pressure:- When semi-pressure membrane respects concentrated solution from weaker solution, pressure in atmosphere required in opposite direction to stop the entry of water molecules.
Uphill transport:- Protein pumps transport substances from low conc. to high conc. These proteins are sensitive to inhibitors that react with protein side chains.