LONG DISTANCE TRANSPORT OF WATER_PART 02

For more information:

http://www.7activestudio.com
[email protected]

http://www.7activemedical.com/
[email protected]

http://www.sciencetuts.com/
[email protected]

Contact: +91- 9700061777,
040-64501777 / 65864777

7 Active Technology Solutions Pvt.Ltd. is an educational 3D digital content provider for K-12. We also customise the content as per your requirement for companies platform providers colleges etc . 7 Active driving force "The Joy of Happy Learning" -- is what makes difference from other digital content providers. We consider Student needs, Lecturer needs and College needs in designing the 3D & 2D Animated Video Lectures. We are carrying a huge 3D Digital Library ready to use.

LONG DISTANCE TRANSPORT OF WATER: A twig bearing white flowers is placed in coloured water. The region through which the coloured water moved, is noticed after few hours. This experiment clearly demonstrates that the path of water movement is through vascular bundles particularly the xylem. Long distance transport of substances within a plant cannot take place by diffusion alone. As diffusion is a slow process, it can account for only short distance movement of molecules. For instance, the movement of a molecule across typical plant cell about 50μm takes nearly 2.5 seconds. At this rate it would take years for the movement of molecules over a distance of 1 metre within a plant by diffusion alone. In huge plants, substances have to be moved across large distances. In some cases the sites of production or absorption and sites of storage are very far from each other; diffusion or active transport would not sufficient. Special long distance transport systems become essential to move substances across long distances at faster rate. Water, minerals and food are generally moved by bulk or en masse from one point to another as a result of pressure differences, whether in solution or in suspension, are swept along at the same pace, as in flowing river. This is unlike diffusion where different substances move independently depending on their concentration gradients. Bulk flow can be achieved either through positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient. The bulk movement of substances through the conducting or vascular tissue of plants is called translocation.
How do plants absorb water? Roots absorb capillary water from soil water present in the form of fine capillaries in micro pores of soil through unicellular root hairs. Root hairs are the walled slender outgrowths of epidermal cells which increase the water absorbing surface area of the root system. Absorption of water takes place on the basis of potential gradient and osmosis. The lower water potential of the root hairs and higher water potential of soil makes water to enter the cells of this gradient increases, the uptake of water also increases. The water absorbed by the root hairs, deeper into root layers by two distinct pathways. They are 1.Apoplast Pathway and 2. Symplast Pathway. Apoplast: Apoplast is the continuous system of cell walls and intercellular spaces in plant tissues. In apoplast, pathway of water movement is through the cell wall without crossing membranes. This movement is dependent on the gradient. The apoplast does not provide any barrier to water movement and water movement is through mass flow. As water evaporates into the intercellular spaces or atmosphere, tensions develop in the continuous stream of water in the apoplast, hence mass flow of water occurs due to adhesive & cohesive properties of water. Most of the water flow in the roots occurs via the apoplast, since the cortical cells are loosely packed, and hence offer no resistance to water movement. In young roots, water enters directly into the xylem vessels or trachieds. These are non-living and so are parts of apoplast.
Symplast: Symplast comprises the net work of cytoplasm of all cells interconnected by plasmodesmata. The movement of water occurs from one cell to another through plasmodermata. During symplastic movement, the water travels through the cells - their cytoplasm; intercellular movement is through the plasmodesmata. Water has to enter the cells through the cell membrane; hence the movement is relatively slower. Movement is down a potential gradient. Symplastic movement may be aided by cytoplasmic streaming. The inner boundary of the cortex, the endodermis is impervious to water because of a band of suberised matrix called casparian strip. Water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes. The water then moves through the symplast & again crosses a membrane to reach the cells of the xylem.