LONG DISTANCE TRANSPORT OF WATER IN PLANT

Long Distance Transport of water

• For understanding of long distance transport we examined an experiment where a plant of white flower placed in colored water and after a few hours we look the flower is colored and when we cut the their twig (stem) transversely then look some spot of color from which water goes called xylem (vascular bundle)
•  For long distance transport diffusion, active transport etc, are not successful because these are the slow process so a it is necessary that should have a transport system called mass or bulk flow system.
•  The mass flow system is the system in which movement of substances (water, minerals and foods) takes place in the bulk from one point to another point with the help of pressure different between two points.
•  Bulk flow can be achieved either by Positive Hydrostatic Pressure Gradient (e.g. garden house) or Negative Hydrostatic Pressure gradient (e.g. suction through a straw).
• The bulk flow system conducted through the vascular tissues from one point to another point called translocation.
• Vascular tissues are of two types as

1.       Xylem:- Translocation of water, mineral salt, some organic nitrogen and hormones from roots to the terrestrial part of plant.

2.       Phloem:- Translocation of many organic and inorganic solutes from leaves to the other parts of plants.

Differences between bulk flow system and Diffusion:-

Bulk Flow System	Diffusion
1.       Lgon distance transport	Short distance transport
2.       All substances move at same place	Substances moves independently
3.       Achieved through positive or negative hydrostatic gradient.	Depends on the concentration gradient

Absorption of Water

• Generally roots absorb water with the help of millions root hairs, arises from the epidermal cells of root and works to absorb water through diffusion.
• Once water absorbs through root hairs can move into roots through following pathways

1. Apoplast Pathway
2. Symplast Pathway

·         Some plants have additional organ to absorb water and some extra substances according to need. E.g. Mycorrhiza in fungus ( because only roots are not sufficient for their absorption)

1.Apoplast Pathway

• Apoplast is the continuous system of adjacent cell walls throughout the plant except at the casparian strips of the endodermis in the roots so called non-living system.
• The apoplastic movement of water occurs through intracellular spaces and permeable cell wall as well as depends on the gradient but not cross the cell membrane.
• It does not interfere in the water movement through mass flow system.
• Some waters are evaporates into the intracellular spaces or atmosphere, tension develop on the continuous streaming of water.
• Most of the water movement in the root occurs through the apoplast because of loose packing of cortical cells but a band of matrix is present in the epidermis called casparian strip which are impervious to water so the water molecules directed to the wall region and a little enters into the cell and again and again then moves through the symplast to cross the endodermis to reach the xylem.

2.Symplast Pathway

• Symplast is the system interconnected protoplasts of cells through some pores called plasmodesmata.
• The symplastic movement of water occurs through cytoplasm of interconnected cells with the help of cytoplasmic streaming.
• Water enters into the cells through cell membrane so the movement is relatively slower.
• It further mixed with apoplastic water and enters into the xylem through endodermis.
• Water moves into the xylem freely and also a non-living so we called it a part of the apoplast.

Movement of water

• There are following type of movement occurs in the plant which are given as 

Upward Movement of water 

• After absorption of water through roots it moves into the vascular tissues and then into the various parts with the help of Root pressure and Transpiration pull.

Root Pressure

• It is caused by active transport of water, minerals and ions through the roots into the xylem.
• It is a positive pressure and is responsible for the pushing of water over a small height but fail to transport in tall trees.
• It not play major majority for the transport of water but most of the plant have need to transpiratory pull.
• For the observation of root pressure we takes an experiment, For this we choose a soft stemmed plant and cut their stem horizontally at atmospheric moisture after a few hours a drop of solution ooze come out at the cut region this is due to positive pressure and when we fix a rubber tube them collect and measure the rate of exudation and composition of exudates.
• In the night and early (low evaporation) at the tip of grasses and leaves of many herbivorous plant have many droplets of water comes from stomata this loss of water called guttation. 

Transpiration pull

• Large plants use the transpiration pull to transport of water upward because it transport the water at the rate of 15m/hours.
• Driven force arises by the leaves responsible for the transpiration pull.
• It is also called cohesion-tension-transpiration pull model of water transport.
• Transpiration pull is made possible by three physical properties of water as following as

1. Cohesion: - Mutual attraction between water molecules.

2. Adhesion:- Attraction between water molecules and polar surfaces of tracheary elements  in a xylem cells like tracheids and vessel elements.

3. Surface tension:- Attraction between water molecules is more in the liquid phase than in the gaseous phase.

• Due to this properties, water have high tensile strength and also gives the ability to resist a pulling force for high capillarity.
• The water moves into the leaves from the xylem by a pulling force generated by evaporation of water through stomata.
• The force generated by transpiration can create a pressure to lift the xylem column of water over 130m high.
•  Less than 1% of water reached at leaves for photosynthesis and growth, most of which are lost through stomata of leaves, this loss of water is called transpiration.
• A fully mature tree losses a thousand litter of water through its leaves on a hot dry day through transpiration

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