Water Transport in Plants : 1 Water Transport in Plants Water reaches to the top of tall tree in the absence of a pumping organ like heart. How does the plant do this?
Transportation of Substances in Plants : 2 Transportation of Substances in Plants Continuous transportation of water, mineral nutrients, organic nutrients and plant growth regulators
Short distance transport –
Diffusion
Cytoplasmic streaming supplemented by active transport
Long distance transport - xylem and phloem
Relationship Between Water and Plants : 3 Relationship Between Water and Plants Limiting element required for the growth of the plants.
Functions performed by water are -:
Acts as a primary component of protoplasm
It is one of the substrate required for plant metabolism
It acts as a solvent promoting absorption and transportation of minerals and nutrients
It keeps the plant in shape (extension)
It helps in maintenance of balance in plant temperature
Water Potential : 4 Water Potential It is the potential energy of water per unit volume.
It is denoted by the Greek letter psi Ψ
It is expressed in units of pressure that is pascals (Pa)
↑Concentration of water – ↑Water potential
Pure water has the highest water potential and its value is considered to be 0 at standard temperature, with no effect of pressure
Factors which Affect the Water Potential : 5 Factors which Affect the Water Potential Addition of solute
↑Concentration of solute ------ ↓Water potential
Solute potential - magnitude of lowering of the water potential due to the addition of solutes.
Represented as ΨS
Always negative
Alternatively known as osmotic potential
Factors which Affect the Water Potential : 6 Factors which Affect the Water Potential 2. Increase of pressure
Under atmospheric pressure -
Water potential ΨW = solute potential ΨS
Application of pressure to the solution ---- increases the water potential
Water potential due to pressure is denoted as ΨP
Total water potential or ΨW = ΨS + ΨP
Osmosis : 7 Osmosis Movement of solvent from a region of higher solvent concentration to a region of lower solvent concentration across a semi-permeable or a selectively permeable membrane
Does not require the input of energy
A physical process which tries to reduce the concentration gradient between the two solutions
Osmosis : 8 Osmosis
Osmotic Pressure : 9 Osmotic Pressure Pressure that is required to maintain a state of dynamic equilibrium between the two solutions
Solute concentration determines the osmotic pressure
Osmotic pressure =Solute potential (numerically)
Osmotic pressure is positive pressure
Solute potential is negative pressure
Osmotic Pressure : 10 Osmotic Pressure
Effect of Osmosis in Plant Cell : 11 Effect of Osmosis in Plant Cell
Transport of Water through a Long Distance : 12 Transport of Water through a Long Distance Diffusion is a slow process which supports a short distance movement of water.
Mass or bulk flow system
This system accounts for the transportation of water, minerals and food over long distances
The driving force is the difference in pressure between the two points
Takes place through vascular tissues like xylem and phloem
It is also termed as translocation
Transport of Water through a Long Distance : 13 Transport of Water through a Long Distance Bulk flow can be achieved in two ways -:
Through a positive hydrostatic pressure gradient
Through a negative hydrostatic pressure gradient
Absorption of Water by Plants : 14 Absorption of Water by Plants Absorption of water takes place through roots.
Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
Root hair cells absorb water through osmosis.
Pathway of Water : 15 Pathway of Water Root hairs →Cortical cells →Endodermis →Pericycle →Xylem
This movement of water is purely dependent on water potential and is brought about in two pathways which are -
Symplast pathway
Apoplast pathway
Symplast Pathway : 16 Symplast Pathway The symplast of a plant is the inner side of the plasma membrane and is made up of interconnected protoplasts.
The interconnections between the neighboring cells are established by plasmodesmata.
During this movement water travels from one cell to another via the connections of plasmodesmata.
The movement is down the potential gradient and slower as it has to enter the cell membrane.
This movement is aided by cytoplasmic streaming.
Apoplast pathway : 17 Apoplast pathway Structurally apoplast is composed of cell walls of continuously placed adjacent cells throughout the plant together with the extracellular spaces.
Apoplastic movement of water takes place through the intercellular spaces and the walls of the cells.
During this movement water does not cross the cell membrane and this movement is largely dependent on the gradient.
A mass flow of water due to adhesive and cohesive properties of water takes place during this pathway.
Apoplast pathway : 18 Apoplast pathway As the cortical cells are loosely packed they offer no resistance to the movement of water and allow most of the water to flow through apoplast pathway.
The cell walls of endodermal cells have depositions of water impermeable material called suberin.
This deposition is known as casparian strip.
The casparian stip blocks the apoplastic pathway of water that is between the endodermal cell and this block is termed as apoplast block.
Apoplast pathway : 19 Apoplast pathway To bypass this casparian strip, water enters the endodermal cells and follows the symplast pathway.
Hence symplast pathway is the only solution for the transport of water and other solutes to the vascular tissues.
The apoplast block inhibits the harmful substances from entering the xylem. Image reference - http://12knights.pbworks.com/w/page/25273922/926-Explain-how-water-is-carried
Details of a Symplast and Apoplast Pathway : 20 Details of a Symplast and Apoplast Pathway Image reference - http://scienceaid.co.uk/biology/plants/waterplants.html
Movement of Water to the Upper Parts of the Plant : 21 Movement of Water to the Upper Parts of the Plant Two forces which allow the movement of water to the upper parts of the plant are –
Transpiration pull
Evaporation of water from the cell walls of the sub-stomatal cavity within the leaf
Water gets pulled through the xylem vessels to the leaves from the roots continuously
This movement of water is according to the cohesion theory which allows continuous movement of water
Majority of water transportation is due to the transpiration pull.
Movement of Water to the Upper Parts of the Plant : 22 Movement of Water to the Upper Parts of the Plant Transpiration pull
Controlling factor for the movement of water in continuous columns is the rate of evaporation of water.
Environmental conditions which have a strong impact on it are –
Air temperature
Relative humidity
Wind speed
Radiation intensity
Movement of Water to the Upper Parts of the Plant : 23 Movement of Water to the Upper Parts of the Plant Osmotic pumping mechanism
This mechanism operates in the roots.
Solute concentration in the cell sap and in the soil water determines the absorption of water by the roots.
The movement of water into the roots allows the building up of pressure within the roots. This positive pressure forces water up the xylem.
This is known as root pressure and it allows the movement of water to small heights.
Movement of Water to the Upper Parts of the Plant : 24 Movement of Water to the Upper Parts of the Plant This mechanism acts continuously but the demand of water is more during the day light due to transpiration.
Hence osmotic pumping mechanism cannot meet the demand of the day light but is the major mechanism acting during the night time.
Primary function of root pressure –
Transpiration creates enormous tension as a result of which water column in xylem breaks. Root pressure tries to re-establish the continuity of the water columns.
Effect of Root Pressure : 25 Effect of Root Pressure Guttation
Collection of excess of water in the form of droplets around special openings of veins
Seen during early mornings and at night time
The rate of evaporation is low
Seen at the tip of the grass blades and leaves of many herbaceous plants
Summary of the Class : 26 Summary of the Class Water is an essential factor for plant growth and is transported in a unidirectional manner from roots to the stem.
Difference in water potential between the soil water and that of the cell sap allows the movement of water from soil to the root hairs through osmosis.
Movement of water through long distance is accomplished by bulk flow or mass flow system.
Bulk flow takes place through vascular tissue called xylem.
The two pathways through which water travels through the plant after entering the root hairs are
Apoplast pathway
Symplast pathway
Slide 27 : 27 Thank you