Introduction:

Water is very important to plant life and well being. It forms 80-90% of fresh weight of plants. Plants develop in soil and absorb water and minerals that is accessible in soil. So water has great significance for plants. Water functions as the reactant and a product. Water and dissolved substances are transported inside and outside cell through procedure of diffusion and osmosis. The understanding of plant water relations relies on understanding of principle of diffusion and osmosis.

Diffusion:

Diffusion is movement of molecules or ions of liquid, solid or gas from area of the greater concentration to the area of lesser concentration. Diffusion continues till dynamic equilibrium is established. At this phase of dynamic equilibrium, movement of molecules is equivalent in both directions thus, diffusion procedure stops because of equal movement of molecules. On account of the kinetic energy that is present in molecules, substances are distributed all through available space. Movement of molecules relies on kinetic energy during diffusion. Movement of molecules happens from the higher energy towards the lower energy. Diffusion of one substance is independent of the other.

Diffusion pressure:

As molecules diffuse from region of higher concentration to area of lower concentration, diffusing molecules likely to apply the type of pressure called as diffusion pressure. This is developed because of difference in concentration of molecules of materials. Diffusion pressure is high at larger concentration and low at smaller concentration. Diffusion pressure of the pure solvent is forever higher than its solution. Water molecules moves from the higher concentration to lower concentration in plants.

Factors affecting rate of diffusion:

Temperature:

Rate of diffusion is directly proportional to temperature, it signifies rate of diffusion increases with increase in temperature. Rate of diffusion α temperature. Diffusion stops roughly at 0°C.

Density:

Rate of diffusion is inversely proportional to square root of the relative density. It signifies rate of diffusion lowers down with increasing density. Diffusion of materials is in following sequence according to the density. Rate of diffusion is greatest in gas, followed by liquid and least in solid.

Pressure:

Rate of diffusion is directly proportional to pressure, it signifies rate of diffusion increases with increase in pressure. Rate of diffusion α pressure.

Size of molecule:

Diffused molecules/atoms/ions are inversely proportional to the size and mass. The rate of diffusion reduces with increase in size.

Importance of diffusion:

• Exchange of gases such as CO₂, O₂ happens through diffusion.
• Absorption of necessary elements from soil occurs through the process.
• Distribution of hormones that are synthesized in plants takes place through diffusion.
• Process of transpiration is the diffusion process. Evaporation of water from intercellular spaces is linked with diffusion during transpiration.
• Process of osmosis is the special kind of diffusion process.

Permeability:

Permeability refers to exchange of materials through the membrane. There are four different kinds of membranes on basis of the permeability:

Permeable membranes:

These are membranes which permit movement of both - solutes and solvent through them, like cellulosic cell wall.

Impermeable membranes:

These are membranes which don't permit movement of substances through them, example cutinized cell wall.

Semi permeable membranes:

These are membranes that permit passage of solvent molecules but don't permit passage of solutes, example parchment membrane, celloidin membrane.

Selectively-permeable membrane or differentially permeable membranes:

These are membranes that permit selective solutes to pass through them along with solvent molecules, like plasma membrane, tonoplast. This membrane is permeable for N₂, O₂, CO₂, gases and alcohol, ether and water; but impermeable for proteins and polysaccharides.

Osmosis:

Osmosis is movement of solvent or water molecules from region of the higher diffusion pressure or free energy to region of the lower diffusion pressure or free energy across the semi-permeable membrane.

When the cell is placed in hypertonic solution, water molecules emerge from cell in outer solution. This is termed Exosmosis. It is movement water molecules out of cell. Due to exosmosis, protoplasm shrinks and leaves cell wall and therefore, cell becomes flaccid, that is known as plasmolysed cell and phenomenon is called plasmolysis. Therefore, exosmosis leads to plasmolysis. Though, endosmosis is entry of water molecules in cell during osmosis. If plamolysed (flaccid) cell is placed in hypotonic solution water enters in cell. Endosmosis makes restores turgidity of cell.

Osmotic pressure:

Osmotic pressure can be stated as pressure that is developed in the solution in which solution and water is divided by semi-permeable membrane or as hydrostatic pressure developed in solution that is just sufficient to stop entry of solvent in solution when two are divided by the semi-permeable membrane. Osmotic pressure of water is zero. This is because of presence of solute in solution. Osmotic pressure of solution is directly proportional to concentration of solute in it. Osmotic pressure illustrates maximum variation in plants cells. Highest osmotic pressure is found in halophytes. Lowest osmotic pressure is found in aquatic plants or hydrophytes. Osmotic pressure of the solution is estimated by osmometer. Value of osmotic pressure relies on concentration of solution, the higher the concentration of solutes in solution, the greater the osmotic pressure. Osmotic pressure of solution is estimated mathematically as follows:

OP = mRT

where m = molar concentration

R = Gas constant [0.082 mole/molecules]

T = Absolute temperature

Factors affecting osmotic pressure

a) Concentration: Osmotic pressure of the solution increases with increase in concentration of the solutes.

b) Temperature: Osmotic pressure of the solution is proportional to temperature. This means that as temperature increases osmotic pressure also increases.

c) Dissociation of solutes also increases osmotic pressure of solution.

Importance of osmosis:

• Root hairs of roots absorb water from soil through process of osmosis.
• Conduction of water from one cell to another cell in plant and distribution of water in plant by phenomenon of osmosis.
• Turgidity is developed by process of endosmosis that assists to maintain the definite shape of leaves, stem and flowers. Turgidity also gives mechanical strength to plants.
• Opening and closing of stomata also relies on process of osmosis.
• Resistance is increased because of high osmotic concentration against dry climate and cold temperature [below 0ºC].
• Development of young cells depends as result of osmosis. Other daily activities also happen by osmosis and plasmolysis.
• Fresh water growing plants and animals either wilt or die when they are remained in marine water.

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