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CHAPTER: 5
Diffusion & Osmosis

Solutions:
•      Solutions are made of solute and a solvent
•      Solvent - the liquid into which the solute is poured and dissolved. Usually we will use water as solvent.

•      Solute - substance that is dissolved or put into the solvent. Salt, sugar and sucrose are solutes.





Significance of Diffusion:
Importance of Diffusion to plant life in general: 
  1. It is an essential process in exchange of gases (02 and CO2) during respiration and photosynthesis. 
  2. Uptake of minerals is also affected by the process of diffusion. 
  3.  It helps in removal of excess water by the process of transpiration. 
  4. Translocation of organic solutes also takes place by diffusion means. 
  5. Fragrance of flowers or sweet scent emitted by flowers spread in air by diffusion means attracts insects to materialize pollination. 



Factor Affecting the Rate of Diffusion:
Ø  Particle size, temperature, concentration difference and diffusion distance affect the rate of diffusion.
1.Particle Size:
•      The particle size highly influences the rate of diffusion. Since the heat of the environment is the energy source for diffusion, a smaller particle at a given temperature moves faster than a larger particle. The rate of diffusion shares an inversely proportional relationship with the particle size.
2. Temperature:
•      Temperature and the rate of diffusion have a directly proportional relationship with one another. The rate of diffusion increases as the temperature increases. At higher temperatures, particles move faster because more energy is available to diffuse them.
3.Concentration difference:
•      The rate of diffusion increases as the concentration difference increases. A substance diffusing between two areas exhibits a concentration difference as the particles diffuse from one side of the wall to the other side. For example, if a semipermeable bag of plain water is placed in salt water, the rate of diffusion increases because the salt water has a higher concentration of particles than the plain water.
4.Density of Diffusing Substance:
The rate of diffusion is inversely proportional to the square root of their relative density of their diffusing substance. The larger the molecule, slower is the rate of its diffusion. 

Diffusion Pressure (DP):
•      The pressure exerted due to the tendency of the particles of a substance to diffuse is called its diffusion pressure.
•      The DP is directly proportional to the concentration of the number of diffusing particles.
•      The greater is the concentration of particles, the greater is their diffusion pressure.
Diffusion Pressure Deficit (DPD) or Suction Pressure:
•      Diffusion pressure of a solution is always lower than its pure solvent.
•      The difference between the diffusion pressure of the solution and its solvent at a particular temperature and  atmospheric condition called as diffusion pressure deficit (DPD).
•      It is increased by the addition of solutes, lowering temperature and pressure.
•      If the solution is more concentrated, its DPD increases, but it decreases with the dilution of the solution.
•       DPD is directly proportional to the concentration of the solution.
•      The DPD of the cell sap or the cells is a measure of the ability of the cells to absorb water and hence it is often called as suction pressure (SP).
•      It is related with osmotic pressure (OP) and turgor pressure (TP) of cell sap and also the wall pressure (WP) as,
                        DPD (or, SP) = OP – WP
                                    WP = TP
                                    DPD = OP – TP
                        In fully turgid cell (endo-osmosis)
                                    OP = TP , therefore DPD = 0
                        In fully palsmolysed cell  (Exo-osmosis)
                                    TP = 0 , Therefore, DPD = OP



•      Osmosis is the movement of WATER across a semi-permeable membrane
•      At first the concentration of solute is very high on the left.
But over time, the water moves across the semi-permeable membrane and dilutes the particles.
  • Osmosis is a special case of diffusion
  • Osmosis involves the diffusion of water through a membrane
  • The membrane may be artificial and  non-living  e.g. Cellophane
  • In biology, the important membrane is the cell membrane

Osmotic Pressure:
As a result of separation of solution from its solvent or the two solution by the semi-permeable membrane, a pressure is developed in solution due to the presence of dissolved solute in it. This is called as osmotic pressure(O.P.) 
Osmotic pressure is directly proportional to the concentration of dissolved solute in the solution.
* More concentration solution has higher osmotic pressure  



Hypotonic – The solution on one side of a membrane where the solute concentration is less than on the other side. Hypotonic Solutions contain a low concentration of solute relative to another solution.




Hypertonic – The solution on one side of a membrane where the solute concentration is greater than on the other side. Hypertonic Solutions contain a high concentration of solute relative to another solution.






Osmosis system & Plant cell:
•      In plant cell semi-permeable membrane is plasma membrane & cell sap
•      The solvent in case of plant is always water
•      If a living plant cell or tissue is placed in water or hypotonic solution water enters into the cell sap by osmosis/end osmosis
Endosmosis & Exosmosis:
•      If a living plant cell or tissue is placed in water or hypotonicsolution(whose O.P. is lower than that of cell sap) water enter into the cell sap by osmosis. This process is called as endosmosis
•      Entry of water into cell sap – pressure developed and presses the protoplasm  against the cell wall – turgid. The pressure is called turgid pressure.
•      If the plant cell or the tissue placed in hypertonic solution (whose O.P. is higher than that of cell sap) the water comes out of the cell sap into the outer solution and the cell become Flaccid. The process is known as exosomosis.

Plant cell in High water potential:
1.     Cell vacuole has lower water potential compared to solutions outside cell
2.     Water enters cell by osmosis.
3.     Vacuole increases in size, pushes against cell wall
4.     Cell wall exerts opposing pressure (against turgor pressure)
5.     Plant cell expands and become turgid (cell does not bursts) .




Plant cell in Low water potential:
  1. Vacuole has higher water potential compared to solution outside cell.
  2. Water leave cells by osmosis
  3. Vacuole decreases in size
  4.  Cytoplasm shrinks away from cell wall ( Plasmolysis.)
Significant of osmosis in plant:
•      Large quantities of water are absorbed by roots from the soil by osmosis
•      Cell to cell movement of water and other substance dissolved in it involves this process
•      Opening and closing of stomata depend upon the turgid pressure of the guard cell
•      The resistance of plants to drought and frost increases with increases in osmotic pressure of their cell.
•      Turgidity of cells of the young seedling allows them to come out of the soil








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CHAPTER: 4
Introduction to Cell Water Relations

Water:

•      Water being considered as universal solvent, occupies 75% of our planet in the form of oceans.
•      The evaporation of water from the surface of ocean, formation clouds and raining, is a natural cycle evolved during course of Evolution of this planet.
•      In the course of Chemical Evolution, the birth of life has chosen H2O as the medium of biochemical activities.  Thus water has become mother of life or “Solvent of Life”.
•      Cells of all organisms are made up 90% or more of water.  And all other components are either dissolved or suspended in water to form protoplasm, which is often referred to as physical basis of life.
•      Water is a remarkable compounded made up of Hydrogen and oxygen (2:1) and it has high specific heat, high heat of vaporization, high heat of fusion and expansion (colligative properties)

Importance of Water:

•      Water is the major component of living cells and constitutes more than 90% of protoplasm by volume and weight.
•      It acts as medium for all biochemical reaction that takes place in the cell, and also acts a medium of transportation from one region to another region. 
•      Water because of its bipolar nature acts as universal solvent for it dissolves more substances than any other solvent.  Electrolytes and non-electrolytes like sugars, and proteins dissolve very well.  Even some hydrophobic lipid molecules show some solubility in water.
•      Water acts as a good buffer against changes in the Hydrogen ion concentration (pH).  This is because of its ionization property.  Certain xerophytes use water as buffer system against high temperature.
•      Water is an important substrate in photosynthesis, for it provides reducing power in CO2 fixation; water is also used in breaking or making chemical bonds of polypeptides, poly-nucleotides, carbohydrates etc.
•      Water also exhibits viscosity and adhesive properties. 
•      Because of hydrogen bonds, water molecules are attracted towards each other, they are held to each other with considerable force.  This force of attraction is called cohesive force. 
•      Thus water possesses a high tensile strength.  If this water is confined in very narrow columns of dimensions of xylem vessels, its tensile and cohesive forces reach very high values (1000-1200 Gms).  And this force is very helpful in ascent of sap. 
•      Water is of great importance in osmoregulation, particularly in the maintenance of turgidity of cells, opening and closing of stomata and growth of the plant body.

Cell:

•      Cells are the basic structural units of organisms, and plant organization varies from single cells to aggregations of cells to complex multicellular structures.
•      With increasing complexity there are increasingly sophisticated systems for absorbing water, moving it large distances, and conserving it but fundamentally the cell remains the central unit that controls the plant response to water.
•      The driving forces for water movement are generated in the cells, and growth and metabolism occur in the aqueous medium provided by the cells.
•      The cell properties can change and result in acclimation to the water environment.
•      As a consequence, many features of complex multicellular plants can be understood only from a knowledge of the cell properties.

Cell Water Relation:

•      The term “Cell water relations” describes plant water status in a cell, individual organ (leaf, internode, flower) or whole plant level, furthering our understanding of basic plant growth and development, and plant response to the environment.
•       At the field level, water use and water use efficiency are the common means of evaluating a crop and its yield performance to seasonal water availability.
•      Water relations encompasses measurement techniques that describe general plant water status, the quantification of water in cell and tissue expansion, maintenance of turgor, and the overall stomatal gas exchange of plants according to moisture in the soil and aerial environment.
•      Water enters plants through the roots, and travels both through and around cells.
•      Water is moved up the plant mainly in the xylem vessels of the stem and in leaf veins, and water is transpired from the plant via stomata on the leaf surface.
•      From the 1960s, the discipline of water relations expanded mainly through the concept of water potential, allowing multidisciplinary research spanning soil, plant and aerial environments, and culminated in the practical management of crop water requirements.
•       In simple terms, water relations allow direct measurement of how much water is in the plant.

•      This information directly indicates how well the plant is performing and how the plant copes with stress – in contrast to indirect measurements of soil moisture content or rainfall deficit.



Ø  The cell water relation can be briefly discussed on the following headings:
  1. Water Potential
  2. Diffusion
  3. Osmosis
  4. Transpiration
  5. Stomatal physiology
  6. Ascent of Sap
Water Potential:

•      The free energy in water that is available to do work is described by water potential.
•      The water content in the soil, plants and atmosphere is usually described as water potential (Ψw).
•      Water in plants and soil moves in response to differences in water potential (Ψw).
•      Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action (which is caused by surface tension).

Ø  Water Potential (Ψw) = Ψs + Ψm + Ψt+ Ψg 
                               where, osmotic/solute potential (Ψs),
                                matrix potential (Ψm) ,
                                turgor potential (Ψt) or pressure potential &
                                gravitational potential (Ψg) significant in tall plant.

Ø  In this context, it is important to be familiar with the term called water potential (Ψw) which refers to the chemical free energy of water. The chemical free energy of pure water or solutes is always expressed in terms pressure units such as bars.
Ø  Purest form means there are no other molecules in it.  the potential of free pure water at atmospheric pressure and at a temperature of 25°C corresponds to 0 (zero) Mpa (mega pascal) or bar.

2. Diffusion:

•      Movement of molecules from an area of high concentration to an area of low concentration. movement from one side of a membrane to another .
•      Diffusion is the process by which substances move down a concentration gradient, from an area of high concentration to an area of low concentration. Diffusion happens in living systems, for example, it explains the movement of carbon dioxide in leaves.
Ø  Having own kinetic energy of water,  water molecules will be in constant motion randomly.



3. Osmosis:

•      If a solution & its pure solvent are separated by a semi permeable membrane the solvent molecule diffuse into solution. The diffusion of solvent molecule into the solution through a semi-permeable membrane is called osmosis  or osmotic diffusion.
•      Water, for that matter any solvent in its pure state has its own chemical potential by virtue of which it exhibits random movement.  This is referred to as chemical free energy or water potential. If such a solvent is separated from a solution (solvent + solute) by a semi permeable membrane, water molecules move from higher chemical or water potential to the lower water potential.  In this case pure water has higher chemical energy than the solution, for the solute present in water lowers the free chemical energy of pure solvent of the solution.



4. Transpiration:

•      Plant absorbs a large quantity of water from the soil by root hairs. Only a small part (1-2%) of this water is utilized by the plant in its life process. The remaining large part (98-99%) of water is lost in the form of vapor from the internal tissue. Loss of water in the form of vapor through the exposed aerial parts of plants is called Transpiration.
•      Transpiration helps in creating suction pressure, which facilitates the ascent of sap as well as absorption of water and to some extent absorption of minerals.  Transpiration keeps the cells in continuous flux.  It has cooling effect on the plant body and also helps in the development of good roof system and also helps in the development of good roof system and mechanical tissues.  Added to this, it helps in the development of good drainage in the soil.
•      Water absorbed by the root system is transported upwards and the same is always lost from the aerial surfaces of the plant body. In fact loss of water facilitates the absorption and translocation of water and minerals in the plant body.
•      If water is lost in the form of liquid, it is called Guttation; on the contrary if water is lost in the form of water vapors, it is considered as Transpiration.



5.Stomatal physiology:

•      When the water vapor escapes into the atmosphere through the stomata, it is called stomatal transpiration. The stomata constitute the chief pathways through which about 90% of the water vapor is lost by the aerial parts of the plant. The stomata are minute pores found on the epidermis. They allow water vapor to escape through the minute opening present between two guard cells.
•      Stomata plays a significant role in transpiration as a major part of water vapor is lost through the stomatal pore. Stomata exhibits periodic opening and closing during day. It depends upon heat, light, water content of the cell and humidity. Generally, stomata are open during the day and close at night. The opening and closing of the stomatal pore regulate the process of stomatal transpiration. The changes in turgor pressure of guard cells cause the opening and closing of stomatal pore.



6.Ascent of Sap:

•      The plants absorb a large quantity of water from the soil by root hairs.
•      From root hairs, it reaches to the top of the plant through xylem, where  approximately the same amount of water is transpired from the surface of the aerial parts of the plants to the atmosphere.
•      Water and minerals absorbed by the root hairs are called sap. The upward movement of sap from the root to the tip of plant is called ascent of sap.
•      Ascent of sap takes place against the gravitational force.






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