Water is a simple compound made up of hydrogen and oxygen in 2:1 ratio.  During the course of evolution of this planet, earth, Hydrogen and Oxygen that escaped into atmosphere reacted with each other to produce water in enormous quantities.  The dense clouds thus formed in the atmosphere came down to the earth in the form of torrential rains, which filled the shallow regions of the earth’s surface, which was still in the formation stage; thus developed lakes and oceans.  In fact 75% of the earth’s surface is underwater.  It is in this fresh water ocean, myriad of chemical reactions that ensued resulted in the formation of organic molecules.  The fresh water ocean with the inflow of minerals from flowing waters, gradually become salty; now the oceans contain salt water.  Finally, about 3.8 billion years ago, life originated in this ocean.  Since then, life forms multiplied and evolved into innumerable kinds.  Thus water has been considered as the mother of life forms of the past and the present.




Water in its fluid state; jach.hawaii.edu


Features and properties of liquids and fluidity; www. web.clark.edu





Properties of Water molecules:

All matters are made up of Elementary particles.  Each of them consists of central protons and neutrons; this is surrounded by electronic cloud revolves around the central nucleus in orbits.  The number of electrons depends upon the protons in the nucleus.  The number of electrons in the orbits is constant and fixed, Change in the number of electron also changes the property of the atom.













approx. 1 amu



approx. 1 amu



  1/1836 amu 






The importance of water to living organism stems from the fact that various properties of water are well suited for life’s various auto-regulatory biochemical activities. It is to be remembered that life originated in water and water has the most influencing properties on life for 90% of all living cell is all water.  Any thing that is not compatible with water, life is totally excluded.


Structurally water molecule has a configuration (structurally), where hydrogen atoms are linked to the central oxygen atoms with negative charge and H with positive charges.  Because of greater negative charges operating on the oxygen atom, electrons of hydrogen atoms are dragged towards Oxygen and the H atoms are more or less rendered positively charged.  So water molecules exhibits dipolar nature with its own dielectric constants.  Because of this property water molecules are held to each other by hydrogen bonding.  Such forces of attraction between two similar molecules are often called cohesive force.  Thus the mass of water is nothing but a network of (billions and trillions or maximum number you count) of water molecules held to each other in a 3-D configuration by hydrogen bonds.  Though the cohesive forces are weak, the overall number of such bonds in a given quantity of water is so large in numbers; one has to apply sufficient force to break the bonds particularly if they are confined in a narrow column of capillary dimensions.








Water molecules because of their electronic properties are held to each other by Hydrogen bonds.intro.chem.okstate.edu


Secondary Science - Particles - Eduwiki;edutech.csun.edu



     Carbon and Life; online.science.psu.edu



In spite of great strength exhibited by cohesive forces that are operating in the network of water molecules in liquid state, except at the freezing temperatures, the hydrogen bonds among water molecules will be constantly breaking and reforming among themselves. That is what gives fluidity status to water. Such a dynamic property provides greater mobility not only to water molecules, but also for other molecules suspended and dissolved in water.  Water provides the necessary molecular environment for substances to go into solution, interaction and reactions. Water is the mother of life but also foster of life.


Further more, because of cohesive forces; the specific heat is significantly high.  The specific heat means the amount of heat required to raise the temperature of one gram of water by 1 degree C.  Similarly, the heat of vaporization is also high.  The amount of heat required to break cohesive bonds from liquid state to gaseous state is referred to as heat of vaporization.


The dipolar property of water renders it as ‘the’ universal solvent for solids liquids and even gasses with different electric charges.  Even some hydrophobic molecules like oil can get into intermolecular spaces found in water to a little extent.


Almost all living cells (except viral particles) contain water more than 90% of its protoplasmic mass.  In such a complex mixture of organic and inorganic molecules with various sizes and dimensions, water provides media and mobility for the molecules to interact with each other.  Water itself is involved in various biochemical reactions.  Many colligative properties like boiling point, freezing point, osmotic pressure etc greatly felicitates the movement and interaction of molecules and they also help to maintain turgidity and shape of the cells.




As the oxygen atom in H2O possesses greater mass and greater negative charge than the hydrogen atom, one of the hydrogen atoms often looses its electron to oxygen and dissociates into a positives charged protoionic hydrogen ion.  The other partner will be rendered as a negatively charged hydroxyl ion.  Under normal circumstances, such free protons do not exist in water but they are immediately attracted by another dipolar H2O molecule to form Hydronium ions. (H3O).


Water in its purest state consists of 1.0 x 10^7 moles of H3O (+) ions and equal number of OH (-) ions are equilibrium state in every liter of water at 25°C.    This is in spite of their active breaking and making of the bonds, this is what we call as the dynamic of water.


K(W) = Equilibrium constant


Ionic product of water; www.chemguide.co.uk





Using the above said equilibrium state of ions; it is possible to obtain an equilibrium constant by simply dividing the number of grams of water per liter i.e. 1000 gms by the mol wt. of water i.e. 18 gms.  The value obtained is called equilibrium constant of water.  The same can be expressed in terms of ionic products (kW) i.e. (H+) x (OH-)


1 liter of water                        =1000 ml

Gram mol wt. of water           =18

Equilibrium constant   =          1000/18 = 55.5 m


The value of KW ionic product at 25 degree C is 1.0 x 10^14.  It means that there are 10-7 of H+ ions and 10-7 OH- ions in one liter of water. The same is also referred to as hydrogen ion concentration or pH.


Solvatin of Ions:  If ions are formed from a neutral compound, as when NaCl is dissolved in water, the oppositely charged cations and anions naturally attract each other, so formation of a dispersed homogeneous solution might appear to be energetically unfavorable. To achieve charge separation of ions in solution, two solvent characteristics are particularly important. The first is the ability of solvent molecules to orient themselves between ions so as to attenuate the electrostatic force one ion exerts on the other. This characteristic is a function of the polarity of the solvent. Solvent polarity has been defined and measured in several different ways, one of the most common being the dielectric constant, ε. High dielectric constant solvents such as water (ε=80), dimethyl sulfoxide (ε=48) & N,N-dimethyl formamide (ε=39), usually have polar functional groups, and often high dipole moments. When subject to the electric field of an ion, such polar molecules orient themselves to oppose the field, and in so doing they limit its reach. Because of electrostatic attraction between these polar groups, the boiling points of these solvents are generally higher than those of similarly sized nonpolar solvents, such as diethyl ether (ε=4.3) and hexane (ε=1.9). 

The second factor important in the stabilization of ions, which also resists their intimate recombination, is called solvation. This refers to the ability of solvent molecules to stabilize ions by encasing them in a sheath of weakly bonded solvent molecules, thus somewhat dispersing the electrical charge. Anions are best solvated by hydrogen-bonding solvents; cations are generally solvated by binding to nucleophilic sites on a solvent molecule Two-dimensional diagrams illustrating the primary solvation shell about Na(+) and Cl(–) are shown here. The water dipoles are drawn as red arrows, and partial charges are noted. Additional water molecules are oriented in secondary and tertiary layers about the ions.


From this description of ion formation in solution, it should be clear that both enthalpy and entropy factors will be important to the outcome of an ionization process. Thus solvation stabilizes and insulates an ion, helping the enthalpic change, whereas the same solvation adds order and structure to the ionic species at the cost of lowering entropy. The outcome of these interactions is discussed below for two typical salts.





The hydrogen ion concentration or pH generally represents the obsolete concentration of hydrogen (acid) ions and hydroxyl (base) ions in a given amount of water or solution.  Aqueous solutions show variations in the concentration of hydrogen and hydroxyl ions.


A Danish biochemist by name S.P.L. Sorenson has plotted the concentration of hydrogen and hydroxyl ions on a negative logarithmic scale at a constant temperature of 25 degree C.  The expression is written as pH=log 10-H+ = log 10 (H+).


As the concentration of H(+) ions is equal to the concentration of OH(-) ions in a neutral solution at 25 degree C, i.e. (H+) = (OH-) =1.0x10^7 the pH is expressed as  7


On a pH scale, pH 7.0 refers to neutral state for at this point; the number of hydrogen ions present is equal to the number of hydroxyl ions.  Hence the pH 1 refers to the higher concentration of hydrogen ions.  Similarly pH 14 refers to highest concentration of hydroxyl ions.  Most of the cellular protoplasmic fluids generally show a range of pH value from 5.8 – 7.2 but variations are always found.




Various components present in a solution are capable of donating protons or accepting protons or vice versa, thus such components act as Acid : Base pairs.  For example, Acetic acid, Phosphoric acid, Ammonia and even water (H2O) at a particular pH show certain dissociation constants, which is referred to as K.  On the basis of the logarithmic expression, the same can be transformed into pK.  The pK value of substances such as H2PO4 (h+) donating components is equal to the number of HPO42- (hydrogen accepting) ions, at which the pH value is 7.2.  So different substances exhibit different pK values where pK’=pH.  Ex., the pK of NH4 is 9.25, acetic acid is 4.76.


The pK value of different compounds can be determined by titrating the said against a known concentration of NaOH, which easily dissociates into Na+ and OH ions.  The midpoint of the titration curve gives the pK value of that compound.


Some of the compounds, if present in an equilibrium state, in a solution, i.e. the proton donating and proton accepting ionic components, present in equal concentrations, the solution resists against the addition of a small amount of acids or bases.  Such a solution is called “buffer”.  Fortunately the cellular protoplasmic fluids, by their inherent chemical composition, exhibit buffering properties, against any small change in the pH.  This property is very important for the enzymes, which depend upon the optimal pH for their activity.

Buffer solutions achieve their resistance to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A-.


HA is in equilibrium with H+ + A-


Acetate buffer; http://images.1233.tw/


Simple buffering agents.


Buffering agent


useful pH range

Citric acid

3.13, 4.76, 6.40

2.1 - 7.4

Acetic acid


3.8 - 5.8



6.2 - 8.2






8.25 - 10.25


Common buffers used-Tris, Tricine, HEPES, Bicine, MOPS and MES and many others





Sustenance of growth and development of plants depends upon the uptake of various nutrients along with water from external sources and synthesize different organic components within the cells.  Nonetheless the uptake and transportation of water in plants is very important, for maintaining adequate supply of water to various parts of the plant body.  To maintain the supply of water to every living cell against adverse environmental conditions is a difficult task.  Still the plants perform this process efficiently and with at most ease. 


Molecules, either in liquid state or gaseous state, exhibit constant, but random movement because of their inherent kinetic energy.  The net movement of molecules is called diffusion. However the direction of diffusion is concentration dependent.  Irrespective of the kind and state, molecules move from higher concentration i.e., higher pressure to lower concentration (low pressure) of the same kind.  In a system containing more than one kind of molecules, each one of them, find their own gradient.  Still the rate of diffusion depends upon many factors like steepness of the gradient, temperature, pressure, the medium in which they operate.



As with dye dropped into a liquid, electrons diffuse because of a concentration difference from place to place. In this example, the dye molecules spread throughout the water in a random fashion; statistically, they should! This is similar with electrons in a semiconductor, which should spread out over time until they reach an equilibrium (are uniformly spread). Source: <http://isite.lps.org/sputnam/Biology...Notes_cell.htm>





Diffusion across a membrane is called osmosis. http://www.toltec.biz/’www.biologyguide.net


 Movement across a semipermeable membrane; http://www.toltec.biz/


Diffusion and Osmosis



Greater the concentration gradient between two systems, greater is the rate of diffusion.  When once the concentration in both the systems is rendered equal, the rate of diffusion of molecules will be at equilibrium state.  Temperature has a profound effect on the rate of diffusion of molecules, because it provides energy for movement.  With the increase in temperature, molecules diffuse at a higher rate than at lower temperature.  The rate of diffusion in turn brings about increased metabolic activity as well as transportation.  The rate of diffusion through solids, liquids and gases vary because of their inter molecular spaces and forces.  Molecules can diffuse faster in gases and liquids than through solids.




The net movement or net diffusion in a pure solvent is always zero, because the molecules will be in equilibrium state.   So the diffusion pressure of such solvent system is also zero. But the same solvent in its purest form shows higher chemical energy of movement, which is very often referred to as chemical potential.  For example, pure water has greater water potential or it can be stated as to have higher chemical energy of movement.  To such a pure solvent, if a known amount of solute is added, the solutes disperse and dissolve in water.  This is greatly facilitated by the dipolar nature of water molecules, where water molecules surround the solutes and bind to them; this is because of the surface charges found on the solutes.  The binding or immobilization of a number of H2O molecules on to solute surfaces ultimately results in the loss of total number of free H2O molecules in its purest form.  So also its total diffusion pressure, chemical energy or water potential decreases.  This decrease or loss is called diffusion pressure deficit.  This is always measured in terms of negative pressures, because the diffusion pressure of pure water is zero.  The units of measurements they are used for living systems are either atmospheric pressures (Atoms) or bars.  Protoplasmic systems always exhibit changes in DPD, which acts as a motive force for the movement of solvents in a living system.


DPD of solution = DP of solvent – DP of solution



            In pure state free molecules collide on the wall of the container exhibiting pressure; such free diffusion of all water molecules enmass and unhindered is called Diffusion pressure; and it is proportional to its concentration. To such a solvent, if few milligrams of sugar or salt are added, water molecules by their dielectric charges bind to the charged surface of solute molecules, either using their negative charges or positive charges, thus help in solubility of the solutes. By binding to the solute surface some water molecules are rendered immobile. That means, from the total number of water molecules, certain number of water molecules that are bound to solute surfaces are not free for movement that means they are lost for free diffusion, which also means loss of some amount of energy or water potential. As the diffusion pressure of water molecules in solution is proportional to the concentration of free diffusible molecules, addition of salt reduces the diffusion pressure of water; this loss is called diffusion pressure deficit (DPD). It is also called as Suction Pressure (SP). So, greater the concentration of solutes higher is the DPD and vice versa. The solute concentration is also called Osmotic Concentration (OC).







In an open system, according to the principle of diffusion, molecules move from higher energy potential to lower energy potential, i.e. from higher concentration to lower concentration till they reach an equilibrium state.  If a living membrane separates two solutions of different concentrations, the principle of diffusion applies to only solvent but not to solute.  Membrane never acts as a barrier for the movement of water and it moves from higher potential to lower potential; but solutes cannot cross through the membranes, thus it acts as a barrier.  Hence the passive diffusion of water from higher water potential to lower potential across a plasma membrane is called Osmosis.

However there specific water transporters in cellular membranes called ‘Aquaporins’-water channels.  They are found in all forms of life. In 2003 Nobel Prize in chemistry was given to Peter Agree for the discovery of them.  The allow water to flow more rapidly than passive diffusion. They are also known as ‘aquaglycoporeins’ they also transport some small uncharged solutes of 150-200Da.


Water Permeation Potential of Mean Force

Water channels;http://www.ks.uiuc.edu/



Osmosis is a kind of diffusion of solvent water molecules across the cellular membranes. The direction of diffusion, according to the law of diffusion, is from higher concentration or potential to lower concentration or potential, but if two solutions of different concentrations are separated by semi permeable membrane only water is allowed to diffuse across the membrane from higher concentration to lower concentration. But the diffusion of solutes across the membrane is prevented by its characteristic semi permeable and selective permeable property. Such differential diffusion of water across the living membrane is referred to as Osmosis.




The amount of solute present in a solution is called Osmotic concentration, which is measured in terms of molarity.  If two solutions of different concentration are separated by living cell membrane; the solution, which has greater water potential, tends to loose water to the solution where the water potential is low.  In the sense the solution that contains greater amount of solutes possesses greater DPD; hence a negative force develops in such solution so the water is diffuses from the solution, which has higher water potential.  Such a force that is generated by virtue of solute concentration is called suction force or osmotic pressure.  Sometimes OP is also defined as a pressure applied on a system sufficient enough to prevent the diffusion of water from the other system.


Though the concentration of solutes determines the osmotic pressure of the system, certain factors like temperature and ionization nature of solutes also contribute to osmotic pressure.  As temperature provides energy for the movement of molecules, increase in temperature increases the rate of movement of molecules, which results in increase of frequency of collision, hence OP increases.  Similarly, ionizable substances like NaCl; KNO3 etc. exert greater OP than non-electrolytes like sugar.  For example, the osmotic pressure produced by 1M NaCl is two times greater than the OP developed by 1M sugar soln.  Because the ionized products like Na and Cl, act as two solute components hence the OP generated by NaCl is more than the non-ionizable compounds.




In living system, as OP is generally expressed in terms of atmospheric pressures i.e. ATM.  One ATM is equivalent to the pressure that is capable of supporting mercury in a column to a height of 760 mm at sea level.  The alternate expressions used to denote ATM (atmospheric pressure) is Psi, Torr or Bar.  Psi (ψ) is expressed as pounds of pressure per square inch.  1 ATM = 14.7 Psi or 1 ATM = 760 Torr. For example, 1 gram mol. Wt. of sucrose dissolved in water and made up to 1 liter with water i.e. 1 Molar solution, because sucrose is a non ionizable solute, it exerts an osmotic pressure or potential of 22.4 atm at room temperature. Osmotic pressure is also one of the colligative properties of solution.  The other colligative features of a liquid are boiling point, heat of vaporization and freezing point.  The presence of solutes changes these properties. Even the nature and concentration of solute molecules exert profound influence on the colligative properties of solutions.


Difference between Osmotic Pressure and Osmotic Potential



Osmotic Pressure (OP)

Osmotic Potential


It is expressed in bars with a positive sign.

It is expressed in bars with a negative sign. It is also known as solute potential.


OP of pure solvent (or water) is zero. The value of OP increases with increase in concentration of solute particles.

Osmotic potential of pure solvent (or water) is zero.

Higher negative value of osmotic potential means greater the concentration of solute particles.


If two solutions having different osmotic concentrations are separated by a semipermeable membrane, the molecules move from the solution having less OP to the solution having more OP.

If two solutions having different osmotic concentrations are separated by a semipermeable membrane, the molecules move from the solution having more osmotic potential to the solution having less osmotic potential.

- See more at: http://www.biology.lifeeasy.org/567/differentiate-between-osmotic-pressure-osmotic-potential#sthash.lkW7INFD.dpuf; http://www.biology.lifeeasy.org/





Osmotic pressure (OP) / Osmotic potential (y P ):



Subscripts; p, m and g indicate solute, pressure, matrix (solid) and gravity; http://prometheuswiki.publish.cistro.au







Osmotic pressure: Osmotic pressure of solution (that is enclosed in membrane immersed impure water) is the pressure applied on the solution just enough to prevent the diffusion of water from outside across the membrane. Higher the solute concentration higher is the osmotic pressure. The OP of a non-electrolyte solution is proportional to the concentration of solutes, but OP of an electrolyte solution, depending upon its ionizing value, varies.


Osmotic potential (y P): It is the water potential of a solution in comparison to the water potential of pure water.  So yW of a solution is always less than zero bars, thus osmotic potential also refers to solute potential. From this context osmotic pressure (OP) is equal to osmotic potential in negative pressures, if OP of a solution is 2 bars, then yW of it is (-) 2 bars.


Exosmosis, Endosmosis and Turgidity:


            When a normal cell is put in a hypertonic solution (solution with high concentration of solutes, than the solute concentration of the cells), a water potential or DPD gradient is created between the cell and the external solution. Hence the water diffuses out of the cell; the process is called Exosmosis or Plasmolysis. As a consequence, the cellular protoplasm collapses and the plasma membrane withdraws from the cell wall and the whole cytoplasm gets concentrated in a corner of the cell. Such a cell is called flaccid cell. In this state, turgour pressure (y p) is zero


            If such a cell is transferred to hypotonic solution i.e. the solute concentration is less than that of a cell. If the solute concentration of the solution is equal to the cell concentration then it is called Isotonic. Movement of water across the cell is equal and opposite.  On the contrary if the solvent water is higher water enters into the cell. This process is called Endosmosis or Deplasmolysis. As a result, the concentration of water or water potential within the cell increases. Increase in the water concentration creates its own molecular pressure within the cell and it is called turgor pressure. With the increase in turgour pressure, the cytoplasm swells and gradually plasma membrane is pushed towards the cell walls. As more and more water enters, more and more of turgour pressure builds up and the cell goes on increasing in the size. The water potential of the cell increases towards zero value.


            As turgour pressure exerts its impact outwardly i.e., on to the cell wall, the cell wall being plastic, exerts counter pressure; this is called wall pressure. When the TP (Ψp) becomes equal to wall pressure, the water potential within the cell and outside the cell reaches an equilibrium state. Such a cell is called turgid cell and w.


            The relation can be expressed in the following formulae: 


            If DPD = OP - TP          If Ψw =  Ψ + Ψp

            So OP = DPD + TP        Ψπ = Ψw - Ψp

            Therefore TP = OP-DPD          Ψp = Ψw - Ψπ


            By determining OP or Ψπ one can determine the DPD or Ψw of any given cell. This can be done by initial plasmolytic method.

            Where M = molarity at which incipient plasmolysis occurs, = t = Room temperature, 273 = Absolute temperature. 


            Ψπ = (-) miRT


            Where Ψπ = osmotic potential, M = molarity at which incipient plasmolysis takes place, I = Ionization constant i.e., one for glucose and sucrose, but for NaCl, it is two, R = gas constant, i.e., 0.083 bars, T = 273 + room temperature say 250C.




Under favorable conditions of water supply most of the plant cells contain more than 90% of water as the protoplasmic mass.  If such cells are placed in a solution containing 1 M NaCl, protoplasmic mass of the cell shrinks because water diffuses out of the cell across the plasma membrane from higher water potential towards lower water potential.  That is why this process is called Exosmosis which is also referred to as plasmolysis.  As a result of exosmosis, DPD of the protoplasm increases.  When such cells are placed in pure solvent, water diffuses rapidly into flaccid cells, which is called endosmosis.  This is because of the steep gradient in chemical energy potential between the external solvent and the internal soln.  As the protoplasmic mass increases in terms of solvent, the plasma membrane is pressed towards the cell wall.  The intracellular pressure that is generated due to endosmosis of water is called turgour pressure.  With further increases in the turgour pressure, the cell wall is also subjected to tension, but as the cell wall has some elastic properties, they tend to counteract on the turgour pressure, which is called wall pressure.  When the turgour pressure and wall pressure of the cell is equal, the cell is said to be in a turgid state; where DPD is zero and the solvent of the protoplasm is in equilibrium state with the solvent of the external solution.


The relationship between the TP, OP and DPD is shown in the following equation.  Based on their relationship an equation can be developed to find out any of the unknown from the known parameters.




It is important to note that in a living system the OP is expressed always in terms of negative atmospheres or bars; but turgour pressure is expressed as positive pressures.  In any given system, the diffusion pressure deficit is always equal to osmotic pressure, because the binding of solvent molecule to solutes results in the loss in chemical free energy of water.  For example, let us use a flaccid cell for calculations.  In a flaccid cell, TP is zero and assume it to be OP is -20 bars.  It should be remembered that OP=DPD.  If such a cell is placed in a pure solvent whose DPD is O, water because of its high chemical free energy or water potential diffuses into the cell across the membrane.  As a result the water potential increases inside the cell till the turgour pressure of the cell increases to 10 bars, i.e. final OP = initial OP + TP which accounts to final OP (-20+10=10).  As the DPD + OP (DPD =10+10), the DPD and water potential becomes zero.  Such a state of the cell is called turgid.


Turgour changes are mainly due to certain variations in the intracellular concentration of osmotically active molecular processes like translocation of water, minerals, organic solutes, opening and closing of stomata, opening and closing of leaf blade, nastic movements, etc.  are all controlled by turgour processes.


Osmotic pressure of a given cell can be determined by plasmolytic methods or by cryoscopic methods.  Cryoscopic method is very easy.  First, the actual freezing point of an unknown solution should be determined, call it δ.  Extrapolate this value with an equation obtained from the theoretical equation, which is partly derived from the OP values obtained for 1 Molar sugar solution at freezing point, as 22.7 bars with a constant.


If the actual observed freezing point of the protoplasm is 0.93, by substituting the values, Op of the said cell can be determined as 11.35 bars.


The plasmolytic method is simple but time-consuming.  In this processes, epidermal peels of any colored leaves such as Rheodiscolour, Tradescantia, virginiana etc., are placed in a series of sucrose or NaCl solutions of different molar concentrations.  After keeping them in the said solutions for about 30 minutes or so, the cells are mounted on the slide in the same liquid and observed.  Whatever concentration of water at which 50% of the cells are plasmolysed, such dilution is taken as the criteria for inducing incipient plasmolysis.  Viewing the following equation the values for OP can be determined.



Difference between Osmotic Pressure and Osmotic Potential



Osmotic Pressure (OP)

Osmotic Potential


It is expressed in bars with a positive sign.

It is expressed in bars with a negative sign. It is also known as solute potential.


OP of pure solvent (or water) is zero. The value of OP increases with increase in concentration of solute particles.

Osmotic potential of pure solvent (or water) is zero.

Higher negative value of osmotic potential means greater the concentration of solute particles.


If two solutions having different osmotic concentrations are separated by a semipermeable membrane, the molecules move from the solution having less OP to the solution having more OP.

If two solutions having different osmotic concentrations are separated by a semipermeable membrane, the molecules move from the solution having more osmotic potential to the solution having less osmotic potential.

- See more at: http://www.biology.lifeeasy.org/567/differentiate-between-osmotic-pressure-osmotic-potential#sthash.lkW7INFD.dpuf; http://www.biology.lifeeasy.org/






Water potential () = pressure potential () + solute potential ();     http://www.phschool.com/









www.dc152.4shared.com; fac.ksu.edu.sa


www.dc152.4shared.com; www.studyblue.com




Free molecules, possessing inherent kinetic energy, exhibit random movements. The net movement of molecules from one point to the other is called diffusion. The rate and the direction of movement depend upon the temperature, concentration gradient and the medium through which it diffuses. Molecules move enmass from higher concentration towards lower concentration.  But in pure state, as the random movement is equal and opposite, the rate of net diffusion is unity. However, the rate of diffusion of different molecules is independent of each other, for each kind finds its own gradient.




www.weloveteaching.com; www.tutorvista.com



Water Potential ΨW:

The water potential of pure water in an open container is zero because there is no solute and the pressure in the container is zero. Adding solute lowers the water potential. When a solution is enclosed by a rigid cell wall, the movement of water into the cell will exert pressure on the cell wall. This increase in pressure within the cell will raise the water potential. http://www.phschool.com/science/biology



            In this context, it is important to be familiar with another 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. (1 bar = 0.987 atmospheric units, 10 bars = 1 Mega Pascal (Mpa). 1 mpa = 106 dynes / cm2, under standard conditions).


            The chemical free energy of water in its purest form is also called water potential (yW).  Purest form means there are no other molecules in it.  The chemical energy of water is maximum and its value is given as 0 bars. Addition of solutes to pure solvent decreases the chemical free energy of pure water, because certain amount of energy of a number of water molecules is used for binding to the surface of solutes. So the total value of water potential of a solution is less than zero; it is always expressed in negative pressure values.  Here it is equal to DPD; if the water potential of pure water is zero and DPD is also zero. But the water potential of solution is less than zero expressed in negative value, but DPD of the solution is expressed in positive value.


DPD of pure water = 0 bars; DPD of a solution = (+) bars

y w of pure water = 0 bars; y w of a solution = (-) bars










Plant parts are mostly made up of cellulose, hemi cellulose and other substances of which majority of the components are water loving in nature.  The hydrophilic property of the cellular components is due to the presence of electrical charges in them.  Water, being a dipolar component, gets attracted towards either the negative charge or the positive charges and ultimately binds to such particles by hydrogen bonding.  Such a phenomenon where organic substances like cellulose, hemicelluloses, lignin, etc., attract as well as adsorbs water molecules to their surfaces is called as imbibition.  In fact, the above cellular components provide a greater negative water potential, so that water rushed to these surfaces gets adsorbed by hydrogen bonding.  The rapid adsorption results in swelling, because the polymers are pushed apart to accommodate more and more water molecules.  This process continues till all their sites on the surface are saturated with water molecules.  The binding of water molecules on to specific sites also release lot of energy which is called imbibitional energy.  That is why when seeds are soaked with water, the temperature of the medium increases, even before the process of respiration is activated to its maximum efficiency.


An interesting feature of this phenomenon of imbibition is the rapidity with which cellular components, particularly dead cellular components like cell walls, lignins, tannins, etc., attract water molecules or absorption.  The attraction can be measured in terms of negative atmospheric pressures.  But if such substances are confined to restricted spaces and still the imbibant water is provided, the binding of water molecule to the cellular components is greatly favored, so enormous amount of positive pressure is developed, which is called as imbibition pressure, which may amount to 1000 to 100900 atom or more.

















Surface tension





www.growflow.com. www.magneticwater.com.au




Cohesion-molecular attraction holds them to each other. www.leavingbio.net


Random movement of molecules but direction and rate depends upon the concentration gradient.