wateranditsproperties-171114151233.pptx

Water and its properties ๏ Without water, life as we know it could not exists. Water is considered to be one of the most importance substances on earth. ๏ Covering 70% of the earth’s surface and making up as much as 95% of the matter of living organisms, it is virtually unique among liquid...

Water and its properties ๏ Without water, life as we know it could not exists. Water is considered to be one of the most importance substances on earth. ๏ Covering 70% of the earth’s surface and making up as much as 95% of the matter of living organisms, it is virtually unique among liquids. ๏ For every gram of organic matter made by the plant, approximately 500 g of water is absorbed by the roots, transported through the plant body and lost to the atmosphere. Even slight imbalances in this flow of water can cause water deficits and severe malfunctioning of many cellular processes. ๏ Water makes up most of the mass of plant cells, as we can readily appreciate if we look at microscopic sections of mature plant cells: Each cell contains a large water-filled vacuole. In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole. ๏ Water typically constitutes 80 to 95% of the mass of growing plant tissues. ๏ Common vegetables such as carrots and lettuce may contain 85 to 95% water. ๏ Wood, which is composed mostly of dead cells, has a lower water content; sapwood, which functions in transport in the xylem, contains 35 to 75% water; and heartwood has a slightly lower water content. ๏ Seeds, with a water content of 5 to 15%, are among the driest of plant tissues, yet before germinating they must absorb a considerable amount of water. ๏ Water fills a number of important roles in the physiology of plants; roles for which it is uniquely suited because of its physical and chemical properties.  The molecular structure of water ๏ The water molecule consists of an O-atom covalently bonded to two H-atoms. ๏ The two O—H bonds form an angle of 105°, on average, closer to the oxygen nucleus than to Diagram of the water hydrogen. molecule. ๏ Because the O-atom is more electronegative than hydrogen, it tends to attract the electrons of the covalent bond. ๏ This attraction results in a partial negative charge at the oxygen end of the molecule and a partial positive charge at each hydrogen. These partial charges are equal, so the water molecule carries no net charge.  The molecular structure of water ๏ This separation of partial charges, together with the shape of the water molecule, makes water a polar molecule, and the opposite partial charges between Diagram of the water neighboring water molecules tend molecule. to attract each other. ๏ The weak electrostatic attraction between water molecules, known as a hydrogen bond, is responsible for many of the unusual physical properties of water. ๏ H-bonds can also form between water and other molecules that contain electronegative atoms (O or N).  The molecular structure of water ๏ In aqueous solutions, H-bonding between water molecules leads to local, ordered clusters of water that, because of the continuous thermal agitation of the water molecules, continually form, break up, and re-form. (A) Hydrogen bonding between water molecules results in local aggregations of water molecules. (B) Because of the continuous thermal agitation of the water molecules, these aggregations are very short-lived; they break up rapidly to form much more random configurations.  The properties of water - Adhesion forces Specific heat capacity Cohesion forces Heat of vaporization Surface tension Heat of fusion Tensile strength Viscosity Water is the universal solvent Volume and density Thermal properties of water Water is transparent  Adhesion force ๏ The adhesiveness of water is explained by hydrogen bonding of water molecules to other polar surfaces such as oxygen-containing molecules. ๏ The strong dipole of water exerts electrostatic and gravitational forces on charged electrovalent compounds and on the dipoles of polar, covalent compounds. ๏ Thus, water will absorb to substances such as cellulose but not to polyesters due to few oxygen availability for H- bonding. ๏ Adhesion is an important factor in the capillary rise of water in small-diameter conduits  Cohesion forces: surface tension ๏ Water molecules at an air–water interface are more strongly attracted to neighboring water molecules than to the gas phase in contact with the water surface. ๏ As a consequence of this unequal attraction, an air–water interface minimizes its surface area. ๏ To increase the area of an air–water interface, hydrogen bonds must be broken, which requires an input of energy. ๏ The energy required to increase the surface area is known as surface tension.  Cohesion forces: surface tension ๏ Surface tension not only influences the shape of the surface but also may create a pressure in the rest of the liquid. ๏ Surface tension at the evaporative surfaces of leaves generates the physical forces that pull water through the plant’s vascular system. ๏ A high surface tension is the reason water drops tend to be spherical or that a water surface will support the weight of small insects.  Cohesive force: tensile strength ๏ Cohesion gives water a high tensile strength, defined as the maximum force per unit area that a continuous column of water can withstand before breaking. ๏ Careful studies have demonstrated that water in small capillaries can resist tensions more negative than –30 MPa (the negative sign indicates tension, as opposed to compression). This value is only a fraction of the theoretical tensile strength of water computed on the basis of the strength of hydrogen bonds.  Cohesive force: tensile strength ๏ The presence of gas bubbles reduces the tensile strength of a water column. ๏ If a tiny gas bubble forms in a column of water under tension, the gas bubble may expand indefinitely, with the result that the tension in the liquid phase collapses, a phenomenon known as cavitation. ๏ Cavitation can have a devastating effect on water transport through the xylem.  Water is the universal solvent ๏ Water is an excellent solvent: It dissolves greater amounts of a wider variety of substances than do other related solvents. ๏ This versatility as a solvent is due in part to the small size of the water molecule and in part to its polar nature. The latter makes water a particularly good solvent for ionic substances and for molecules such as sugars and proteins that contain polar —OH or —NH groups. 2 ๏ H-bonding between water molecules and ions, and between water and polar solutes, in solution effectively decreases the electrostatic interaction between the charged substances and thereby increases their solubility.  Water is the universal solvent ๏ Water has the ability to partially neutralize electrical attractions between charged solute molecules or ions by surrounding the ion or molecule with one or more layers of oriented water molecules, called a hydration shell. ๏ H-bonding between macromolecules and water reduces the interaction between the macromolecules and helps draw them into solution, thereby reducing the probability that ions can recombine and form crystal structures.  Water is the universal solvent ๏ The polarity of molecules can be measured by a quantity known as the dielectric constant. Water has one of the highest known dielectric constants. ๏ The dielectric constants of alcohols are somewhat lower, and those of non-polar organic liquids such as benzene and hexane are very low. ๏ Water is thus an excellent solvent for charged ions or molecules, which dissolve very poorly in non-polar organic liquids. Many of the solutes of importance to plants are charged.  Specific heat capacity ๏ Water has the highest specific heat capacity of the common liquids. ๏ Specific heat is defined as the amount of energy required ◦ to raise the temperature of one gram of substance by 1 C ◦ (usually at 20 C).  Specific heat capacity ๏ When the temperature of water is raised, the molecules vibrate faster and with greater amplitude. To allow for this motion, energy must be added to the system to break the H-bonds between water molecules. Thus, compared with other liquids, water requires a relatively large energy input to raise its temperature. ๏ Because of its highly ordered structure, liquid water also has a high thermal conductivity. This means that it rapidly conducts heat away from the point of application. ๏ The combination of high specific heat and thermal conductivity enables water to absorb and redistribute large amounts of heat energy without correspondingly large increases in temperature.  Specific heat capacity ๏ This high specific heat capacity has importance in terms of heating and cooling of aqueous bodies. ๏ For plant tissues that consist largely of water, this property provides for an exceptionally high degree of temperature stability. ๏ Localized overheating in a cell due to the heat of biochemical reactions is largely prevented because the heat may be quickly dissipated throughout the cell. ๏ In addition, large amounts of heat can be exchanged between cells and their environment without extreme variation in the internal temperature of the cell.  Heat of vaporization ๏ Just as hydrogen bonding increases the amount of energy required to melt ice, it also increases the energy required to evaporate water. ๏ The heat of vaporization of water, or the energy required to convert one mole of liquid water to one mole of water vapor, is about 44 kJ mol at 25 C, a process that occurs −1 ◦ during transpiration. ๏ Because this energy must be absorbed from its surroundings, the heat of vaporization accounts for the pronounced cooling effect associated with evaporation.  Heat of vaporization ๏ Evaporation from the moist surface cools the surface because the most energetic molecules escape the surface, leaving behind the lower-energy (hence, cooler) molecules. ๏ The high latent heat of vaporization of water enables plants to cool themselves by evaporating water from leaf surfaces, which are prone to heat up because of the radiant input from the sun. ๏ Transpiration is an important component of temperature regulation in plants.  Heat of fusion ๏ The energy required to convert a substance from the solid to the liquid state is known as the heat of fusion. ๏ The heat of fusion for water is 335 J g−1 , which means that 335 J of energy are required to convert 1 gram of ice to 1 gram of liquid water at 0◦C. ๏ The heat of fusion of water is one of the highest known, second only to ammonia. ๏ The high heat of fusion of water is attributable to the large amount of energy necessary to overcome the strong intermolecular forces associated with hydrogen bonding.  Heat of fusion ๏ An interesting application of this high heat of fusion for water is in frost production. Citrus grooves are frequently protected from frost injury by flooding. When the water freezes, that heat liberated during fusion (i.e,. during freezing) add heat to the grooves and protects the trees.  Viscosity ๏ The viscosity of water, or resistance to flow is higher in water than in most liquids, again because of the H- bonding. ๏ The viscosity of water at 20℃ (actually at 20.20℃) taken as reference, is 1.0 centipoise. Ethanol has a viscosity at 20℃ of 1.2 centipoise and hence is slightly more viscous, nut ethyl ether has a viscosity of 0.2 centipoise at 20℃. ๏ Viscosity decreases with increasing temperature. The increasing tendency to flow (decrease in viscosity) as temperature increases comes about by thermal disruption of the H-bonds. Temperature effects on the transport of liquid within plants can be partially accounted for by viscosity changes.  Volume and density ๏ The density of ice is another important property. ๏ At 0℃, the density of ice is less than that of liquid water. ๏ As temperature increases or decreases from 4℃, the volume occupied by 1 g of water increases. Thus water, unlike other substances, reaches its maximum density in the liquid state (near 4℃), rather than as a solid. ๏ This occurs because molecules in the liquid state are able to pack more tightly than in the highly ordered crystalline state of ice.  Volume and density ๏ Hence water freezes from the top down rather than from the bottom up. ๏ Consequently, ice floats on the surface of lakes and ponds rather than sinking to the bottom where it might remain year-round. ๏ The frozen water at the surface tends to insulate the water below, preventing large bodies of water from freezing solid and killing aquatic life. ๏ This is extremely important to the survival of aquatic organisms of all kinds.  Water is transparent ๏ The transparency of water to visible light enables sunlight to penetrate the aqueous medium of cells where it can be used to power photosynthesis or control development. ๏ For instance, ๏ phytoplankton need light for photosynthesis, ๏ light can pass through the eyeball to receptor cells in the back in jelly-fish  Water as a buffer ๏ Water ionizes into H+ and OH- ๏ Water acts as a reservoir of H+ ions, donating or removing then from solution as necessary. ๏ offers protection from extreme pH levels. ๏ Organisms do not tolerate much pH change. ๏ Cells function best within a narrow pH range.