Lecture03 - Water Water comprises about 80-95% of 95% the...

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Unformatted text preview: Water Water comprises about 80-95% of 95% the the mass of the young herbaceous plants Root, Root, carrot Stem, asparagus Leaf, lettuce Fruit, watermelon Seed, corn (dry) Water Water Content of Plant Tissues 88% 88% 95% 92% 11% Water Water is a key limiting factor for productivity of agricultural crops and natural ecosystems Why? Blaming photosynthesis! 500:1 ratio for H2O/CO2 Transpiration Major Major functions of water in plants 1] 1] Water constitutes the large Central Vacuole in most plant cells. Central This accounts for approximately 90% of the Protoplast. Cytoplasm Cytoplasm accounts for the remaining 10% and it also contains water. 2] 2] It is the most abundant and best Solvent in nature. Solvent It is the medium for major transport processes in plants. It It affects the structure of proteins, carbohydrates, & nucleic acids. 3] 3] Water is the medium in which biochemical reactions occur in plants. in 4] 4] Water participates directly in some important biochemical reactions. biochemical The oxygen we breathe comes from the splitting of water inphotosynthesis. Water is continuously absorbed and lost by plants. A leaf may loose 100% of its water content in one hour under dry, hot conditions. A plant may loose its entire fresh weight 100 times during its lifetime. 5] 5] Water evaporation is important for regulating Leaf Temperatures. regulating Transpiration is the process by which leaves regulate water loss. Evaporation Evaporation cools leaves and prevents them from overheating. This may dissipate 50% of the heat from solar irradiation. Water Water is important because it is polar and readily forms hydrogen bonds 20 kJ/mol . 1. Water molecules are attracted to one another due to the bipolar nature of the molecule. 2. This weak "electrostatic" attraction is called a "Hydrogen Bond". 3. much weaker than covalent bonds, however, many H-bonds can be "strong". Polarity Polarity makes it an excellent solvent--shells of hydration 1. Water dissolves substances to which it can readily hydrogen bond 2. For a substance to dissolve in water, the substance must displace water-to-water hydrogen bonds 3. Consequently, for a substance to readily dissolve in water, it must be something that water will hydrogen bond to at least as well as water hydrogen bonds to itself 4. Furthermore, the substance must also more-readily hydrogen bond to water than it interacts with molecules of its own kind; that way the molecule tends to leave the solid state and enter into solution 5. Within solution, such a substance will be surrounded by water molecules which are hydrogen bonded to it 6. This surrounding array of water molecules is called a hydration shell Hydrogen Hydrogen bonding is also responsible for other properties of water 1. Cohesion (Mutual attraction) Cohesion refers to the tendency of water molecules to hydrogen bond to each other It explains the ability of water to be siphoned as well the related property of transport of water from the roots to the leaves of plants 2. Adhesion Adhesion is the tendency of water to stick to substances other than water 3. Surface tension (the energy required to break H-bonding…) Surface tension is an emergent property of water that results from the tendency of water molecules to stick to each other (by hydrogen bonding) better than they adhere to air molecules QuickTime™ and a d ecompressor are needed to see this picture. Capillary action is the result of adhesion, cohesion, and surface tension. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward. Adhesion Adhesion can be quantified by by the contact angel “from the solid surface through the liquid to the gas-liquid interface” Temperature buffering and cooling properties of water High specific heat The heat energy required to raise the temperature of a substance by 1 OC 1 calorie is required to raise 1 g of pure water 1OC this value is higher than that of any other substance except liquid ammonia water (1.00 cal/g/deg) > alcohol (0.58) > air (0.25) > copper (0.09) @ standard pressure, Thus water can absorb large quantities of energy without much temperature increase Such a thermal property is important to buffer temperature fluctuations High latent heat of vaporization the energy needed to separate molecules from the liquid phase and move them into the gas phase at constant temperature For water at 25OC, the heat of vaporization is 44 kJ mol-1 Help plants to cool themselves by evaporating water from leaf surfaces. Water Water has a high tensile strength Defined as the maximum force per unit area that a continuous column of water can withstand before breaking Cohesion gives water a high tensile strength. A simple demonstration of water tensile strength Hydrostatic pressure (+) Tension (-) -30 Mpal [1 MP = 9.9 atomspheres Pressure = force/unit area 1 Pal = 1N/m2 = 1 J/m3 J/m The The presence of gas bubble residues reduces the tensile strength of a water water column (cavitation). Water transport mechanisms 1. Diffusion driven by concentration gradient 2. 2. Bulk flow driven driven by pressure gradient 3. Osmosis driven by a water potential gradient Water Water moving from the soil through the plant to the atmosphere passes through a widely variable medium such as cell wall, cytoplasm, membrane, membrane, and air spaces, and the mechanisms of water transport also varies with with the types of medium Diffusion Diffusion is the movement of molecules by random thermal agitation resulting resulting in the net movement of substances from net of areas areas of High Concentration to areas of Low Concentration. Frick's Frick's First Law The The Rate of Movement (the amount of substances crossing a unit area per unit time) is directly proportional to the Concentration Gradient Concentration flux density Js = -Ds [Cs]/[x] The Diffusion The Diffusion coefficient (Ds) is a proportionality is constant that measure how easily a given substance moves moves through a particular medium The Ds is related to molecular size, the medium in is which it moves, and the temperature. Diffusion Diffusion is rapid over short distances but extremely slow over long distances The The average time needed for a particle to diffuse a distance L is equal to t = L2/Ds where Ds is the diffusion coefficient It takes 2.5 sec. for a glucose molecule to diffuse from one side of a 50 micron cell to the other. However, However, it would take 32 years for the molecule to travel 1 meter by diffusion! 32 for The The rate of Diffusion can not match the requirements for the sustenance of large plant Bulk Bulk Flow or Mass Flow Bulk Bulk flow is the concerted movement of molecules en masse. concerted This This is usually due to a Pressure Gradient Pressure 1. Bulk Flow occurs regardless of solute concentration gradients! 2. The Rate of Bulk Flow through a hollow cylinder or pipe is related to many parameters. Volume Volume flow rate = [r4/8][p/x] (Poiseuilles Equation). r: radius of the tube , the viscosity of the liquid [p/x], the pressure gradient that drives the flow x], the Thus, Thus, the Radius or Diameter of the pipe is the most important factor Radius is If If the Radius of a pipe is doubled the rate of Bulk Flow is increased by 16X! Question Why do Vessel Members transport more water than Tracheids? Osmosis is the movement of a solvent like water through a Membrane. Membrane The The plasma membrane of plant cells are selectively permeable selectively It is permeable to small, uncharged molecules. It It is relatively impermeable to large molecules and charged molecules like ions. The direction and rate of water flow across a membrane are determined by both concentration gradients and pressure gradients. concentration and pressure The The Total Driving Force for Osmosis is called the Water potential. Water Movement Movement will occur from areas of High Water Potential towards areas of Low Water Potential. High If If the stem of the funnel is removed, there will be no outlet for the Positive Hydrostatic Pressure that is produced by the influx of Water. If the semi-permeable membrane is flexible and stretchable, it will become become bow towards the outside. If the membrane is delicate, like the Plasmalemma, it will break. Plasmalemma If If the membrane remains intact, water influx will continue until the Hydrostatic Pressure Hydrostatic inside inside the funnel makes the Water Potential of both compartments equal. Water Water can cross plant membranes by diffusion of individual water molecules through diffusion the the membrane bilayer and by microscopic bulk flow of water molecules through a bulk water selective pore formed by integral membrane proteins such as aquaporins Three Three major factors contribute to cell Water Potential Water (free energy of water/unit volume; J m-3) w = s + p + g s Solute potential or osmotic potential p Hydrostatic pressure of the solution g Gravity potential s = -RTCs W here Where R is the gas constant [8.32 J mol-1K-1] is T is the absolute temperature is Cs is the solute concentration of the solution It It is very important to remember that [For [For ionic solutes that dissociate into 2 or more particles, C s must be multiplied by must the the # of dissociated particles] The sign The - sign indicates that dissolved solutes reduces the water potential potential of a solution is relative to that of water The water potential of pure water is 0 Pressure p The The term Hydrostatic Pressure is used to indicate Hydrostatic is the the pressure component of Water Potential. Positive Pressures increase the Water Potential. [Positive [Positive Pressure would bring Water Molecules closer together. This This would foster a quasi-crystalline organization which has less crystalline Entropy Entropy and greater Potential Energy] Negative Pressures reduce Water Potential. [They [They pull water molecules further apart. This disrupts Hydrogen Bonding & increases Entropy. This This lowers the potential energy of water] The The terms Tension & Negative Hydrostatic Pressure are used interchangeably. Tension Turgor Pressure is the Positive Hydrostatic Pressure that occurs inside plant cells. Gravity g g = wgh Gravity causes water to move downward unless the gravity force Is opposed by an equal and opposite force The The term g depends on The height The height (h) of the water above the reference-state water The density of water (w) The acceleration due to gravity (g) Gravity Gravity is not a major consideration for plants less than 5 meters in height. It It is significant for taller trees. Examples Examples illustrating the concept of water potential and its components 0.3 M Sucrose Examples Examples illustrating the concept of water potential and its components One important point to remember: Water flow is a passive process, water moves in response to physical forces, Toward regions of low water potential or low free energy. ...
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This note was uploaded on 11/30/2011 for the course BIOLOGY 321 taught by Professor Min during the Winter '11 term at University of Michigan.

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