Chapter 3 Water and plant cells

Importance of water on crop yield

Productivity of various ecosystems as a function of annual precipitation

  Most (~97%) of the water absorbed by roots is carried through the plant and evaporates from leaf surfaces. => transpiration   In contrast, only a small amount of the water absorbed by roots actually remains in the plant to supply growth (~2%) or to be used in photosynthesis and other metabolic processes (~1%).   The uptake of CO 2 is coupled to water loss.   The concentration gradient for water loss from leaves is much larger than for CO 2 uptake.

Physical and chemical properties of water   Polar   H-bonds   High melting point   High boiling point   High specific heat   High thermal conductivity   High heat of vaporization : energy required to convert a substance from the liquid to the vapor state (The heat of vaporization accounts for the cooling effect associated with heat loss as water evaporate from leaves.)   An excellent solvent => hydration shell   Cohesion ( 內聚力 )   Adhesion ( 附著力 ) (continued)

Physical and chemical properties of water   Cohesion ( 內聚力 ) : mutual attraction between water molecules   Surface tension : cohesion force between water molecules is higher than that between water and air   Tensile strength ( 抗張強度 ) : the ability to resist a pulling force The maximum force for a continuous column of water to withstand before breaking   Adhesion ( 附著力 ) : attraction of water to a solid phase such as cell wall

The Solubilization of Sodium Chloride

  Pushing on the plunger compresses the fluid, and a positive pressure builds up. => positive hydrostatic pressure ( 流體靜壓 )   Pulling on the plunger causes the fluid to develop a tension, or a negative pressure. => negative hydrostatic pressure.   The hydrostatic pressure is positive in plant cells and is referred as turgor pressure (  p ).

The combination of cohesion, adhesion, and tensile strength result in capillarity ( 毛細管作用 ).

Translocation of water  Diffusion  Bulk flow

The size of the pore is about 0.3nm = the size of a water molecule. Flow rate = 1 X 10 9 molecules/sec

Diffusion ( 擴散 )   Diffusion means molecules move down a concentration gradient.   Fick’s first law (1855) : The rate of diffusion is directly proportional to the concentration gradient and is inversely proportional to the length of the path.   Diffusion is rapid over short distances but extremely slow over long distances.

Bulk flow   also called “mass flow”.   Movement of groups of molecules en mass in response to a pressure gradient.   Bulk flow is driven by pressure.   Bulk flow of water is the predominant mechanism responsible for long-distance transport of water in plants via the xylem and the water flow through the soil and the cell walls of plant tissue.

Osmosis   Movement of a solvent (water) through a selectively permeable membrane is called osmosis ( 滲透 ).   Occurrence of osmosis is dependent on the gradient in free energy of water across the membrane.   The free-energy gradient of water = driving force of water movement.   In osmosis, both concentration gradient and pressure gradient will influence transport.   The forces involved in osmosis is called osmotic pressure.   For an isolated solution, it has only an osmotic potential. Osmotic potential ( 滲透勢能 ) is the negative of the osmotic pressure, since they are equal but opposite forces.

Osmotic pressure Hydrostatic pressure developed in the tube

Osmosis   The magnitude of the osmotic pressure is a function of solute concentration.   If addition pressure were applied, we might expect the net movement of water to reverse its direction and instead flow out of the solution.   Therefore, osmosis is driven by the solute concentration as well as by the pressure differences.   In osmosis, both concentration gradient and pressure gradient will influence the overall chemical potential of water (water potential), which is the ultimate driving force for water movement in plants.   Osmosis is driven by a water potential gradient.

Water potential   Also called chemical potential of water.   A quantitative expression of the free energy associated with water   Free energy means a potential for performing work or the energy that is free and available for performing work.   Water potential is a measure of the free energy of water per unit volume (J/m 3 )   Water moves down a chemical potential gradient from a region of high chemical potential to a region of low chemical potential.

Water potential ( 水分勢能 )   Water potential in plants is influenced by three major factors :   Concentration   Pressure   gravity

   w =  s +  p +  g    w : water potential    s : osmotic potential (solute potential)    p : hydrostatic pressure of the solution    g : the effect of gravity on water potential

   w inside plant cells is negative.    w is proportional to the work required to move 1 mole of pure water.  w : water potential

  The effect of dissolved solutes on water potential   Dissolving of solutes increases the disorder of water and reduce the free energy of water by diluting the water.   Solutes reduce the vapor pressure of a solution, raise its boiling point, and lower its freezing point.    s = -RTC s R : gas constant T: absolute temp. C s : osmolality ([Solute]/L) The minus sign indicates that dissolved solute reduces the water potential of a solution.  s : osmotic potential (solute potential)

 p : hydrostatic pressure of the solution   Positive hydrostatic pressures raise the water potential; negative hydrostatic pressure reduce the water potential.   The positive hydrostatic pressure within plant cells is referred as turgor pressure.   Water moves from high turgor pressure to the region with low turgor pressure.   The xylem and cell walls between cells have negative hydrostatic pressure (or tension).   During daytime, transpiration increases =>  p decrease (negative)   During nighttime, transpiration decrease =>  p increase (plants rehydrated)

 p => +

 g : the effect of gravity on water potential    g is influenced by height (h) of the water, the density of water (  w ), and the acceleration due to gravity (g).    g =  w gh   Gravity causes water to move downward.   When dealing with water transport at the cell level, the gravitational component (  g ) is generally omitted because it is negligible compared to the osmotic potential and the hydrostatic pressure.

  Water flow is a passive process.   Water moves in response to physical forces toward region of low water potential or free energy; that is the direction of water flow is determined by the direction of the gradient, which is the driving force for transport.

  The rate of water transport depends on   Driving forces (Δ ψ w ) Water potential difference across the membrane   Hydraulic conductivity (Lp) = membrane permeability   Flow rate = driving force X hydraulic conductivity

Hydraulic conductivity   Volume of water per unit area of membrane per unit time per unit driving force (MS -1 MPa -1 )   The larger the hydraulic conductivity, the larger the flow rate.

Why do we study water potential?   Water potential is the quantity that govern water transport across cell membranes.   Water potential is usually used as a measure of the water status of a plant.

Water potential of plants under various growing conditions

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The concentration gradient of a solute that is diffusing according to Fick’s law

The pressure chamber method for measuring plant water potential. The diagram at left shows a shoot sealed into a chamber, which may be pressurized with compressed gas. The diagrams at right show the state of the water columns within the xylem at three points in time: (A) The xylem is uncut and under a negative pressure, or tension. (B) The shoot is cut, causing the water to pull back into the tissue, away from the cut surface, in response to the tension in the xylem. (C) The chamber is pressurized, bringing the xylem sap back to the cut surface.

Measuring cell turgor pressure