Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Water, Diffusion and Osmosis

Water, of its very nature, as it occurs automatically in the process of cosmic evolution, is fit, with a fitness no less marvelous and varied than that fitness of the organism which has been won by the process of adaptation in the course of organic evolution.

L. J. Henderson
The Fitness of the Environment, 1913

I. Water is absolutely essential for all living organisms

The evidence:

1. Most organisms are comprised of at least 70% or more water. Some plants, like a head of lettuce, are made up of nearly 95% water;
   
2. When organisms go dormant, they loose most of their water. For example, seeds and buds are typically less than 10% water, as are desiccated rotifers, nematodes and yeast cells;
   
3. Earth is the water planet (that's why astronomers get so excited about finding water in space).
   
4. Water is the limiting resource for crop productivity in most agricultural systems (see text for supporting data)

Tom Robbins, author of Even Cowgirls Get the Blues, has eloquently stated the importance of water:

"Water - the ace of elements. Water dives from the clouds without parachute, wings or safety net. Water runs over the steepest precipice and blinks not a lash. Water is buried and rises again; water walks on fire and fire gets the blisters. Stylishly composed in any situation - solid, gas or liquid - speaking in penetrating dialects understood by all things - animal, vegetable or mineral - water travels intrepidly through four dimensions, sustaining, destroying, and creating. Always in motion, ever-flowing (whether at stream rate or glacier speed), rhythmic, dynamic, ubiquitous, changing and working its changes, a mathematics wrong side out, a philosophy in reverse, the ongoing odyssey of water is virtually irresistible."

    Now, let's change our perspective for a minute and put ourselves in the place of water. According to Robbins, water is so important that "[i]t has even been suggested that life evolved as a means to transport water." Well, this is certainly good fodder for a late night discussion, but I doubt that anyone would question Frank Salisbury’s and Cleon Ross’s (1992) statement that "Plant physiology is ... the study of water."

II. Water is important because it is polar and readily forms hydrogen bonds

A. Water is Polar
    In other words, the water molecule has a positively-charged (hydrogen side) and negatively-charged side (oxygen). This occurs because:

1. the hydrogen atoms are arranged at an angle of about 105 degrees;
   
2. the covalent bond between O-H is polarized. This is caused by an unequal sharing of electrons between these atoms which, in turn, results in a slight negative charge on the oxygen atom (electronegative) and slight positive charge on the hydrogen;
   
3. oxygen has an unshared pair of electrons (the molecule is tetrahedral-shaped).

B. Hydrogen Bonds
    The 'fancy' definition of a hydrogen bond is that it is a weak bond that forms between a hydrogen atom that is covalently bonded to an electronegative atom (like oxygen) and another electronegative atom. In other words, a positively-charged hydrogen atom is attracted to a negatively-charged oxygen.

    The end result is that water readily forms hydrogen bonds with itself and other polar molecules. When likes attract it is termed cohesion (i.e., hydrogen bonds between water molecules). When unlikes attract, it is called adhesion (i.e., when a paper towel absorbs water, water and cellulose adhere to one another). Cohesion and adhesion are responsible for capillary action, the movement of water up a thin tube.

    In liquid water, hydrogen bonds between water molecules are continuously made and broken. The molecules can even form temporary "quasi-crystalline" areas. Individually, each hydrogen bond is weak (20 kJ mol^-1), but collectively they give water many unique properties (a Marxist molecule!).

A. Water is a liquid at physiological temperatures (i.e., between 0-100 C). 
    In other words, water has a high boiling point and a high melting point when compared to other similar-sized molecules such as ammonia, carbon dioxide, hydrogen sulfide. These other molecules are gases at room temperature. This is important because if life exists anywhere, we predict that it occurs between approx. 0 and 100 C. Temperatures much below 0 are too cold to permit significant chemistry for metabolism; temperatures above 100 tend to disrupt bonds.

B. Water has a high heat of vaporization.  
    In other words, it takes a lot of energy (ca. 44 kJ mol^-1) to convert water from a liquid to a gas; or stated another way, Water resists evaporation. This property is responsible for water's use in evaporative cooling systems, hence the reason dog's pant, people perspire, and leaves transpire.

C. Water has a high specific heat (heat capacity).
    It takes a lot of energy (4.184 J g^-1 C^-1, or the non-SI unit is the calorie where 1 cal = 4.184 J) to raise the temperature of water (because it requires a lot of energy to break/make hydrogen bonds). Thus, water is slow to heat up and cool down, or stated another way, water resists temperature changes. This is why you can swim reasonably comfortably in the Sag in late fall but not the spring. In contrast, a sidewalk has low specific heat - it heats up quickly (try walking barefoot on a sidewalk in summer on a sunny day), but cools down quickly. This property is important in water's role as a thermal buffer. It's not surprising that desert plants are succulent - to help resist temperature fluctuations.

D. Water has a high heat of fusion. 
    It takes a lot of energy to convert water from a solid to a liquid, or put another way, water resists freezing. Energy is required to break the collective hydrogen bonds holding water in its solid configuration. Conversely, a lot of energy (6 kJ mol^-1) must be released by water to freeze. This property is used by citrus growers - prior to a light freeze they spray fruit with water; ice forms releasing the heat of fusion which will help protect the crop from serious damage.

E. Water has a high surface tension. 
    It takes a lot of energy to break through the surface of water, because water molecules at the surface are attracted (cohesion) to others within the liquid much more than they are to air. Thus, water acts as though it has a skin. This phenomenon is important at air/water interfaces and explains why: (1) water rises up a thin tube (capillary action); (2) raindrops are round (the molecules at the surface attract one another); (3) water striders and other bugs can "walk on water"; (4) a meniscus forms; and (5) a belly-whopper into a pool of ammonia would not hurt nearly as much as one into water.

F. The density of water decreases on crystallization. 
    Good thing too, or else ice fishing would be a moist business. This occurs because when ice forms each water molecule is hydrogen bonded to exactly four others. At four degrees, water is it's densest, and each water molecule is attracted to slightly more than four others. Thus, as water cools it gets denser and denser until it reaches 4 C, then, it gets less dense. And ice floats.

G. Water is a universal solvent. 
    Water dissolves more different kinds of molecules than any other solvent. Hydrophilic (water-loving) molecules dissolve readily in water (likes dissolve likes), hydrophobic (water-fearing) ones do not.

H. Water has high tensile strength and incompressibility. 
    In other words, if you put water in a tube and put a piston on either end, you won’t be able to push the pistons together. Thus, water is good for hydraulic systems because when it is squeezed it doesn't compress and produces positive pressures (hydrostatic pressures). This pressure provides the driving force for cell growth and other plant movements. The pressure is measured in units of Pascals (or actually MegaPascals, MPa). One MPa is approximately equal to ten atmospheres or 10 bars.

    In a similar fashion, if you fill the tube with water, remove any air bubbles, and then pull the pistons away from one another, the water column resists breaking.  This will result in a suction on the water column - just like putting your finger on the end of a syringe and pulling back the plunger.  Negative pressures (tensions) can develop in the water column. Very sizable tensions can be generated in a thin water column.  However, cavitation, when air comes out of solution at negative pressures, can be a problem.

I. Water is transparent to light. 
    This is important because chloroplasts (inside a cell) are obviously surrounded by water.  If water were opaque, plants couldn't photosynthesize.  From an ecological perspective, the penetration of in water determines the distribution of aquatic plants.

J. Water is chemically inert. 
    It doesn't react unless it is enzymatically designed to do so.

K. Water dissociates into protons and hydroxide ions. 
    This serves as the basis for the pH system (see below).

L. Water affects the shape, stability & properties of biological molecules.
    For example, many ions (such as sodium) and molecules (such as DNA and wall components) are normally hydrated.  This means that water is hydrogen bonded to them and in some cases (i.e., sodium) forms a hydration shell around them.  

IV. Functions of Water. In addition to the functions mentioned above, water:

1. is a major component of cells
   
2. is a solvent for the uptake and transport of materials
   
3. is a good medium for biochemical reactions
   
4. is a reactant in many biochemical reactions (i.e., photosynthesis)
   
5. provides structural support via turgor pressure (i.e., leaves)
   
6. is the medium for the transfer of plant gametes (sperms swim to eggs in water, some aquatic plants shed pollen underwater)
   
7. offspring (propagule) dispersal (think "coconut")
   
8. plant movements are the result of water moving into and out of those parts (i.e., diurnal movements, stomatal opening, flower opening)
   
9. cell elongation and growth
   
10. thermal buffer
    
11. perhaps most importantly, water has directed the evolution of all organisms. You can think of morphological features of organisms as a consequence of water availability. For example, consider organisms growing in xeric (dry), mesic (moderate) and hydric (aquatic) environments.
    

V. Acids and Bases (a review from introductory biology)
    Water ionizes to a small degree to form a hydrogen ion (or proton) and hydroxide ion (OH-). In reality, two water molecules form a hydronium ion (H₃0⁺) and a hydroxide ion (OH^-).

Thus:

1. An acid is a substance that increases the [H⁺], or as the chemists say, is a proton donor.
       eg. HCl → H⁺ + Cl^-
   
   
2. A base is a substance that increases the [OH^-]; or from the perspective of a proton, a base is a substance that decreases the proton concentration; it is a proton acceptor.
       e.g. NaOH →  Na⁺ + OH^- (accepts protons to make water)
       e.g. NH₃ (ammonia) + H⁺ →  NH₄⁺ (ammonium ion)

The pH Scale:
    pH is the scale to express the degree of acidity (or alkalinity) of a solution. The scale ranges from 0 to 14 where 1 is highly acidic, 7 is neutral, and 14 is highly alkaline.

As the pH increases, the [H⁺] decreases and the [OH^-] increases
As the pH decreases, the [H⁺] increases and the [OH^-] decrease

pH = - log[H+]

Points to note:

1. the pH scale is based on proton concentration; and
   
2. the pH scale is logarithmic, there is a 10-fold difference in concentration between each pH unit.

    The products of [H⁺] x [OH^-] always equals 10^-14. Thus, you can always determine concentration of one if you know the other. For example, if the [H⁺] = 10^-2, then the [OH^-] is 10^-12.

VI. Living systems are very sensitive to pH
    Organisms must maintain pH within tolerable ranges. This is a good example of homeostasis.

    A buffer is a solution that resists fluctuations in pH when additional OH- or H+ are added. They maintain a constant pH and usually consist of a proton donor and a proton acceptor. [e.g., blood pH must be between 7.36 (venous) and 7.41 (arterial). The carbonic acid/bicarbonate buffer helps to maintain pH:

    H₂CO₃ (carbonic acid; proton donor) → H⁺ + HCO₃^- (bicarbonate ion; proton acceptor)

 VII. Water Movement - There are two major ways to move molecules:

A. Bulk (or Mass) Flow.
    This is the mass movement of molecules in response to a pressure gradient. The molecules move from hi → low pressure, following a pressure gradient. A good example would be a faucet. When you turn a faucet on, water comes out. This occurs because the water in the tap is under pressure relative to the air outside the faucet. A toilet is another example; high pressure in the tank/bowl but lower pressure in the sewer system. Some molecular movements rely on bulk flow which requires a mechanism to generate the pressure gradient. For example, animals have evolved a pump (i.e., heart) that is designed for the bulk flow of molecules through the circulatory system.

B. Diffusion
    The net, random movement of individual molecules from one area to another. The molecules move from [hi] → [low], following a concentration gradient. Another way of stating this is that the molecules move from an area of high free energy (higher concentration) to one of low free energy (lower concentration). The net movement stops when a dynamic equilibrium is achieved.

    Imagine opening a bottle of perfume containing volatile essential oils in a very, very still room. Initially, the essential oils are concentrated in a corner of the room. As the molecules move randomly, in every different direction, over time they will eventually appear throughout the room. Ultimately the essential oils will reach a point, dynamic equilibrium, at which they are evenly distributed throughout the room. At this point the molecules are still moving. They continue to move randomly in every direction. The only difference is that there is no net change in the overall distribution of the perfume in the room.

    Now imagine that the room is divided by a partition with holes (which is analogous to a membrane). If we place a drop of perfume on one side of the partition and then count at intervals the number of essential oil molecules on either side of the partition and graph the results:

insert:  plot # molecules vs. time on both sides of the partition. (not included)

    We will observe that the number of molecules on one side will decrease while the other will increase until they reach dynamic equilibrium. At equilibrium the molecules continue to move randomly, back and forth from one side of the partition to the other.  Hence the number of molecules on either side of the partition at any given time is simply chance. The number oscillates about the midpoint.

A caveat
    Although this theoretical example can help us to better understand the nature of diffusion, it is technically wrong. The molecular movement attributed to diffusion in this example is really due to air movements in the room, or convection. True examples of diffusion are hard to come by (see Vogel, 1994; Wheatley, 1993). Nevertheless, it serves our purpose to illustrate the general concept of diffusion.

C. Osmosis
    This is a specialized case of diffusion; it represents the diffusion of a solvent (typically water) across a membrane.

D. Dialysis
    Another specialized case of diffusion; it is the diffusion of solute across a semi-permeable membrane. Example – consider a cell containing a sugar dissolved in water. If water (the solvent) moves out of the cell into the surroundings it moves osmotically; if the sugar (solute) moves into the surroundings, it is an example of dialysis.

VII. Factors influencing the rate of diffusion - Several factors influence the rate of diffusion. These include:

A. Concentration Gradient.
    As previously stated, solutes move from an area of high concentration to one of lower concentration; in other words, in response to a concentration gradient (ΔC). Although this is true for most solutes, it is NOT important for water. The concentration of water (55.2 - 55.5 mol L^-1) is nearly constant under all conditions (i.e., MW = 18 g/mol, and 1000 g/liter; thus, 1000/18 = 55.5 mol/L).

Fick’s Law - is an equation that relates the rate of diffusion to the concentration gradient (C1 – C2) and resistance (r). Diffusion rate, also called flux density (J_s, in units of mol m^-2 s^-1) can be expressed in the simplified version of Fick's equation as:

J_s = (C1 - C2) / r

The take-home-lessons from this equation are that:

1. the rate of diffusion is directly proportional to the concentration gradient. The greater the difference in concentration between two areas, the greater the rate of diffusion. Thus, when the gradient is zero, there will be no net diffusion; diffusion will only occur so long as a concentration gradient exists;
   
2. the rate of diffusion is indirectly proportional to resistance. In other words, the greater the resistance to diffusion, the lower the rate of diffusion. Resistance refers to anything that reduces the rate of diffusion such as the partition in our perfume example. The width of the partitions is a resistance; the wider the partitions, the lower the resistance. And, the membrane is a resistance to the movement of ions and other charged substances in or out of cells; and
   
3. the rate of diffusion is inversely proportional to distance traveled (also a function of resistance). For example, some typical diffusion rates for water are 10 �m - 0.1 sec; 100 �m -1 sec; and 1 mm - 100 sec. As the text demonstrates nicely, diffusion is effective over short distances, but is pathetically slow over long distances.

B. Molecular Speed. According to kinetic theory, particles like atoms and molecules are in always in motion at temperatures above absolute zero (0 K = -273 C). The take-home-lesson is that molecular movement is:

1. directly proportional to temperature; and
   
2. indirectly related to molecular weight (heavier particles move more slowly than lighter, smaller ones). At room temperature, the average velocity of a molecule is fast - about 2 km/sec (=3997 mph!).

C. Temperature - increases the rate of molecular movement, therefore, increases the rate of diffusion

D. Pressure - increases speed of molecules, therefore, increase the rate of diffusion

E. Solute effect on the chemical potential of the solvent. Solute particles decrease the free energy of a solvent. The critical factor is the number of particles, not charge or particle size. Essentially solvent molecules, such as water in a biological system, move from a region of greater mole fraction to a region where it has a lower mole fraction. The mole fraction of solvent = # solvent molecules/ total (# solvent molecules + # solute molecules). This is particularly important in the movement of water. Water moves from an area of higher mole fraction or higher energy to an area of lower mole fraction or lower energy.

VIII. Water Potential
    Water potential is a measure of the energy state of water. This is a particularly important concept in plant physiology because it determines the direction and movement of water.

A. First, some definitions

1. Free energy of water - energy available to do work (J = n m)
   
2. Chemical potential (�) - free energy/unit quantity (usually per mole) (J mol^-1)
   
3. Water potential (Ψ_w) - chemical potential of water, compared to pure water at the same temperature and pressure. The units are in pressure because: (a) plant cells are under pressure (remember the wall?); and (b) it is easier to measure pressure.  
   
4. Derivation of units - Water potential is official defined as the chemical potential of water (J mol^-1).  When divided by the partial molar volume (L mol^-1):
   
   J mol^-1/ L mol^-1 = n x m mol^-1/ m³ mol^-1  =  n  m^-2 = MPa
   
                or looking at it another way
   
   J mol^-1/ L mol^-1 =  J / liter = energy / volume = (weight x distance)/area x distance = force/area = pressure units  
   
5. Pressure is measured in MPa (megapascals). 1 MPa = 10 bars = 10 atm.  (as an aside, 1 atm = 760 mm Hg = 14. 7 lbs sq in^-1)

B. Equation for water potential (must account for the factors that influence the diffusion of water):

Ψ_w   = Ψ_p + Ψ_s + Ψ_g

where Ψ_w = water potential; Ψ_p = pressure potential; Ψs = solute or osmotic potential; and Ψ_g = gravity potential.

1. Solute (or osmotic) potential (Ψ_s)
    This is the contribution due to dissolved solutes. Solutes always decrease the free energy of water, thus there contribution is always negative. The solute potential of a solution can be calculated with the van’t Hoff equation: Ψ_s = - miRT where m = molality (moles/1000 g); i = ionization constant (often 1.0); R = gas constant (0.0083 liter x MPa/mol deg); and T = temperature (K).

2. Pressure (or Pressure Potential; Ψ_p)
    Due to the pressure build up in cells thanks to the wall. It is usually positive, although may be negative (tension) as in the xylem. Pressure can be measured with an osmometer.

3. Matric potential
    This is the contribution to water potential due to the force of attraction of water for colloidal, charged surfaces. It is negative because it reduces the ability of water to move. In large volumes of water it is very small and usually ignored. However, it can be very important in the soil, especially when referring to the root/soil interface.

4. Gravity (Ψ_g)
    Contributions due to gravity which is usually ignored unless referring to the tops of tall trees.

C. The water potential of pure water is zero. Water potentials in intact plant tissue are usually negative (because of the large quantities of dissolved solutes in cells).

D. Water potential is the sum of the contributions of the various factors that influence water potential
         where:  Ψ_w = Ψ_p + Ψ_s + etc.

E. Measuring Water Potential - we will discuss the following techniques in class/lab:

1. Pressure bomb - a steel chamber that can be pressurized, usually with nitrogen.  The sample is placed in the chamber with the petiole or surface exposed through a hole in the lid.  The sample is pressurized and the pressure that is required to force water to appear on the cut surface is assumed to be equivalent to the water potential of the tissue.

2. Chardakov Method - Dye Drop method

3. Gravimetric method

F. Measuring Solute Potential
    Solute potential can be measured by:

1. Freezing Point Depression - dissolved solutes lower the freezing point of a liquid (think salt and MN roads in the winter). A 1 molal solution with an osmotic potential of -2.27 MPa lowers the freezing point (fp) by 1.86 degrees.  We can use this relatinoship to set up a ratio:   -2.27 MPa/1.86 degree = unknown Ψ_s / fp.  Rearranging we get the equation:   Ψ_s (MPa) = -1.22 x fp
   
2. Incipient Plasmolysis - a tissue is incubated in a series of solutions of known water potential.  The point at which membrane just pulls away from tissue is "incipient plasmolysis" and considered to the equivalent to the osmotic potential of the tissue.
   
3. Vapor pressure osmometer - dissolved solutes increase the boiling point or decrease the vapor pressure of a liquid. A thermocouple hooked to a recorder is placed in an airtight chamber with the tissue sample or standard.  The thermocouple is also linked to a reference junction.  Thermocouples are made of two different metals (constantan and chrome) and a current will flow if there is a difference in temperature between the reference junction and thermocouple.  A water droplet or KCl (aq) solutions of known osmolarity are placed on the thermocouple.  Depending on the osmotic potential of the solution, water will either evaporate from or condense on the droplet.  This is turn causes a current change in the thermocouple which can be detected by a meter.  The cooling rate is plotted vs. Ψ_s or [KCl] to yield a standard curve from which the Ψ_s of the tissue is determined.

IX. The Movement of water across a membrane is a combination of diffusion and bulk flow
    Individual water molecules diffuse across the membrane. In addition, there are integral proteins in the membrane that form a channel or pore through which water moves. These pores are important and water molecules essentially move through these pores by bulk flow. The proteins are called aquaporins and are essentially water transport channels. Water is moving passively (following a gradient of free energy).

References:

• Boyer, JS (1969) Measurement of the water status of plants. Ann. Rev Plant Physiol. 20: 351 - 364.
• Hebrank, MR. 1997. Reduce confusion about diffusion. American Biology Teacher 59: 160.
• Odom, AL. 1995. Secondary & College Biology Students’ misconceptions about diffusion & osmosis. American Biology Teacher 57: 409 - 415.
• Vogel, Steven. 1994. Dealing Honestly with diffusion. American Biology Teacher 56: 405-407.
• Wheatley, D. 1993. Diffusion theory in biology: its validity and relevance. Journal of Biological Education 27: 181-187.
• Zuckerman, JT. 1994. Problem solvers’ conceptions about osmosis. American Biology Teacher 56: 22-25.

Water Unit Questions:

Click here for some study questions 

Last updated:  02/13/2009     � Copyright  by SG Saupe