Water Movement through Vascular Plants

{NOTE at the outset of our tour, that only the most successful and common plants have vascular tissue. The earliest and most primitive plants (mosses and liverworts) don't have vascular tissue, and simply absorb moisture directly through the surfaces of their bodies.]
If one is to understand how water moves through plants (who have no muscles or nerves!),then one must first know a little bit about that magical molecule WATER.

WATER is a compound whose chemical formula is H2O, which means it's composed of two hydrogens attached to an oxygen atom.

Recall our discussion of POLARITY in molecules, and remember that water is POLAR: one end (the hydrogen end) is relatively POSITIVELY CHARGED compared to the other end (the oxygen end), which is NEGATIVELY CHARGED.

Everyone knows that OPPOSITES ATTRACT. Water molecules are attracted to each other at their opposite ends, and water in its densest forms (liquid and solid (ice)) is simply a network of water molecules being attracted to each other's opposite ends. Let's have a LOOK.

The relatively weak attraction between the positive and negative ends of the water molecules are called HYDROGEN BONDS.
The water molecule's structure is auspicious! It results in two important properties of water that make life on earth possible!

COHESION - The aforementioned property of water molecules to stick to each other via hydrogen bonds.

ADHESION - The property of those polar water molecules sticking to *other* types of polar molecules, also via hydrogen bonds.

While we're on the subject of water, let's mention a few other very important properties of this amazing substance, and will come into play later, when we discuss energy flow.

  • Anything that is composed of matter has POTENTIAL (stored) ENERGY.
  • Matter can exist as SOLID, LIQUID or GAS, depending on the temperature and pressure of the environment in which it exists.
  • Each element and compound has its own characteristic freezing points (conditions under which it becomes solid), melting points (conditions under which it goes from solid to liquid) and boiling points (conditions under which it goes from liquid to gas/vapor).
  • Anything that moves has KINETIC ENERGY, the energy of movement.
  • The faster something moves, the more kinetic energy it has.
  • HEAT is a measure of how much kinetic energy something has.
  • TEMPERATURE measures the intensity of heat due to the average kinetic energy of the molecules making up a substance

  • The SPECIFIC HEAT of a substance is defined as the amount of heat that the substance must absorb for ONE GRAM of that substance to increase in temperature by ONE DEGREE CELSIUS. (On the Celsius scale, water freezes at 0o0oC and boils at 100oC).
  • The specific heat of water is one calorie, and it's water that is actually used to DEFINE the calorie. (The dietary Calorie you're used to hearing about is actually 1000 calories, or one kilocalorie.)

    Strangely enough, water, when compared to other substances, has a very HIGH specific heat! For example, it takes relatively little energy to raise the temperature of ethyl alcohol. It takes a LOT of energy to raise the temperature of water! This means that water can absorb a LOT of energy (e.g., from the sun) without changing its temperature very much. This is, in part, due to the hydrogen bonds, which are so strong that they prevent the water molecules from moving quickly, despite the input of energy into the system. (Think about it: you can burn your finger on the metal handle of an iron pot of water while the water in the pot is still lukewarm. That's because the specific heat of water is 10x greater than that of iron!)

    Water is the SOLVENT OF LIFE.

    If you put a spoonful of sugar into a glass of water, the sugar will dissolve, and eventually there will be a uniform distribution of sugar and water molecules in the glass, due to the slow, random movement of molecules (Brownian motion) in the glass.

  • a SOLUTION is a completely homogeneous liquid mixture of two or more different kinds of molecules.
  • a SOLVENT is the dissolving agent of a solution (e.g., water)
  • a SOLUTE is the substance dissolved in a solution (e.g., sugar)
  • a solution in which the solvent is water is called an AQUEOUS SOLUTION.
    As we've already discussed, some substances are HYDROPHILIC ("water loving") and will readily mix with water. (e.g., salts, sugars, proteins and other polar substances) Others are HYDROPHOBIC ("water fearing"), and do not readily mix with water (e.g., oils, waxes and other non-polar substances)
    BIOLOGICAL CHEMISTRY IS WATER CHEMISTRY! Just about all of the chemical reactions that take place in living organisms involve aqueous solutions.
    The movement of water through plant vascular tissue is intimately connected with the very nature of water itself.

    Water movement can also be understood by knowing something about the Laws of Thermodynamics (which we'll meet again when we discuss energy flow):
  • First Law of Thermodynamics - Energy can be changed in form, but it cannot be created or destroyed. (All the energy in the universe is constant)
  • Second Law of Thermodynamics - All systems tend to move from a state of order to a state of greater disorder, or chaos. A quantitative measure of disorder/chaos is ENTROPY.

    ****WATER FLOW THROUGH PLANTS HAS EVERYTHING TO DO WITH THE SECOND LAW OF THERMODYNAMICS*******

    Can you stand one more concept from the Magical Land of Physics? Here goes...

  • A system which is more organized has more POTENTIAL ENERGY than a similar system which is less organized. (Let's do an analogy...)
  • If the two systems (one more organized than the other) are close to one another, or even somehow touching, then there is a difference in potential energy between them which can be thought of as a GRADIENT (like the gradient of a hill, in which one end is higher, and the other end is lower).
  • If systems always move from a state of greater order to a state of greater disorder, then it stands to reason that when the two adjacent systems are actually *joined*, that the highly ordered system's energy will flow into the more chaotic system, until both are at a state of equilibrium, and there is no further NET change between them. (Let's do another analogy...)

    WATER POTENTIAL is a measure of the difference in potential energy between two aqueous systems. For example, an partitioned aquarium filled with water. (Let's do a few solution manipulations of the aquarium...)

    Got all that? Now let's move on and discuss a few more terms...

  • bulk flow: overall movement of water in response to differences in the potential energy of water. (i.e. in response to differences in water potential).

    examples: water behind a dam; water in your bladder...

    Water always moves FROM an area of low water potential TO an area of lower water potential.

    (Water potential is usually measured in units of the pressure needed to stop water movement: hydrostatic pressure. (Common units include the bar or megaPascals...)

    By convention, pure, 100% water (nothing dissolved within it) is said to have a potential of ZERO.

    This is the maximum water potential possible, since the liquid contains the maximum amount of water molecules possible. If you dissolve any substance into the pure water, the water potential of that solution DECREASES. (There are relatively fewer water molecules in it than the same volume of pure water. Less water, less water potential energy.)

    HENCE, any aqueous solution has NEGATIVE water potential relative to 100% water.

    In solution, water molecules cluster around a polar solute molecule, creating a HYDRATION SHELL.

    **************

  • a HYPOTONIC (= hypoosmotic) solution is one which has relatively more water molecules (and fewer solutes) than a HYPERTONIC (= hyperosmotic) solution.

    two ISOTOIC (isoosmotic) solutions have equal concentrations of solutes.

    *******

    If you understand that Second Law of Thermodynamics, you will already have figured out that water molecules will always tend to move from a hypotonic area (relatively high water potential) to a hypertonic (relatively low water potential) area. This is known as DIFFUSION.

  • osmosis: movement of water from high potential to low potential across a selectively permeable membrane.

  • dialysis: movement of solute molecules from area of high concentration to low concentration across a selectively permeable membrane.

    Recall: water molecules are highly cohesive (they stick together) and adhesive (they stick to other polar substances).

    Osmotic pressure: a measure of the tendencey of a solution to take up water molecules when separated from pure water by a selectively permeable membrane.

    In living cells, the plasma membrane serves as the selectively permeable membrane.

    ******************

    If a plant cell is placed in a very hypertonic solution, and left long enough, it may lose so much internal water (via osmosis) that it becomes plasmolyzed (i.e., torn away from the cell wall)--beyond repair.

    In an isoosmotic solution, a plant cell is somewhat flaccid (i.e., it's walls have a bit of "give" and "bounce").

    In a hypoosmotic solution, a plant cell may take up so much water that it becomes very turgid. (If the hypoosmotic solution is very different from the plant cell, the cell may rupture).
    Have a LOOK.

    The turgor pressure of a cell is equal and opposite to the wall pressure (as long as the cell membrane is intact).

    (This is the main source of structural support in herbaceous plants)

    ********

    Water movement through xylem...

    Plants imbibe (take up) and transpire (release via the stomates) more water than animals do, as they have no re-circulation system. About 99% of all water entering the roots leaves the leaves via the stomates without ever taking place in metabolism.

    Water movement is due to differences in potential between soil, root, stem, leaf and atmosphere.

    Under normal circumstances, the water potential in soil is higher than that in root fluids.

    For example, typical moist soil might have a water potential of abou -1.0 bar; root tissue about -4.0 bar; stem about -8.0 bar; leaf about -15.0 bar and typical "sunny day" atmosphere about -800 bar!

    Water follows the potential gradient from soil to atmosphere, and is pulled together by the cohesiveness and adhesiveness of the molecules themselves! SHOOT TENSION.

    ***************

    Generation of transpirational pull is due to the flow of water from vascular tissue into the spongy mesophyll spaces in the leaf.

    *Water diffuses from the xylem into the spaces inside the spongy mesophyll--> stomates-->atmosphere.

    * Water coating the surface of the mesophyll spaces forms crescent-shaped menisci as it enters the air spaces.

    * As water molecules are pulled from between the cells, the menisci become more curved, and water tension increases (due to the cohesiveness of the water molecules).

    * Meniscus tension is inversely proportional to the radius of its curved surface: as the meniscus becomes more concave, its radius decreases and the tension increases.

    * Tension is a NEGATIVE PRESSURE: it essentially "pulls" water from areas of greater hydrostatic pressure (i.e., fluid-filled areas such as xylem vessels and interstitial areas filled with water) into the areas of lower hydrostatic pressure (stomates).

    This is the basis of "transpirational pull".

    Bubbles sometimes can form in the water column in the xylem (this is more common in vessel elements than in tracheids--why?).

    cavitation: rupture of the water column

    embolism: filling of a vessel or tracheid with air.

    Shoot tension is capable of lifting water up to 500 feet from the ground, which is more than enough to account for the tallest land trees' (redwoods) ability to lift water all the way to their tips. (350 feet).

    Of lesser importance is ROOT PRESSURE.

    *at night, stomates are usually closed and transpiration stops.

    * endodermis-ringed root stele cannot "leak" ions, so water potential decreases as water from the soil osmoses into the stele.

    * this effectively "pushes" water up the stem and into the leaves.

    Some herbaceous plants have special openings on the leaf margins called hydathodes. These allow root pressure water to escape, forming lovely little "beads" of "dew" overnight, and preventing cell rupture due to too much water pressure.

    (GUTTATION)

    Root pressure is effective only over very short distances; guttation can be observed only very early in the morning.