Water, H2O, is a molecule with polarity. The central oxygen atom is more electronegative than the two hydrogen atoms, and so the electrons shared in the two bonds spend more time around the oxygen atom than around the hydrogen atoms. This structure is shown below:
This electron distribution gives the central part of the water molecule a partial negative charge. The terminal hydrogen atoms thus have a partial positive charge. These partial charges of course mean that the hydrogen ends of one water molecule are attracted to the central oxygen portions of a neighboring water molecule. This kind of attraction is called hydrogen bonding:
The polarity of the water molecule gives it the ability to dissolve polar molecules as well as less-polar molecules. In fact, among known liquids, water dissolves the widest range of chemical solutes. This makes water a medium for chemical transport and exchange. For plants, water dissolves soil minerals and carries them up the plant in the transpiration stream in the xylem; photosynthesis produces carbohydrates which are dissolved in water and carried from the leaf to the rest of the plant in the translocation stream in the phloem. Solubility in water causes phospholipids to orient themselves into membrane bilayers, and causes amino acid R-groups to twist in space to bring about protein conformation.
Not only does water dissolve solutes, it serves as a medium for chemical reactions, and can react chemically with solutes. Groups of reactions in plant cells are named "hydrolysis" or "dehydration" reactions because of this chemical participation by water. Water is a reactant in photosynthesis (CO2 + H2O + light → O2 + CH2O) and a product of respiration (CH2O + O2 → CO2 + H2O + energy).
The polarity of water and the resulting hydrogen bonding among water molecules means that it takes much heat (one calorie) to raise the temperature of 1 mL of water just 1° C. The hydrogen bonding has to be given a lot of energy to get them to vibrate and generate the temperature change. This property of water is called specific heat. It means that this liquid can absorb much heat from the various chemical reactions occuring in cells without temperature change; it is a heat buffer. It helps maintain an even plant body temperature.
Among liquids, water has the highest latent heat of vaporization (44 kJ • mol-1) which is also known as heat of fusion. This means that when water goes from liquid to gas it takes a lot of energy. This property can obviously be traced directly to hydrogen bonding again. As the highest energy molecules in the liquid achieve what it takes to move away as a gas, their energy is removed from the liquid and it gets cooler. We sometimes refer to this as evaporative cooling. This is a critical property in maintaining the temperature of dark green leaves essentially "parked" in sunshine. A green car parked in the sunshine demonstrates the greenhouse effect; the leaf would do the same except that the water evaporating out of the stomata in the epidermis carries away the excess heat. The water is replaced by the transpiration stream in the xylem.
The partial polarity of the water molecule makes it attractive to polar and less-polar surfaces. Water adheres to and climbs up materials like glass (forming a meniscus). The fact that water molecules attract each other makes them cohesive. These two properties allow water to climb up small-diameter tubes and remain in an unbroken fluid column; this is called capillarity. The column of water will climb inside the small tube to a height determined by:
rise in m = 14.9 • 10-6 m2 (radius in m)-1
Obviously water will climb higher in tubes of smaller radius that those of larger radius. In fact given a tracheid with a radius of 14.9 µm, the rise of water in that tracheid would be: 14.9 • 10-6 m2 (14.9 • 10-6 m)-1 = 1 meter! That of course is enough height to explain how water might get to the top of an herb in the xylem. It would fail to explain completely how water gets to the top of a 70 meter tall pine tree. We shall see later that in addition to capillary climb, the water is pulled through the xylem by evaporation. This evaporative pull is the major force in movement to the top of tall trees. This is accomplished by capillary movement of water away from the xylem along tiny intercellular spaces. The water bathing the mesophyll cells occupies even smaller spaces among the cellulose and hemicellulose polymers of the cell wall. These are in the range of 10-8m in radius. The evaporative pull is achieved in large part by capillarity of the very tiny cell-wall spaces.
The cohesion of water molecules to each other relative to the much less polar N2 and O2 of air or other surfaces leads to the property called surface tension. This property is responsible for the beading-up of water on leaf epidermal waxes. The drop rounds up because of its cohesiveness and the lack of adhesion to the non-polar wax allows the drop to roll off of the leaf and to drip off, onto the soil below the leaf. This of course provides the water for the root to take up nutrients, and to cool that leaf ultimately by evaporative cooling.
The cohesive property of water keeps the column of water in the xylem unbroken all the way up to the top of a tree. A failure to do this would produce cavitation in the xylem and this would stop all flow of water up the tree in that column of xylem elements. In very small capillary tubes, the backwards-pull (tension) of the weight of the column of water below a given point may reach -30 MPa (megapascals) without breaking the column. This means that such a narrow column of water is about 1/10 as strong as copper or aluminum wire of similar diameter!
While gases can be compressed into smaller and smaller spaces, liquid water is not so compressible. Thus compression of water into a space surrounded by a cell wall produces turgor pressure. This form of hydraulic pressure is critical for cell growth, for the opening and closing of stomata, flow processes in translocation in the phloem, exchange of materials within and between cell compartments, and for the rigidity and support for herbaceous (not supported by lignin in wood) plants. Turgor keeps petals and leaves extended into the air and prevents wilting. Units of pressure are:
1 atmosphere = 14.7 lbs in-2 = 760 mm Hg = 1.013 bar = 0.1013 M Pa
typical tire pressure is 0.25 MPa.
Because of the water cycle, rainfall and ground water are linked through the transpiration and evaporation from plants. Water is a free medium for cell and body enlargement. Vacuole water provides up to 90% of the volume of a cell. Perhaps 5% more is found in the water of the cytosol. This very high water content of plant cells makes plants excellent "diet food." Of course foolish humans douse this effect by piling ladles full of salad dressing on their vegetables.
While water is often commonly available, water can be insufficient during important times in the life of a plant. A drought is common in late summer and can reduce the yield of crop plants quite significantly. Because plants need water for photosynthesis, for evaporative cooling, and a host of other uses, they can run through a staggering amount of water. On a warm, dry, sunny day a plant can lose 100% of its water content in one hour! This all has to be replaced from the soil at these rates to keep up with the water demand. The figures below show how water is critical for both crop yield and ecosystem productivity.
From a strictly human-perspective, then, water must be kept at reasonable levels to support high yields of essential food crops.
There are two fundamental methods by which water can move from one place to another: Diffusion and Bulk Flow. These are often confused, particularly in introductory biology books.
The passive movement of any material from an area of higher concentration to an area of lower concentration is often called diffusion. The basis for this movement is the kinetic energy of individual molecules. As these molecules collide with each other they will disperse into space, perhaps among molecules of another type. For a cell:
The rate of movement in diffusion is shown by Fick's Law:
Js = -Ds • ΔCs • Δx-1
Js is the rate of movement or flux density usually measured as the moles of substance s crossing a square meter of area per second. Ds is the diffusion coefficient indicating how easily substance s moves through the medium. If the medium is air, then the coefficient is high and movement is rapid. In liquid, the coefficient is low and movement is slow by comparison. ΔCs is the concentration difference between the area of high and area of low concentration. The negative sign indiates that the movement is from the area of high to the area of low concentration. Δx is the distance between the areas of high and low concentration. Another way to say all this is that material moves faster when the substance shows good fluidity, the concentration gradient is steeper, and the needed travel is short.
How short is short for travel? If we think about sucrose, the transport form of photosynthate in plants, moving in a plant by diffusion, the distance must be very short indeed. For sucrose the Ds is 0.5 • 10-9m2s-1, the diameter of a cell is 50 µm (= 50 • 10-6m)...
Some algebra applied to Fick's law gives us:
t = x2 • Ds-1
Now we plug in the values above:
t = (50 • 10-6m)2 • (0.5 • 10-9m2s-1)-1 = 5 seconds
So diffusion can explain a reasonably sensible rate of movement for a sucrose molecule across a cell. But diffusion will fail to explain movement when the distance gets larger. To test that out, imagine sucrose made in the tip of a sugar cane leaf diffusing to the base of that leaf. That distance is about 1 meter. When you plug 1 meter in for the distance in the formula above, the time calculates out to 63.42 years! Now everyone knows that a sugar cane leaf never lasts 63 years...more likely less than one year. So diffusion is too slow to explain how sucrose gets out of a sugar cane leaf.
Many books in describing diffusion use an analogy of dropping some dye in the corner of a swimming pool and that with time this dye would become evenly distributed in the pool. Now you realize that this will take more than a lifetime if the movement of the dye were due exclusively to diffusion. Obviously these books are wrong about what moves the dye around in the pool to see it happen in your lifetime.
Now to get really ridiculous, we can calculate the time for sugar to diffuse from the bottom of a huge tree to its apical bud (say 70 meters). That distance calculates out to 310,755.96 years! Even the oldest bristlecone pine tree is "only" 5000 years, and the tallest coastal redwood must transport sugar over even greater distances than 70 meters to get sucrose to the roots. So diffusion will not explain sugar movement over tissue and organ distances.
Does all of this diffusion concept apply to water movement? That is not quite so easy to answer. First, does water move from an area of greater water concentration to an area of lower water concentration? Well what IS the concentration (molarity) of pure water?
density = 1 gram mL-1
molecular weight = 18 gram mol-1
1 gram mL-1 • 1000 mL L-1 • 1 mol (18 gram)-1 = 55.6 mol L-1
Now calculate the molarity of sea water given that:
density = 1.025 gram mL-1 = 1025 gram L-1
concentration NaCl = 29.5 gram L-1
weight of water = 1025 - 29.5 = 995.5 gram L-1
999.5 gram L-1 • 1 mol (18 gram)-1 = 55.3 mol L-1
Thus the difference between the concentration of distilled water and sea water is not impressive at all (55.6 vs 55.3 M). Certainly it is insufficient to explain the diffusion of water we observe in osmosis for example. Yet many books erroneously describe it precisely this way.
Movement of water must be more than diffusion based on concentration of water! We shall learn about the diffusion of water as we talk in more detail about osmosis. For now we will say that the explanation is based upon the potential energy of water and let us turn to the other process by which water can be moved.
Bulk flow is the movement of a substance under influence of pressure from an area of greater pressure to an area of lesser pressure. Rather than individual molecules moving on the basis of their own kinetic energy, large volumes of molecules move together in bulk. Typically we describe this movement as through a pipe (plumbing), a tube (phloem), or a channel (river), but it applies equally well to convection currents. Aha! this is how the dye moves in the pool and how sugar gets from the end of a leaf to its base or from leaves to roots.
The rate of bulk flow is shown in the Poiseuille Equation:
Flow = π r4 (8η)-1 • ΔΨp Δx-1
The rate of flow is proportional to the fourth power of the radius of the pipe (channel, convection current, etc.). In other words, increasing the pipe radius by a factor of two will increase the flow rate by a factor of 16. The rate of flow is inversely proportional to 8 times the viscosity of the fluid. A more viscous fluid (maple syrup) will move more slowly than maple sap in the pipe. The rate of flow is directly related to the pressure difference between the ends of the pipe. Increasing the pressure at one end of the pipe increases the rate of flow.
You should notice that the solute concentration has no effect on bulk flow.
Bulk flow can obviously work in phloem and xylem as these are basically pipes. Moreover as short plants evolved into tall trees, the xylem had to increase the number of pipes feeding the canopy. This explains the evolution of secondary growth. It is also not surprising that as dicots evolved into trees with massive canopies, tracheids became eclipsed by vessels with much greater radius. These changes help supply the water needs of a massive canopy such as those in tropical trees.