a. carrot root is about 85% water

b. young lettuce leaves can contain 95% water

c. dry seed may be only 5% water (usually around 10 to 15%)

The chemical properties of water that make it great stuff derive from it's dipolar nature

Unequal electron sharing and polarity: When atoms on the left side of the periodic table share electrons with atoms on the right of the table, they do not share them equally. The tendency of the elements from the left to give up electrons persists and such elements are considered "electropositive". The tendency of elements on the right to take up electrons also persists and such elements are considered "electronegative". As a result, the electron being shared spends more time around the electronegative element than around the electropositive element and the covalent bond takes on some ionic character. Such bonds are somewhat intermediate between the electron sharing of the covalent bond and the electron exchange character of the ionic bonds and are called polar covalent bonds.

ionic bond = Na⁺Cl^-

covalent bond = H : H (dots represent shared electrons)

polar covalent bond = -O-H

Polar covalent bonds play extremely important roles in determining the physical properties of substances. For example, the presence of hydroxyl groups (OH) determins the properties of a large variety of molecules. The molecule with the highest proportion of OH groups is, of course, water and water is one of the most polar substances known and it is the most important molecule in biology.

Hydrogen bonds: The oxygen atom of water is strongly electronegative and tends to draw electrons away from the hydrogen atoms. The oxygen thus has a partial negative charge and the two hydrogen atoms each have a partial positive charge. The positive charges of the hydrogens are electrostatically attracted to the negatively charged oxygens of two neighboring water molecules leading to hydrogen bonding between water molecules. Hydrogen bonds are rather weak (20 kJ/mol) but significant and in large numbers, provide strength.

• At the melting point of water (0°C) only about 15% of the H-bonds are broken
  
• Random thermal motion results in constant breaking and forming of H-bonds - at 25°C more than 80% of the H-bonds are present at a given instant (probability)

Some features of water that are the result of its dipolar nature and hydrogen bonding are:

Cohesion - intermolecular attraction between like molecules in the liquid state.

Adhesion - attraction between water and other molecules (e.g., cell walls)

Heat of fusion - when ice melts at 0°C, some of the H-bonds rupture

- only about 15% of the H-bonds are broken upon melting using 6.03 kJ/mol

- when water freezes, the energy is liberated as H-bonds form.

Thermal buffering - Heat of vaporization - rupturing all of the H-bonds to form water vapor requires ~40kJ/mol.- this is great for evaporative cooling. High thermal conductivity, high heat capacity and high melting point also contribute to the thermal buffering capacity of water.

Universal solvent - Its polar character causes water to surround charged ions and molecules. In addition to being the solvent for most biochemical reactions, water is often a direct participant (e.g., H and e^- donor in many reaction, source of oxygen for photosynthesis and source of hydrogen for CO₂ fixation).

Compressibility is slight

Density - highest at 4°C

Transparent to visible irradiation (light)

Water movement to a seed or root and through a plant can proceed by diffusion or bulk flow.

Diffusion: net movement of molecules driven by random thermal motion (kinetic energy) from a region of high concentration, or high chemical potential, to a region of low concentration, or low chemical potential.
- Diffusion is very important at the microscopic level

Diffusion is described by the following equation:

where:    V = average velocity (cm/s)    R = molar gas constant    T = absolute temp (°K)    MW = g/mole    p = 3.1416....

According to this equation:

- it would take about 1 sec for 50% of the molecules to diffuse 50 µm (distance across a leaf cell).

- it would take 3 * 10⁶ sec (about 2 months) for 50% of the molecules to diffuse 1 meter.

Bulk flow: flow of a substance driven by differences in pressure.
- bulk flow is important at the macroscopic level

Surface tension: Water has extremely high surface tension. Surface tension can be defined as the force per unit length pulling perpendicularly to a line in the plane of the surface. Because of its high surface tension, water can support fairly large objects placed carefully on its surface. Surface tension is due to the water-water attraction (cohesion) of its molecules relative to the water-air molecule interactions (adhesion). Surface tension can be greatly affected by certain solutes such as fatty acids and lipids because they may become concentrated at interfaces. These "surfactant" molecules usually have both polar and nonpolar regions.

Capillary rise: When the adhesive attraction between the liquid and the solid phase is appreciable, the solid is said to be wettable. In a capillary made of a wettable material, water will rise in the capillary until the force of gravity counters the movement. When the adhesion is weaker than the water-water interaction then the upper level of the liquid in the capillary is lower than the surface of the free solution. In xylem vessels adhesion is very strong and capillary action can contribute to the upper movement of water in plants. Theoretical considerations indicate that water could rise about 0.75 meters in a xylem vessel having a lumen radius of 20 um. This would be sufficient for the extent of upward movement in small plants but certainly not for a 30 m tree.
In a xylem vessel containing water the appreciable water-wall interaction that develops at the top of the vessel and in the numerous interstices of its cell wall are the key factors supporting water at great heights. The upward force, transmitted to the rest of the solution in the xylem vessel by water-water cohesion, overcomes the gravitational pull downward. If air gets into the lumen of the vessel capillary action will not be sufficient to refill most air filled xylem vessels greater than about 1 m long.

Tensile Strength: The pulling on water columns that occurs in capillaries and in sustaining water in xylem requires that water be put under tension, or negative pressure. The tensile strength of water is the maximum tension (force per unit area) that it can withstand before breaking. The intramolecular hydrogen bonds lead to this tensile strength by resisting the pulling apart of water in a column. The experimentally determined tensile strength of water is nearly 10% of that for copper or aluminum. The great cohesive forces between water molecules thus allow an appreciable tension to exist in an uninterrupted water column in a wettable capillary or tube such as a xylem vessel.

Viscosity: For water to flow, hydrogen bonds must be broken. Fortunately on the average they are relatively easy to break because in liquid water each hydrogen bond is shared by two other molecules causing each bond to be somewhat weakened. Thus water can flow readily through plants. Viscosity is markedly altered by temperature.

Electrical Properties and Solubilities: The partial negative and partial positive charges of water molecules form electrostatic bonds with the charged ions of salts so that the salts become "hydrated". This hydration is often extensive enough that the oppositely charged ions of the salt dissociate from each other and disperse throughout the water phase. Furthermore, the dissociated and dispersed ions of salts tend to be surrounded by "hydration shells" and are therefore functionally much larger in solution than in the solid form. The electrostatic interaction between ions and water partially cancels or screens out the local electrical fields of the ions. The resultant screening diminishes the electrical interaction between the ions and allows more of them to remain in solution, in part, by preventing the ions from interacting with each other and precipitating or crystallizing.

In a similar manner, water tends to associate with other polar substances, such as other substances also carrying -OH groups, and therefore polar substance tend to be soluble in, or miscible with, water (e.g., alcohols and sugars).

It must be kept in mind that hydroxyl groups are not the only polarity-inducing groups and that hydrogen bonding is not the only instance of bonding by partial charges. Amino groups (-NH2) can also form hydrogen bonds, especially with carbonyl groups, -C=O. This point will is extremely important when considering the structure of proteins since the polypeptide backbone of proteins is composed of a series of -C-NH- links.

Hydroxyl groups and amino groups also have a net electronegative effect that tends to convert entire molecules to which they are attached into larger dipoles. This produces a number of biologically important physical effects on substances rich in -OH or -NH₂ groups. For instance the surfaces of insoluble polyhydroxy compounds (e.g., cell wall materials) are usually negative when they are immersed in aqueous media and their surfaces bind a layer of water.

The existence of polar and non-polar substances divides them into solubility classes. Of course the polarity of substances varies widely and the solubility classes (polar and non-polar) are matters of degree with all kinds of intermediates.

Surfactants and membrane formation: It is the distinction between polar and non-polar substances which makes possible much of the structures of living things. To have structure, it is obviously necessary that the parts do not mix and therefore some mutual insolubilities are needed. To have complex structure, a great deal of compartmentalization is also necessary. In living systems, compartmentalization is a multiplicity of phases separated by fairly permanent barriers called membranes.

The structure of membranes is largely based on the polar nature of certain large molecules, molecules that are very polar at one end and very non-polar at the other end. The properties of such molecules are quite different from the properties of smaller molecules of intermediate polarity. These large molecules do not dissolve very well in either polar or non-polar solvents. Instead the polar end dissolves in polar solvents and the non-polar end dissolves in non-polar solvents. As a result such hybrid molecules tend to accumulate at interfaces between other polar and non-polar solvents. Since they accumulate at surfaces they are called surfactants.

This tendency of hybrid polar-non-polar substances to accumulate between oils and water has far-reaching consequences, for it is this property of the phospholipids and related compounds that results in membrane formation. These are made up of a glycerol molecule with two of its three hydroxyl groups esterified to fatty acids but with the third hydroxyl esterified to some exceedingly polar group (for example phosphoryl-choline, which at physiological pH's is both positively and negatively charged.

Ionization of Water and pH: Some of the water molecules separate into [H⁺] and [OH^-] ions. The tendency of these ions to recombine is a function of the chances for collisions between them, which depends on the number of ions in solution. This is the mass law relationship and may be expressed mathematically by saying that the product of the molar concentrations equals a constant: [H⁺]*[OH^-] = K. This constant, K, is equal to 10^-14 with the reaction, H⁺ + OH^- --> H₂O, proceeding in such a way as to assure this constancy. So in pure water both [H⁺] and [OH^-] = 10^-7 molar (M). However, water is seldom pure enough to contain equal numbers of hydrogen and hydroxyl ions. There are almost always some ions dissolved in water and the amount of cations and anions in solution determine the ratio of H⁺ and OH^- and, thus, the pH of the solution.