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Water



Life began in water and evolved there for 3 billion years before spreading onto land.  Life remains tied to water.  Most cells are surrounded by water, and cells contain from about 70-90% water. 

Growth peaks when soil moisture is at 80% of soil field capacity.  Note that photosynthesis stops before wilt symptoms are seen.  Outside, an average of 535mm rain and irrigation during the growing season gives maximum growth rates.  Besides soil water and plant hydration, careful cultivators must account for atmospheric water.  Grows best at a relative humidity (RH) between 40-80%, but RH over 60% promotes grey mould/ Botrytis, so a RH between 40-60% is optimal during flowering.

Cohesion
The water molecule, shaped like a right angle, is a polar molecule, that is it has opposite electrical charges on opposite ends.  Each water molecule can form Hydrogen bonds to a maximum of 4 neighbours.  Organisms depend on the cohesive nature of water molecules which stick together due to the hydrogen bonding.  These bonds are very fragile/weak, a twentieth as strong as covalent bonds, lasting a trillionth of a second constantly reforming with neighbour molecules.  Collectively, the hydrogen bonds hold the substance together.  In plant xylem, water reaches the leaves through the microscopic vessels that extend contiguous upward from the roots.  Water that evaporates from a leaf through the stomata is replaced by water from the vessels in the vein.  Hydrogen bonds cause the molecules leaving to pull on molecules further down in the vessel.  This upward pull is transmitted all the way down to the root.  Adhesion of the water to the walls helps counteract the downward pull of gravity.  The H-bonding is also what creates the surface tension of water.

Temperature
Water stabilises air temperatures by buffering temperature changes through its unusually high specific heat.  This allows it to absorb relatively large amounts of energy for a small increase in temperature. Evaporative cooling.............Ice

Solvent of Life

Water is unusually versatile solvent because its polarity (H+ region being slightly positive, and O2- slightly negative) attracts it to charged and polar substances.  When ions or polar substances are surrounded by water molecules, they dissolve - ionic bonds are broken and the ions or polar substances are pulled into solution.  Even large molecules as certain proteins dissolve in water because the ionic and polar regions on the surface of the large molecule.

Hydrophilic(water-loving) substances have an affinity for water, for example - cellulose has numerous regions of partial positive and negative charges associated with numerous polar bonds, the cellulose fibres adhere to water but do not dissolve as they are too large and the internal bonds are too strong.  Cellulose is present in the water conduit vessels where the adhesion of water to the walls functions in transport.

Hydrophobic (water-fearing) substances neither dissolve nor have affinity for water, they are non-ionic, non-polar substances such as oils. These hydrophobic oils/lipids are important constituents of cell membranes.

pH

Occasionally, a hydrogen atom shared between two water molecules in a hydrogen bond shifts from one molecule to another, leaving behind an electron.  What is transferred is a hydrogen ion (H+), a single proton with a charge of +1.  The water molecule that loss the proton is now a hydroxide ion (OH-1), with a charge of -1.  The proton binds to the other water molecule, making a hydronium/oxonium ion (H3O+).  It is more useful to think of the dissociation of water into a H+ and OH-:

H2O <----------> H+  +  OH-

This is a reversible reaction, with water dissociating/ionising at the same rate that it is being reformed from H and OH ions. When in equilibrium the concentration of water molecules is very large compared to that of the ions.  In one litre of water there are only 10-7 moles of each ion, whereas there are about 50 moles of water molecules.  H and OH ions are very reactive with even slight changes in their concentrations affecting cell proteins and complex molecules.

Since water is the common solvent in life, we use it as the reference when describing solutions as being neutral.  A neutral solution is one in which the concentration of H+ is equal to that of OH-.  If H+ is in excess the solution is described as being acidic, and if OH- is in excess the solution is described as being alkaline.  Because the H and OH ion concentrations of solution can vary by a factor of 100 trillion or more, we express this by use of the pH scale, which ranges from 0-14.  The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration:

pH  =  -log[H+]

For neutral solutions, [H+] is 10-7M, giving:

-log 10-7  =  -(-7)  =  7

It is important to remember that each pH unit represents a tenfold difference between H and OH ion concentration.

Biological fluids resist changes in their own pH when acid or bases are introduced because of buffers.  A buffer, usually a weak acid and base pair, works by accepting H ions from the solution when they are in excess and donating H ions when they have been depleted.

pH5.5-6.0 is optimal for nutrient absorption in most mediums.

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