A high percentage of living things, both plant and animal, is found in water. All life on the Earth is thought to have arisen from water. The bodies of all living organisms are composed largely of water. About seventy to ninety percent of all organic matter is composed of water.
The chemical reactions in all plants and animals that support life take place in a water medium. Water not only provides the medium to make this life-sustaining reaction possible, but water itself is often an important reactant or product of these reactions. In short, the alchemy of life is found in the chemistry of water.
On land the greatest threat to life is desiccation. Water is lost in several different ways - by evaporation from respiratory surfaces, by evaporation from the skin, through elimination of feces and excretion of urine. Lost water must be restored if life is to continue. It is replaced by drinking it, by eating foods containing water, and by oxidation of nutrients (water is one of the products of cell metabolism).
The Universal Solvent
Due to the marked polarity of the water molecule and its tendency to form hydrogen bonds with other molecules, water is called the universal solvent. A water molecule, expressed in the chemical symbol H20, consists of two hydrogen atoms and one oxygen atom.
Standing alone, the hydrogen atom contains one positive proton at its core, with one negative electron revolving around it in a three dimensional shell. Oxygen, on the other hand, contains eight protons in its nucleus with eight electrons revolving around it. This is often shown in chemical notation as the letter O surrounded by eight dots representing four sets of paired electrons.
The single hydrogen electron and the eight electrons of oxygen are the key to the chemistry of life, because this is where hydrogen and oxygen atoms combine to form a water molecule, or split to form ions.
Hydrogen tends to ionize by losing its single electron and form single H+ ions, which are simply isolated protons since the hydrogen atom contains no neutrons. A Hydrogen bond occurs when the electron of a single hydrogen atom is shared with another electronegative atom such as oxygen that lacks an electron.The Polarity of Water Molecules
Hydrogen atoms, because their electrons are closer to the oxygen atom, take on a small electropositive charge. This means water molecules have a tendency to form weak bonds with other water molecules because the oxygen end of the molecule is negative and the hydrogen ends are positive.
A hydrogen atom, while remaining covalently bonded to the oxygen of its own molecule, can form a weak bond with the oxygen of another molecule. Similarly, the oxygen end of a molecule can form a weak attachment with the hydrogen ends of other molecules. Because water molecules have this polarity, water is a continuous chemical entity.
These weak bonds play a crucial role in stabilizing the shape of many of the large molecules found in living matter. Because these bonds are weak, they are readily broken and reformed during normal physiological reactions. The disassembly and rearrangement of such weak bonds is in essence the chemistry of life.
Basically, reduction means the addition of electron (e-) and its converse, oxidation, describes the removal of an electron. The addition of an electron (reduction) stores energy in the reduced compound. The removal of an electron (oxidation) liberates energy from the oxidized compound. Whenever one substance is reduced, another is oxidized.
To clarify these terms, consider any two molecules, A and B, for example:
Ae- + B >>> A + Be-
When molecules A & B come into contact, here is what happens:
B grabs an electron from molecule A.
Molecule A has been oxidized because it has lost an electron.
The net charge of B has been reduced because it has gained a negative electron (e).
In biological systems, removal or addition of an electron constitutes the most frequent mechanism of oxidation-reduction reactions. These oxidation-reduction reactions are frequently called redox reactions.
Acids and Bases
An acid is a substance that increases the concentration of hydrogen ions (H+) in water. A base is a substance that decreases the concentration of hydrogen ions, in other words, increasing the concentration of hydroxide ions (OH-).
The degree of acidity or alkalinity of a solution is measured in terms of a value known as pH, which is the negative logarithm of the concentration of hydrogen ions:
pH=1/log [H+] = -log [H+]
What is pH?
The pH scale ranges from 0 on the acidic side to 14 on the alkaline and a solution is neutral if its pH is 7. At pH 7, water contains equal concentrations of H+ and OH- ions. Substances with a pH less than 7 are acidic because they contain a higher concentration of H+ ions. higher concentration of OH- than H+. The pH scale is a log scale, so a change of one pH unit implies a tenfold shift in the concentration of hydrogen ions.
The Importance of Balancing pH
Living things are extremely sensitive to pH and function best (with certain exceptions, such as different portions of the digestive tract) when solutions are nearly neutral. Most interior living matter (excluding the cell nucleus) has a pH of about 6.8.
Blood plasma and other fluids that surround the cells in the body have a pH of 7.2 to 7.45. A blood pH of 6.9 could induce coma and possibly death to the body. So the body possesses numerous special mechanisms to aid in stabilizing these fluids so that cells will not be subject to appreciable fluctuations in pH. Substances which serve as mechanisms to stabilize pH are called buffers. Buffers have the capacity to bond ions and remove them from solution whenever their concentration begins to rise. Conversely, buffers can release ions whenever their concentration begins to fall, thus helping to minimize the fluctuations in pH. This is an important function because many biochemical reactions normally occurring in living organisms either release or use up ions.