All chemicals on the body divide into organic and inorganic compounds. Both organic and
inorganic compounds are equally vital.
Organic compounds – contain carbon. Organic compounds are covalently bonded, large molecules.
Inorganic compounds – all other chemicals in the body.
Inorganic compound include water, salts, acids, and bases.
Vital properties of water:
1. High heat capacity (=thermal capacity).
Heat capacity is the amount of energy required to change the temperature of a substance by a given
amount.
Water will absorb or release a lot of heat before it temperature begins to change. This prevents sudden
changes in body temperature and helps to maintain temperature homeostasis.
2. High heat of vaporization – makes water an effective coolant for the body. (Heat of
vaporization is the amount of energy required to heat water to the temperature at which it changes its
state from liquid to gaseous. At this temperature most of hydrogen bonds between water molecules
break.)
3. Polar solvent properties.
Biological molecules chemically react when they are placed in a solution. Chemical reactions in the
body depend on water's solvent properties. Solvent properties of water are dictated by its molecule's
bent shape (polarity – positive on the hydrogen side, negative on the oxygen side). This property
explains why salts and acids and bases dissociate in water.
Water also forms hydration layers around larger charged molecules (proteins, for example). This
prevents them from settling out in a solution.
Due to its solvent properties, water also serves as a major transport medium. Metabolic wastes dissolve
in water and are excreted from the body in urine.
4. Reactivity.
Water serves as a reactant in many chemical interaction. Many molecules are broken by addition of
water (hydrolysis reaction) or constructed by removal of water components from reacting sites of
smaller molecules (dehydration reaction).
5. Cushioning.
Water forms a protective layer around body organs that cushions them against physical trauma.
Salts –
ionic compounds (dissolve in water to form anions and cations).
All ions are electrolytes.
Electrolytes –
substances that conduct electrical current in a solution.
Common salts in the body: NaCl (sodium chloride), KCl (potassium chloride), CaCO3 (calcium
carbonate). Na+ , K+ , and Ca2+ are important for conductivity of nerve impulse and muscle contraction.
Na+ and Cl- are also important in fluid balance.
Oxidation-reduction (redox) reactions –
exchange reactions in which electrons are exchanged between the reactants. Electron donor is oxidized,
electron acceptor is reduced.
Redox reactions are also decomposition reactions.
Ionic compounds are formed in redox reactions:
NaCl → Na+ + Cl-
(sodium is oxidized, chlorine is reduced)
Some oxidation reactions do not involve complete transfer of electron. A substance may be oxidized by
losing a hydrogen atom and by combining with oxygen.
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
glucose + oxygen → carbon dioxide + water + energy
(glucose is oxidized to carbon dioxide by losing hydrogen, oxygen is reduced to water by accepting
hydrogen)
Acids and bases are electrolytes: dissociate in water, separate into anions and cations, and conduct
electricity. (Also, recall physical properties of acids and bases.)
Acid –
substance that releases H+ in detectable amounts; also called proton donors.
Base –
take up H+ in detectable amounts; proton acceptors.
Proton concentration determines the acidity of a solution. Hydroxyl ions OH- concentration
determines how alkaline (=basic) the solution is.
Common inorganic bases are hydroxide: Mg(OH)2 and NaOH. Examples of bases in the body: HCO3(bicarbonate ion) and NH3 (ammonia).
HCL → H+ + Cldissociation of hydrochloric acid
+
NaOH → Na + OH
dissociation of sodium hydroxide
Acids destroy chemical properties of bases, and bases destroy chemical properties of acids in a
displacement reaction. Acids and bases react with each other to form water and salt. This is called
neutralization reaction.
HCl + NaOH → NaCl + H2O
hydrochloric acid + sodium hydroxide → sodium chloride + water
Acid reaction with carbon containing ions will produce salt, water, and carbon dioxide.
Example: vinegar and baking soda reaction:
CH3COOH + NaHCO2 → CH3COONa + H2CO3
acetic acid + sodium bicarbonate → sodium acetate + carbonic acid
Carbonic acid then quickly decomposes into water and carbon dioxide
H2CO3 → H2O + CO2
pH –
stands for 'potential of Hydrogen'. pH scale shows how acidic or basis a solution is.
Acidity is determined by H+ ions concentration: the more H+ a substance releases, the more acidic the
solution is. Alkalinity of a solution depends on how much (OH)- hydroxyl ions the solution contains.
On pH scale, the degree of acidity or alkalinity ranges from 0 (acid) to 14 (base).
At pH 7, the amount of H+ is equal to the amount of (OH)- which means that the solution is neutral:
neither acidic or alkaline. Pure water is neutral at 25ºC (77ºF), and its pH is 7. Human blood pH is 7.40
which is almost neutral (slightly on alkalinic side).
Body pH is regulated by kidney, lungs and buffers.
Buffers –
chemical systems (for example, proteins) that resist large swings in body pH by releasing H+ when pH
starts to rise, or binding to H+ when pH starts to drop.