The chemistry of life
Chemistry is the scientific discipline that deals with the composition and
structure of substances and with the many reactions they undergo. A basic
knowledge of chemical principals is essential for understanding anatomy
and physiology.
Basic Chemistry
Matter is anything that occupies space. An element is matter composed of
atoms of only kind. Atoms are the smallest particles into which an
element can be divided using conventional chemical means. For example,
oxygen (02) is an element composed of only oxygen atoms.
The Structure of Atoms
The three major types of subatomic particles that make up a:onis are
neutrons, protons, and electrons. Neutrons have no electrical charge.
protons have a positive electrical charge, and electrons have a negative
electrical charge. The number of protons in an atom is ecual to the
number of electrons. Consequently atoms are electrically newt-a!.
Protons and neutrons are organized within atoms to form a central
nucleus, and the electrons move around the nucleus. The reon where each
electron is most likely to be found is that orbital Th rnzn&r of protons in
an atom is called its atomic number. Differer: &erts bye different
number of protons and therefore different atoaiic rEc-.
Chemical Bonds
Chemical Bonds are formed when the outermost electrons are
transferred or shared between atoms. The resulting combination of atoms
is called a molecule. If a molecule has two or more different kinds of
atoms, it may be referred to as a compound. A molecule can be
represented by a chemical formula, which consists of the symbols of the
atoms in the molecule plus the number of each type of atom. For
example, the chemical formula for glucose is C6H1206 Thus glucose has
6 carbon (C), 12 hydrogen (H), and 6 oxygen (0).
An ionic bond result s when one atom loses an electron and another
atom accept an electron. For example, sodium (Na) can lose an electron
that can be accepted by chlorine (Cl). The molecule that is formed is
sodium chloride (NaC1), or table salt.
A covalent bond results when two atoms share a pair of electrons. For
example, two hydrogen atoms can share their electrons to form a
hydrogen molecule (H2).
.
Chemical Reactions
A chemical reaction is the process by which atoms or molecules
interact to form or break chemical bonds. The atoms or molecules present
before the chemical reaction occurs are the reactants, and those
produced by the chemical reaction are the products. For example, the
reactants sodium and chlorine combine to form the product sodium
chloride.
Classification of Chemical Reactions
When two or more atoms, ions, or molecules combine to form a new and
larger molecule, the process is called a synthesis reaction. This can be
represented symbolically as follows:
A+B—*AB
An example of a synthesis reaction in the body is the formation of
adenosine triphosphate (AlP):
A-P-P + P A-P-P-P
(ADP) (Phosphate group) (ATP)
In a decomposition reaction, larger molecules are broken down into
smaller molecules, ions, or atoms. A decomposition reaction is the
reverse of a synthesis and can he represented in this way:
AB-A+B
The breakdown of ATP to ADP and a phosphate is an example.
A-P-P-P A-P-P + P
(ATP) (ADP) (Phosphate group)
An exchange reaction is a combination of decomposition and a synthesis
reaction. In decomposition, larger molecules are broken down. In
synthesis, the products of the decomposition reaction are formed into new
molecules. The symbolic representation of an exchange reaction is:
AB+CD—*AC+BD
The reaction of hydrochloric acid (HC 1) with sodium hydroxide (NaOH)
to form table salt (NaC1) and water (H20) is an exchange reaction.
HCI + NaOH NaC1 + H20
—f
—*
—
Acids and Bases
An acid is a proton donor. Because a hydrogen atom its electron is a
proton, any substance that releases hydrogen ions in water is an acid. For
example, hydrochloric acid (HC 1) in the stomach forms hydrogen (H+)
ions and chloride (Cl-) ions.
HC1 —>H+ClA
base is a proton acceptor. For example, sodium hydroxide (NaOH)
forms sodium (Naj ions and hydroxide (OH-) ions. It is a base because
the hydroxide ion is a proton acceptor that binds with a hydrogen ion to
form water.
NaOH Na + OH
2O
-
The pH Scale
The symbol pH stands for the power (p) of hydrogen ion (H)
concentration. The pH scale, which ranges from 0 to 14, indicates the
hydrogen ion concentration of a solution. Pure water is defined as a
neutral solution. A neutral solution has an equal number of hydrogen
ions and hydroxide ions and has a pH of 7.0. Solutions with a pH less
than 7.0 are acidic, and they have a greater concentration of hydrogen
ions than hydroxide ions. Alkaline or basic solutions have a pH greater
than 7.0, and they have fewer hydrogen ions than hydroxide ions.
The survival of an organism depends on its ability to regulate body fluid
pH within a narrow range. One way normal body fluid pH is maintained
is through the use of buffers. A buffer is a chemical that resists changes
in pH when either an acid or a base.
Salts
A salt is a molecule consisting of a positive ion other than hydrogen and
a negative ion other than hydroxide. Salts are formed by the reaction of
an acid and a base. For example, hydrochloric acid combines with sodium
hydroxide to form the salt sodium chloride.
HC1 +NaOH—>NaC1 +H2O
(Acid) (Base) (Salt) (Water)