The unity of life
At the cellular level of organization, the main
chemical
processes of all living matter are similar, if not identical. This is true for
animals
,
plants
,
fungi
, or
bacteria
; where variations occur (such as, for example, in the secretion of
antibodies
by some
molds
), the
variant
processes are but variations on common themes. Thus, all living matter is made up of large molecules called
proteins
, which provide support and coordinated movement, as well as storage and transport of small molecules, and, as
catalysts
, enable chemical reactions to take place rapidly and specifically under mild temperature, relatively low concentration, and neutral conditions (i.e., neither acidic nor basic). Proteins are assembled from some 20
amino acids
, and, just as the 26 letters of the alphabet can be assembled in specific ways to form words of various lengths and meanings, so may tens or even hundreds of the 20 amino-acid “letters” be joined to form specific proteins. Moreover, those portions of protein molecules involved in performing similar functions in different organisms often
comprise
the same sequences of amino acids.
There is the same unity among cells of all types in the manner in which living organisms preserve their individuality and transmit it to their offspring. For example, hereditary information is encoded in a specific sequence of bases that make up the
DNA
(deoxyribonucleic acid)
molecule
in the
nucleus
of each cell. Only four bases are used in synthesizing DNA:
adenine
,
guanine
,
cytosine
, and
thymine
. Just as the
Morse Code
consists of three simple signals?a dash, a dot, and a space?the precise arrangement of which
suffices
to convey coded messages, so the precise arrangement of the bases in DNA contains and conveys the information for the synthesis and assembly of cell components. Some primitive life-forms, however, use
RNA
(ribonucleic acid; a
nucleic acid
differing from DNA in containing the
sugar
ribose
instead of the sugar
deoxyribose
and the base
uracil
instead of the base thymine) in place of DNA as a primary carrier of genetic information. The replication of the genetic material in these organisms must, however, pass through a DNA phase. With minor exceptions, the
genetic code
used by all living organisms is the same.
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The chemical reactions that take place in living cells are similar as well. Green plants use the energy of
sunlight
to convert
water
(H
2
O) and
carbon dioxide
(CO
2
) to
carbohydrates
(sugars and starches), other organic (
carbon
-containing)
compounds
, and molecular
oxygen
(O
2
). The process of
photosynthesis
requires energy, in the form of sunlight, to split one water molecule into one-half of an oxygen molecule (O
2
; the oxidizing agent) and two
hydrogen
atoms (H; the reducing agent), each of which dissociates to one
hydrogen ion
(H
+
) and one
electron
. Through a series of
oxidation-reduction reactions
, electrons (denoted
e
?
) are transferred from a donating molecule (oxidation), in this case water, to an accepting molecule (reduction) by a series of chemical reactions; this “reducing power” may be coupled ultimately to the reduction of carbon dioxide to the level of carbohydrate. In effect, carbon dioxide accepts and bonds with hydrogen, forming carbohydrates (C
n
[H
2
O]
n
).
Living organisms that require oxygen reverse this process: they consume carbohydrates and other organic materials, using oxygen synthesized by plants to form water, carbon dioxide, and energy. The process that removes hydrogen atoms (containing electrons) from the carbohydrates and passes them to the oxygen is an energy-yielding series of reactions.
In plants, all but two of the steps in the process that converts carbon dioxide to carbohydrates are the same as those steps that synthesize sugars from simpler starting materials in animals, fungi, and bacteria. Similarly, the series of reactions that take a given starting material and synthesize certain molecules that will be used in other
synthetic
pathways are similar, or identical, among all cell types. From a metabolic point of view, the cellular processes that take place in a
lion
are only marginally different from those that take place in a
dandelion
.
Biological
energy
exchanges
The energy changes associated with physicochemical processes are the province of
thermodynamics
, a subdiscipline of
physics
. The first two
laws of thermodynamics
state, in essence, that energy can be neither created nor destroyed and that the effect of physical and chemical changes is to increase the disorder, or randomness (i.e.,
entropy
), of the universe. Although it might be supposed that biological processes?through which organisms grow in a highly ordered and complex manner, maintain order and complexity throughout their life, and pass on the instructions for order to succeeding generations?are in contravention of these laws, this is not so. Living organisms neither consume nor create energy: they can only transform it from one form to another. From the
environment
they absorb energy in a form useful to them; to the
environment
they return an equivalent amount of energy in a biologically less useful form. The useful energy, or
free energy
, may be defined as energy capable of doing work under isothermal conditions (conditions in which no temperature differential exists); free energy is associated with any chemical change. Energy less useful than free energy is returned to the environment, usually as
heat
. Heat cannot perform work in biological systems because all parts of cells have essentially the same temperature and
pressure
.