The fundamental viewpoint of the next five chapters can be stated
simply: Life is a behavior pattern that chemical systems exhibit
when they reach a certain kind and level of complexity.
What is this behavior pattern, and what is its chemical basis?
This is a difficult question, and is less of an outline for five
chapters on biochemistry than a blueprint for the future of chemistry.
We cannot yet provide the answers, but we can outline the areas
in which these answers some day will be found. This viewpoint is
helpful in seeing what, in the broad field of life, is relevant
to the chemist and is susceptible to explanation by chemical methods.
Although we have little trouble distinguishing between the living
and the non-living, it is difficult to set down a hard and fast
list of criteria for life. Most living things move, react to stimuli,
breathe, eat and excrete, grow, propagate, and eventually die. Unfortunately,
we can find apparent exceptions to all of these criteria. Most plants
do not move, except between generations in the form of seed dispersal.
Some lower plants do not react overtly to common stimuli, although
most plants exhibit phototropism and geotropism-growth responses
to light and gravity. Gangrene bacteria and many other anaerobic
microorganisms not only do not breathe, they are killed by the mere
presence of oxygen.
Viruses neither breathe, nor eat, nor excrete, nor grow. They
do little else except blunder into host cells and induce them to
make more viruses. Amoebae and other budding or fissioning organisms
do not die of old age in the true sense of the word. However, one
feature is universal: All living systems propagate. To add to the
confusion, some of the properties on our list also are shared by
nonliving things. Sand dunes, supersaturated clay soils, and undermined
seashore palisades react to mechanical stimuli and move, often abruptly.
A crystal in solution grows by taking up molecules or ions from
its surroundings. If chipped at a corner, it will add more molecules
selectively to that corner and "heal" itself. Stars are
born out of matter from older stars; they grow and develop through
predictable stages, and finally die. In spite of these phenomena,
no one would claim that sand dunes, crystals, and stars are alive.
We must be more critical in our definition of life.
One tentative definition of life is the following: Living organisms
are complex, organized chemical systems that propagate, grow, metabolize,
use their environment and protect themselves from it, and evolve
and change in response to long-term changes in the environment.
Each of these properties is worth examination to see how our definition
of life stands up.