Biology · Ch 1 — Living World
Basic Principles of Life
Basic Principles of Life
What Makes Something "Living"?
Planet Earth is made up of two kinds of components: abiotic (non-living, e.g. rocks, water, air) and biotic (living organisms). At first glance it seems obvious which is which — yet the boundary is not always so simple. A crystal can "grow," a sand dune can pile up into a mound, and a machine can respond to a signal. So what single property, or set of properties, truly marks something as alive?
The chapter opens by posing exactly this puzzle: can a metabolic reaction carried out in a test tube ("in vitro") be called living? And is a "brain-dead" patient on life support, who shows no self-consciousness, living or non-living? There is no single easy answer — which is why biologists look at a combination of properties rather than any one property alone.
The Basic Principles of Life
A. Metabolism. Once an organism is born, it must survive, and survival needs a continuous supply of energy and chemical building blocks. This is achieved through metabolism — the sum of all the chemical reactions occurring inside a living body. Metabolism has two complementary halves: catabolism, the breakdown of larger molecules into smaller ones (usually releasing energy), and anabolism, the building of new, larger molecules from simpler ones (usually consuming energy). Every living cell, all the time, is carrying out both.
B. Growth and development. From the moment of birth, an organism shows a well-orchestrated tendency to grow and develop — its cells increase in number and size, and its body plan matures in an organised, internally-driven sequence. This is different from the "growth" of a sand mound or a boulder, which simply piles up material from outside itself (accretion) rather than growing from within through its own cellular processes. That is precisely why a growing sand mound is not considered a living thing, even though it is getting bigger.
C. Ageing. Growth and development do not continue forever. At some point, the molecules, organs and organ systems of the body begin to lose their efficiency, and the organism enters the ageing process.
D. Reproduction. For life to continue (to "perpetuate" itself), an organism must produce offspring resembling itself — through reproduction, which may be asexual (a single parent) or sexual (two parents). This is what ensures the continuity of a species/race across generations. Interestingly, some individuals — such as mules and sterile worker bees — cannot reproduce at all, yet nobody doubts that they are alive. This is exactly why reproduction, though a very important property of life in general, cannot by itself be treated as the single all-inclusive defining characteristic of every individual living being.
E. Death. As the body gradually loses its capacity to carry out metabolism, the organism eventually dies — no living being is immortal.
F. Responsiveness (irritability). Every living being responds to changes — thermal, chemical or biological — in its surroundings. This property of responding to a stimulus is often called irritability, and it is one of the most immediately observable signs of life (for instance, in deciding whether an organism found lying still is alive or dead).
Why Study Life's Diversity?
Life on Earth is enormously diverse, and organisms of many different kinds have coexisted since time immemorial. To understand how living things relate to each other, and to non-living components of the environment, a systematic study of this diversity is essential — this knowledge also has direct value for agriculture and industry. Detailed laboratory and field study, aimed at identifying and classifying organisms, forms the foundation for meaningfully using all this collected information. To carry out this kind of study, biologists use a set of taxonomical aids: herbaria, botanical gardens, museums, biodiversity parks, zoological parks, and taxonomic keys — each of which is introduced in the sections that follow.