Characteristics of Living Organisms

Characteristics of Living Organisms

Before any organism can be classified, biologists first have to agree on what counts as "alive". Two properties are used together, because neither one alone is a safe test.

• Metabolism — the sum of all chemical reactions occurring inside a cell or organism, including the breakdown of molecules to release energy (catabolism) and the building-up of complex molecules (anabolism). Every known living cell shows metabolism; it never stops for as long as the cell is alive. In isolation, a purified enzyme extracted from a cell can also carry out a biochemical reaction in a test tube, so metabolism alone is not a perfectly unique marker of "life" — it is a necessary but not sufficient criterion.
• Growth — an increase in mass and number of cells. Living organisms grow from the inside, by addition of new cytoplasm and new cells (intussusception), which changes their internal organisation as they grow. Non-living objects (like a crystal or a stalactite) can also "grow" by the addition of material, but this happens from the outside, by simple deposition of matter on the surface (accretion), without any internal reorganisation. This inside-versus-outside distinction is the practical test used to separate biological growth from the growth of non-living matter.

Taken together — internal, self-sustained metabolism plus growth by intussusception — these two features are used as the working definition of a living system.

Biogenesis is the principle that life arises only from pre-existing life, never spontaneously from non-living matter. It replaced the older idea of spontaneous generation (abiogenesis), which held that living organisms could arise suddenly from non-living material such as decaying matter. Louis Pasteur's swan-neck flask experiments are credited with settling this question in favour of biogenesis, showing that microbial growth in a nutrient broth appeared only when the broth was exposed to airborne organisms, never when it was sealed off from them. Biogenesis underlies the modern understanding that every cell arises from a pre-existing cell, and every organism from pre-existing organisms.