Unlike non-living objects, living organisms share a constellation of distinctive characteristics that together define the state of being alive. There is no single criterion, but a combination of the following:
- Cellular Organization: All living things are composed of one or more cells—the fundamental structural and functional unit of life. A bacterium is a single cell, while a human is a trillion-celled collaborative.
- Metabolism: Living entities constantly perform biochemical reactions. This includes anabolism (building up complex substances, like photosynthesis) and catabolism (breaking down substances to release energy, like respiration). The sum total of these reactions is metabolism.
- Growth: Living beings exhibit growth, which is an irreversible increase in mass and number of cells. A crystal can grow by accretion, but only living organisms grow from within.
- Reproduction: The ability to produce progeny, ensuring the continuity of species. It can be asexual (a single parent, as in bacterial fission) or sexual (involving two parents, leading to genetic variation).
- Consciousness & Response to Stimuli: Perhaps the most definitive characteristic. All organisms, from plants bending towards light to animals fleeing danger, respond to environmental stimuli. In higher animals, this is mediated by a sophisticated nervous system.
- Homeostasis: Living organisms maintain a stable internal environment despite changes in the external environment (e.g., constant body temperature in birds and mammals).
- Evolutionary Adaptation: Over generations, populations of organisms evolve traits that enhance their survival and reproduction in a specific environment. This results in the magnificent fit between form and function we see in nature.
- Movement: While obvious in animals, movement in plants is slower and often growth-related (tropisms). Even protists exhibit locomotion via flagella or cilia.
These processes are orchestrated by genetic material (DNA or RNA), which carries the blueprint for life and is passed on through reproduction.
