Assumed audience
General adult, no prior biology.
The characteristics of life
All living organisms share certain properties — cellular organization, metabolism, homeostasis, growth, reproduction, response to stimuli, and evolution. No single criterion is sufficient (fire grows and reproduces; crystals grow and have structure), but the combination is what defines life.
Cellular organization
All known living things are composed of one or more cells. The cell is the basic unit of life. Some organisms are a single cell (bacteria, amoebas); others are trillions of cells organized into tissues and organs.
Metabolism
Life requires continuous chemical activity. Organisms take in energy and materials from their environment, transform them through chemical reactions catalyzed by enzymes, and use the products to maintain themselves, grow, and reproduce. This network of chemical reactions is metabolism.
Homeostasis
Organisms maintain stable internal conditions despite changing external environments. Body temperature, blood pH, water balance — all regulated through feedback mechanisms. This is homeostasis.
Reproduction
Organisms produce new organisms. Some do this sexually (combining genetic material from two parents), some asexually (producing genetically identical offspring), many do both. Reproduction is how life perpetuates itself.
Evolution
Populations of organisms change over generations through natural selection and other mechanisms. This is evolution — the process that produced the diversity of life on Earth.
Why this matters
These characteristics provide the conceptual foundation for everything that follows in biology.