Molecules

• Properties of life


  • Organization
  • Metabolism
  • Homeostasis
  • Growth
  • Reproduction
  • Response
  • Evolution
• Living things are highly organized, meaning they contain specialized, coordinated parts
• All living organisms are made up of one or more cells, which are considered the fundamental units of life
• In multicellular organisms, similar cells form tissues, and tissues collaborate to create organs (body structures with a distinct function)
• Metabolism is the sum total of the biochemical reactions occurring in an organism
• Anabolism


  Organisms make complex molecules from simpler ones
• Catabolism


  Organisms break down complex molecules into simpler ones
• Homeostasis is the maintenance of a stable internal environment, even in the face of a changing external environment
• Living organisms undergo regulated growth, where individual cells become larger in size, and multicellular organisms accumulate many cells through cell division
• Living organisms can reproduce themselves to create new organisms, through either asexual or sexual reproduction
• Living organisms show "irritability", meaning that they respond to stimuli or changes in their environment
• Populations of living organisms can undergo evolution, where the genetic makeup of a population may change over time
• The list of properties is not foolproof, as some living things may not exhibit all of them, but it provides a reasonable set of criteria to distinguish between living and non-living things
• Humans, dogs, and trees


  • Easily fulfill all seven criteria of life
  • Made of cells
  • Metabolize
  • Maintain homeostasis
  • Grow
  • Respond
  • Reproduce
  • Undergo biological evolution
• Nonliving things


  • May show some, but not all, properties of life
  • Crystals of snow are organized but don't have cells, can grow but don't meet other criteria of life
  • Fire can grow, reproduce by creating new fires, and respond to stimuli but is not organized, does not maintain homeostasis, and lacks genetic information required for evolution
• Living things may keep some properties of life when they become nonliving, but lose others
• Wood in a chair under a microscope may show traces of the cells that used to make up the living tree, but the wood is no longer alive and can no longer grow, metabolize, maintain homeostasis, respond, or reproduce
• Robot


  Could show organization, respond to stimuli, and have a metabolism, maintain homeostasis, but would not grow, reproduce, or be part of an evolving population, and thus wouldn't be considered alive
• A sophisticated enough computer or robot could start to bend the definition of life
• Viruses have many properties of life but do not have a cellular structure, cannot reproduce without a host, and it's unclear if they maintain homeostasis and carry out their own metabolism
• Simpler molecules like self-replicating proteins and self-replicating RNA enzymes have some, but not all, properties of life
• Extraterrestrial life may or may not share the same characteristics as life on Earth
• NASA's working definition that "life is a self-sustaining system capable of Darwinian evolution" opens the door to many more possibilities than the criteria defined above
• Future discoveries may call for revisions and extensions of the definition of life
• Traits die. Species evolve because they are alive; they are not alive because they evolve. The individual organism itself does not evolve; it simply lives and dies.
• Evolution shouldn't be on the list of qualities of living things.
• Qualities of living things


  • Synthesis
  • Transport
  • Excretion
  • Regulation
  • Nutrition
  • Growth & development
  • Respiration
  • Reproduction
• Cells of living organisms are "organized".
• "Organized" cells


  The way the cells are "positioned" in the body of an organism, similar to how bricks make up a wall
• DNA is a macromolecule.
• Viruses, though not considered living organisms, can hijack our cells' DNA to program it to make more copies of the virus.
• Humans have 'human awareness' - the ability to experience one's own "internal states" as intentional states, which is not something animals can do.
• Reproduction is a defining, but not all-inclusive trait of living beings. Not all beings need to reproduce to be considered living, but all reproductive beings are considered to be living.