Characteristics of living things

While some scientists and organizations list as few as five traits, many authorities agree on eight characteristics of living things. The eight characteristics of living things are not a hierarchy. One trait does not outrank another.

8 Characteristics of life

Since human life began, we have attempted to define what life is. Our current scientific understanding shows eight characteristics of life:

  1. Adaptation through evolution
  2. Cellular organization
  3. Growth and development
  4. Heredity
  5. Homeostasis
  6. Metabolism
  7. Reproduction
  8. Response to stimuli

8 Characteristics of Life

We explained each of them below in alphabetical order to avoid bias.

Adaptation through evolution

Successful organisms use adaptation through evolution to survive. Organisms that cannot evolve cannot keep up with changing environments, become extinct. This is what we call natural selection.

Characteristics of life - Adaptation through evolution

All forms of life evolve. They adapt to the external environment, change their heritable traits, and prepare future generations for more efficient life processes. This characteristic has been seen in the laboratory in a fruit fly experiment that showed evolution at work within a span of 60 years.

Cellular organization

So far, everything biologists have found to be alive uses cell structure to organize molecules into useful tissues and organs. All life forms use cells, whether they are single-celled organisms or multicellular organisms such as plant cells or animals.

The general structures move along a line from cell to tissue to organ to being, which gives us the word “organism,” a living thing with organ systems. Cellular organization is seen in something as simple as a fungus cell. From simple bacteria up to mammals, life uses cellular organization.

Characteristics of life - Cellular organization

Growth and development

Living things grow. To conserve resources, organisms reproduce with immature and small copies of themselves. Without straining the parent organism, these small copies gather their own resources to grow, enlarge, mature, age, and reproduce themselves. Humans are excellent examples of growth and development. Eventually, organisms die, returning their gathered resources to the earth for reuse by new organisms. The cycles and stages of life are growth and development.

Characteristics of life - Growth and development

Heredity

Life transfers characteristics to offspring via deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); these are the building blocks of life. From viruses to humans, traits that benefit the parents are transferred in genes to the offspring. For humans, this genetic material holds our genetic information such as eye color, skin color, and hair type, just to name a few.

Characteristics of life - Heredity

Unfortunately, mistakes, mutations, and traits that harm can also be transferred through our genetic material. Whether for good or bad, life exists through heredity.

Homeostasis

Maintaining a stable internal environment is called homeostasis. Your body, like that of a cat or a cactus, must maintain a stable environment inside. For every inhalation, for example, you need to exhale. If you take in food, you must eliminate waste. If you lack enough water for cellular functions, you will get thirsty. Body temperature, a homeostatic biological process, has its mechanical equivalent in the thermostats, regulating room temperature in buildings and homes.

Characteristics of life - Homeostasis

Homeostasis is how living organisms maintain their internal systems.

Metabolism

Chemical reactions inside cells, tissues, organs, and living beings perform various actions that keep the organism alive. These reactions break down incoming food, send nutrients to cells, remove waste products, transform energy, and synthesize new chemicals. Together, these processes lead to growth, system repair, and excretion. Photosynthesis in plants is a metabolic process. Together, an organism’s chemical reactions are its metabolism.

Characteristics of life - Metabolism

Reproduction

Successful organisms reproduce. From fleeting mayflies living one day to Aldabra tortoises reaching 120 years to the ancient Methuselah bristlecone pine (4,800+ years), all life engages in reproduction.

Characteristics of life - Reproduction

[Insert side-by-side drawings of mayflies, Aldabra tortoise, and bristlecone pine]

Response To Stimuli

Scientists have been able to coax crystal structures to change behavior when exposed to blue light. Though this does not mean the crystals are alive, they exhibit a response to a stimulus, just as you respond to having your name called or your leg pinched. Phototropism is a plant’s response to stimuli (turning toward the light). Everything alive shows a response to stimuli.

Characteristics of life - Response to stimuli

Characteristics of life examples

Biologists, biology students, and thinking people all ponder the meaning of life. What is life? When is something alive, and when does it only appear to be alive?

The list of characteristics of life is not finished. As humans explore beyond our reach, we find more puzzles.

Extremophiles have been discovered living around hydrothermal vents on the seafloor where no light reaches. How can they, or acid-loving, radiation-resistant organisms like exist?

What are the characteristics of a cell?

The basic organizing structure of living things is the cell, a small version of the larger organism’s processes. Individual cells carry on life processes themselves. The characteristics of a cell:

  1. Cells separate themselves from their environment with cell walls, called cell membranes
  2. Cells carry on homeostasis with cytoplasm, the liquid inside the membrane and the medium holding the organelles, or cell structures
  3. Organelles carry on metabolism and other life processes within the cell membrane
  4. All cells carry their own heredity markers, DNA, at some stage of their development (though some, like red blood cells in humans, do not have DNA at maturity)

Cells differentiate and build on these basic functions by either not having a nucleus (prokaryotic) or having a nucleus (eukaryotic).

Are crystals alive?

We mentioned that scientists coaxed crystals into showing response to a stimulus. So are crystals alive? What about crystals “growing” in caves, or sugar crystals forming in sugar water to make rock candy?

This is the danger of relying on only one of the eight characteristics of life. For something to be alive, it must exhibit all eight characteristics.

Crystals accrete, which could appear to show growth and development, but they cannot reproduce, cannot metabolize resources, cannot excrete waste, and have no cell structures to store DNA.

Some spiritual adherents view crystals as being “alive” with energy and auras, but biologically, crystals are not alive.

Is bacteria living or nonliving?

To learn if bacteria are living or nonliving things, let’s go through the list:

  1. Adaptation through evolution – Bacteria evolve; they evolve quickly because they can reproduce at an incredibly fast rate (#3, #4, and #6)
  2. Cellular organization – Bacteria, by definition are single cells, but they are cells with cell walls, ribosomes, flagella, and DNA
  3. Growth and development – Bacteria do not show growth and development the same way higher forms of life do, but one bacterium splits into two daughter cells as part of a life cycle of lag phase, logarithm phase, stationary phase, and death phase
  4. Heredity – Bacteria contain DNA and, typically, RNA
  5. Homeostasis – Bacteria use their cell membranes to perform homeostasis
  6. Metabolism – Autotrophy (unique to bacteria), photosynthesis, anaerobic respiration, heterotrophic metabolism, fermentation, and the Krebs cycle are all aspects of bacterial metabolism
  7. Reproduction – Through binary cell division, bacteria reproduce in exponential numbers in very short periods compared to higher forms of life
  8. Response to stimuliChemotaxis describes bacteria moving toward or away from chemicals, such as Escherichia coli moving toward glucose

Bacteria meet all the requirements for being considered living things. Bacteria are alive.

Are viruses living organisms?

Viruses are trickier to classify than bacteria. Let’s use our trusty list again:

  1. Adaptation through evolution – Viruses can mutate and evolve; this is one aspect of their behavior that makes them so difficult to fight when they infect populations
  2. Cellular organization – Viruses have no cells
  3. Growth and development – Viruses show evidence of growth and development
  4. Heredity – Viruses contain either DNA or RNA; seldom do they possess both
  5. Homeostasis – Viruses cannot perform homeostasis
  6. Metabolism – Viruses do not have metabolism
  7. Reproduction – Viruses can only reproduce inside the cells of other organisms, which they do at astoundingly high rates
  8. Response to stimuli – Viruses do not respond to stimuli

Viruses have all of the characteristics of living things except cellular organization, homeostasis, metabolism, and response to stimuli.

Currently, most biologists, virologists, doctors, and general scientists say viruses are not alive. That thinking may change as more evidence, such as the giant mimivirus, is revealed.

Which characteristics of life does a computer have?

It may sound silly, but a good test of life's definition is to apply it to things we think of as non-living. How does a computer stack up? Once more, to the list!

  1. Adaptation through evolution – Computers can evolve
  2. Cellular organization – Computers have no cells  
  3. Growth and development – Computers do not grow and develop
  4. Heredity – Computers contain no DNA or RNA
  5. Homeostasis – Computers cannot perform homeostasis
  6. Metabolism – Computers obtain and use energy to function
  7. Reproduction – Computers cannot reproduce
  8. Response to stimuli – Computers can respond to stimuli

Since computers have only evolution, a kind of metabolism, and response to stimuli, they cannot be considered alive. They lack five of the eight characteristics of life.

Characteristics of life quiz

See how much you know about life. Try these questions and then check our answers against yours.

  1. Name any five features of living organisms.
  2. Which of the following are properties of life:
    • Reproduction
    • Hunting of prey
    • Response to stimuli
  3. Which characteristics of life does an automobile have?

Behold, the answers as we know them today:

  1. Five features of living organisms you may have listed could include any of these: adaptation through evolution, cellular organization, growth and development, heredity, homeostasis, reproduction, metabolism, and response to stimuli.
  2. Of the traits listed, choice b., hunting of prey, is not a property of life.
  3. The single characteristic of life that an automobile might have is metabolism, in that it can take in and use energy (gasoline or diesel) while producing waste exhaust and water). It cannot do this on its own, and it has no other characteristics of life. An automobile is not alive.

What you learned:

After working your way through this lesson and video, you have learned:

  • The eight characteristics of life: adaptation through evolution, cellular organization, growth and development, heredity, homeostasis, reproduction, metabolism, and response to stimuli.
  • All organisms we consider to be living show all eight of these traits.
  • Bacteria, plants, fungi, mammals – they perform all eight functions and characteristics.
  • Things on the edge of life, such as viruses, may have some of these traits, but not all eight.
Instructor: Malcolm M.
Malcolm has a Master's Degree in education and holds four teaching certificates. He has been a public school teacher for 27 years, including 15 years as a mathematics teacher.

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