What are the characteristics of a living organism?

Question

It seemed fairly basic at first, and yet I have to ask because even a quick Google search reveals that it doesn't seem to be quite so straight-forward anymore. After more research, I found multiple lists, some with six points, others seven or eight. Some lists shared six points and only differed on one, but others had five different points and divided and named the points differently such as Respiration vs Metabolism, Adapt vs Evolve, Homeostasis vs simply consisting of cells, etc.

Maybe it never really was simple, but when I learned basic middle school life science in the 1990s I seem to remember a clear and concise list of 7-8 distinct terms. I know they were just the summary given by the author of whatever textbook we used, but I remember several sources having similar lists (cannot say for sure if they were exactly the same), including a textbook, an educational video game, and a poster.

I'm also told that this has become a bit of a debate in the biology community, whether such a black and white distinction of "living" or "not living" is prudent. Questions such as this one on viruses are an example of how it is difficult to categorize life. However, I noted in my research that there was still clearly overlap in meaning of definitions, it was just in description and division of points that they differed. This suggests to me there is still a generally agreed upon core definition that is simply described by different people, specifically authors, with a few areas of possible debate.

• What are the current generally agreed upon characteristics, the common denominators? (They all reproduce?)
• What characteristics are commonly debated and therefore would unwarranted to claim? (Specifying cell structure?)

With the understanding that an answer will require an objective synthesis of a range of content and opinions, debate on what should or should not be characteristics is discouraged. Rather, an impassioned summary of the current scientific consensus on this question is what I am looking for.

It seems to me that even if this is a topic of opinionated debate, it is certainly at the core of Biology and would be worthwhile to be addressed in an academic manner, be accessible, and available to reference on this site.

(PS - Not in any way a homework question, never was, just a curious 30-year father. Unless you are counting questions I have to answer from a curious little boy.)

Answer 1

First of all, you have to note that there is no such thing as a scientific, objective definition of life, and that many scientists disagree on a given definition, and that every definition has it's limits.

However, the one from wikipedia

Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:[10][12][13][14][15][16][17]

Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature

Organization: being structurally composed of one or more cells — the basic units of life

Metabolism: transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.

Adaptation: the ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.

Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.

Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

Encyclopedia Britannica

• Metabolism

• Respiration

• Sensitivity (responding to stimuli like gravity and sunlight)
• Nutrition
• Excretion (getting rid of CO2)
• Growth
• Reproduction
• Homeostasis