How would you define 'living'? Are molecules alive?

[Darkusvader]

I have been reading a thread regarding the difference between rocks and people?! A lot of the comments reffered to various understandings of various definitions. I am considering the definition of 'living'. Do vibrating molecules indicate life? Or do we refer to cell structures only (I.e. plants and animals)?

 

[jim mcnamara]

See: last post

https://www.physicsforums.com/threads/scientifice-definition-of-life.452946/#post-3014773

@SW VandeCarr gave a good definition. However because these threads tend to become philosophical rather than scientific, I would suggest you let it go at the one good definition.

 

[Ygggdrasil]

There is no universally accepted, scientific definition of life. Here is one attempt at forming a consensus definition 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:[11][13][14][15][16][17][18]

1. Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
2. Organization: being structurally composed of one or more cells — the basic units of life
3. 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.
4. 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.
5. 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.
6. 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.
7. Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.

These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.

https://en.wikipedia.org/wiki/Life

The article also describes some alternative definitions. However, the above definition given provides many differences between rocks, vibrating molecules and people to answer your original question.

 

[rootone]

Whatever definition you use there will be borderline cases.
Typically a virus falls in that category.
While it can reproduce, it can only do so by hijacking the cellular machinery of another life form.
Some can exist for a long time in a suspended state with no significant chemistry occurring, a bunch of complex molecules, but no more alive than a rock.

 

[FactChecker]

The definition may even go beyond viruses. Mad Cow Disease led to reconsidering the definition to include prions. A prion can cause a related chemical to transform to the prion. So it duplicates itself in the right environment. The prion of mad cow disease is very hard to destroy and decontamination is difficult.

 

[BillTre]

I also like to consider viruses when thinking about this question:
Are they only not alive (AKA dead)?
Are they only alive when infecting a cells and then not alive when the complete virus is outside of the cell?
Are they alive all the time?
A related question might be is a frozen cell (which could be revived by thawing) dead or alive?

I like to consider frozen cells and viruses alive (but maybe in suspended animation) all the time (till destroyed) since they can both reproduce when in the right environment (which for a virus would be in a proper cellular environment).
However, I could certainly understand someone saying "no it has to be a cell", but I would say the virus has a cell that it takes over.
Given that they have mutable genetics, this allows evolution.

There are lot of things that are relatively inert but then "spring to life" in the right conditions (like seeds and spores).

To me, these could all fulfill @Ygggdrasil's list of requirements, but only at certain times.

 

[newjerseyrunner]

To answer the original question, there is nothin special about our molecules. We're made up of the same stuff as everything else, it's just how those atoms are arranged so that the chemistry regulates itself.

 

[Darkusvader]

To answer the original question, there is nothin special about our molecules. We're made up of the same stuff as everything else, it's just how those atoms are arranged so that the chemistry regulates itself.

So if atoms self regulate, would you consider them alive or dead?

 

[rootone]

However well regulated an atom may be, it certainly doesn't reproduce.
As for states other than alive or dead there are the borderline cases mentioned, which could be assigned as being something like 'potentially living'.
There is no such scientific term though, and there isn't even a strong definition of what 'definitely alive' is.
The list of properties posted by Ygggdrasil is as close as there is to a consensus.

 

[FactChecker]

Personally, I would not like any definition that did not include a virus as living. Just my two cents.

 

[berkeman]

Closed for Moderation...

 

[Ryan_m_b]

The definitions according to the field and their pros and cons have been elaborated on. The thread is straying into personal beliefs and philosophy, neither of which are appropriate for the forum. Since the question is answered the thread will remained closed.