Cell Biology/Introduction/What is living

From Wikibooks, the open-content textbooks collection

The question, "What is life?" has been one of many long discussions and the answer may depend upon your initial definitions.

Some definitions of life are:

1. The quality that distinguishes a vital and functional being from a non-living or dead body or purely chemical matter.
2. The state of a material complex or individual characterized by the capacity to perform certain functional activities including metabolism, growth, and reproduction.
3. The sequence of physical and mental experiences that make up the existence of an individual.

[edit] Seven Criteria

A cell undergoing mitosis. Reproduction is one of the seven criteria for life.

In biology, whether life is present is determined based on the following seven criteria:

1. Homeostasis: The entity in question has the ability to maintain its interior in a constant state.

2. Organization: Structural composition and appropriate differentiation of one or more cells.

3. Metabolism: Ability to break down or build up nutrients to release or store energy based on need.

4. Growth: Increasing in size with time in an advantageous manner.

5. Adaptation: Ability to change in response to the environment.

6. Response to Stimuli: Ability to respond to environmental stimuli on demand (as opposed to adaptation, which occurs over time).

7. Reproduction: Entity can reproduce a clone of itself (asexual/viral) or create a recombinant of the same species (sexual).

[edit] Virus Controversy

Are these bacteriophages technically alive?

A long-standing debate dating back to the very first years after the virus was discovered, viruses fall somewhere between being alive and being coagulates of organic matter. Viruses maintain some degree of homeostasis (1), being able to keep its protenatious and nucleic machinery separated from the outside world. Viruses also show adaptation(5), with their ability to mutate in order to affect new organisms. Finally, they display response (6) and reproduction (7) as they attach to cells and "hijack" their organelles to reproduce their own parts. However, they fail to meet the other requirements, showing no cellular organization (2) (or indeed cells at all), metabolism (3), or growth (4).