Life• What is the definition of life?

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

• What characteristics are required for something to be alive?

• What has some characteristics of life but is not alive?

Life

• Life itself is a set of processes that are carried out by an organism causing it to survive.

• What are those processes?

• Although there is no universal agreement on the definition of life, scientists generally accept that life exhibits the following:

1. Homeostasis2. Organization3. Metabolism4. Growth5. Adaptation6. Response to stimuli7. Reproduction

Processes

1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to cool off.

Processes

2. Organization: Being composed of one or more cells, which are the basic units of life.

Processes

3. Metabolism: Production of energy by converting nonliving material into cellular components.

• Eating

Processes

4. Growth: Maintenance of a higher rate of synthesis (growth) than catalysis (decomposing).

Processes

5. Adaptation: The ability to change over a period of time in response to the environment.

• Mouse fur color changes over many years based on what the ground color is.

• A Salmon being able to deal with salt water after being born in fresh water.

Processes6. Response to stimuli: A response

is often expressed by motion.

• contraction of a unicellular organism when touched.

• complex reactions involving all the senses of higher animals, running away.

• leaves of a plant turning toward the sun• an animal chasing its prey.

Processes

7. Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual.

Exceptions to the conventional definition

• It is important to note that life is a definition that applies primarily at the level of species, so even though many individuals of any given species do not reproduce, (such as ant workers), these are still considered forms of life.

• One could say that the property of life is inherited; hence– sterile hybrid species such as the mule

are considered life although not themselves capable of reproduction.

– It is also worth noting that non-reproducing individuals may still help the spread of their genes through such mechanisms as kin selection.

• For similar reasons, – Viruses– prion proteins

• are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without host cells or proteins.

Some individuals contest such definitions of life on philosophical grounds, and offer the following as examples of life:

1. viruses which reproduce2. flames which "grow“3. certain computer software programs which are

programmed to mutate and evolve4. machines which can move

Still, most scientists would not call such phenomena expressive of life. Generally all six characteristics are required for a population to be considered a life form.

Descent with modification• A useful characteristic upon which

to base a definition of life is that of descent with modification:– the ability of a life form to produce

offspring that are like its parent or parents, but with the possibility of some variation due to chance.

ICE

• Is ice alive?

• Is it:• Organized?• Grow?