What is Life?
- From: "AleXX" <dalaibaru@xxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 16 Jun 2011 04:20:58 +0800
There is no hard-and-fast distinction between a life and nonlife. Early
thought there was a "vital principle", called quintessence or pneuma, that
present in every living thing, and absent in everything nonliving. This has
proven to be true. A growing conviction, intuitively felt by many
biologists, is that
no clear line can be drawn between the living and the non-living. The bridge
between what is and is not obviously alive consists of a range of
agents, including viruses, prions, and transposable genes, which appear to
living at times and non-living at other times.
The following is a very general characterization of what defines a living
.. Birth and Death
.. Need for an Energy Source
Birth and Death
Living things are born and living things die. Although true by definition,
underlies an essential property of living things: they are organized. As
long as we
maintain a certain level of organization, we are alive. When this
breaks down, we die.
By reproduction, growth, and development, living things impose
organization on nonliving matter. In death and decomposition, this
An expression of this organization is homeostasis, the constant internal
environment maintained by organisms.
Living things exist in a state of dynamic equilibrium-organized systems that
requires a constant input of energy to maintain themselves.
Growth and Development
Living things grow, and multicellular organisms develop as well. Growth is
simply adding new material to our bodies. Development is the changes in
structure we undergo as we grow and age.
Most multicellular organisms become more complex as they grow.
Differentiation is the changes in structure and function our cells undergo
assume roles and functions in our increasingly complex bodies.
Living Things Require Energy
All living things require a constant input of energy to survive.
Like other animals, humans use a chemical reaction called respiration to
energy (some other organisms use fermentation, photorespiration, etc..)
Respiration requires free oxygen, as well as carbohydrates, lipids, or amino
break down into simpler molecules. As humans, we consume dead plant and
animal material for energy (we call it "food"), and also for nutrients and
Humans, like other animals, are heterotrophs. We process energy that was
originally captured by other living things. Unlike plants, we cannot fix
from sunlight, nor can we fix energy by reducing hydrogen sulfide these
organisms are autotrophs and chemoautotrophs respecitively.) All of the
energy we use to survive, and most of the nutrients, were taken from another
All living things are able to make copies of themselves. Some living things
reproduce rapidly, others take a very long time.
.. Examples: E. coli bacteria can replicate once every 20 minutes. Paramecium
aurellia, a protist, can replicate once every 10 hours under ideal
.. Mice take about 4 months. Seagulls take 7 years. Humans take 16-20 years
In general, generation time is inversely proportional to size.
Living things tend to reproduce until some factor limits the growth of their
populations: disease, predation, or limited resources. All organisms have an
environmental carrying capacity, a maximum number that the resources in the
environment can sustain.
Living things use a template to impose order on nonliving things and to
maintain order within their own bodies. In all present-day living things,
template is DNA. This template makes proteins, which are responsible for our
structure, function, and metabolism. It is copied every time living things
DNA is the information template for life on earth. Without DNA, living
could not reproduce or function.
Living things evolve and adapt to the environment over time. An individual
an essentially fixed set of information, called a genome, which does not
individuals cannot evolve (individuals develop). Errors during reproduction
genomes, competition among different genomes, and sampling error cause
evolution - evolution is the genetic change of a species over time.
The Unity of Life
All terrestrial organisms share an amazing number of other things in common;
common genetic code, a similar set of amino acids and basic building blocks,
common ancestry with the first, primitive organisms.
Through the process of adaptation, evolution has produced organisms suited
a wide range of environments. Other differences are the product of
history, and are not necessarily adaptive.
Some basic attributes of organisms that differ from one species to the
The presence of a nucleus-Prokaryotes, the most primitive, widespread, and
metabolically diverse organisms on the planet, lack an organized nucleus.
include bacteria and archaeans. Eukaryotes have an organized nucleus
Organisms can be multicellular, single-celled, or colonial (multicellular,
lacking true division of labor)
Organisms may be autotrophs, heterotrophs, chemoautotrophs, or
mixotrophs, depending upon how they get the energy and organic compounds
they need to survive.
Modes of reproduction vary. Eukaryotes can reproduce sexually, asexually, or
alternate between asexual and sexual reproduction. The timing and method of
sexual reproduction vary tremendously among organisms. Prokaryotes generally
reproduce asexually by fission, but may exchange genes.
All organisms have a life history, and life histories differ. A life history
timing of devleopment and reproduction based upon the probability of dying
given age, and the method of reproduction. In addition, many organisms go
through different stages in their life cycle. These stages may differ in
number of chromosomes, the resources they exploit, and the places they
Many organisms also have resistant, resting stages able to survive tough
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