What is Life?

There is no hard-and-fast distinction between a life and nonlife. Early
scientists

thought there was a "vital principle", called quintessence or pneuma, that
was

present in every living thing, and absent in everything nonliving. This has
not

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
problematic

agents, including viruses, prions, and transposable genes, which appear to
be

living at times and non-living at other times.

The following is a very general characterization of what defines a living
thing.

.. Birth and Death

.. Need for an Energy Source

.. Reproduction

.. Information

.. Evolution

Birth and Death

Living things are born and living things die. Although true by definition,
this

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
organization

breaks down, we die.

By reproduction, growth, and development, living things impose

organization on nonliving matter. In death and decomposition, this
organization

breaks down.

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
as they

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
obtain

energy (some other organisms use fermentation, photorespiration, etc..)

Respiration requires free oxygen, as well as carbohydrates, lipids, or amino
acids to

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
water.

Humans, like other animals, are heterotrophs. We process energy that was

originally captured by other living things. Unlike plants, we cannot fix
energy

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

organism.

Reproduction

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
conditions.

.. Mice take about 4 months. Seagulls take 7 years. Humans take 16-20 years
to

reproduce.

In general, generation time is inversely proportional to size.

Carrying Capacity

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.

Information

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,
this

template is DNA. This template makes proteins, which are responsible for our

structure, function, and metabolism. It is copied every time living things

reproduce.

DNA

DNA is the information template for life on earth. Without DNA, living
organisms

could not reproduce or function.

Evolution

Living things evolve and adapt to the environment over time. An individual
has

an essentially fixed set of information, called a genome, which does not
change -

individuals cannot evolve (individuals develop). Errors during reproduction
of

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;
a

common genetic code, a similar set of amino acids and basic building blocks,
and a

common ancestry with the first, primitive organisms.

Through the process of adaptation, evolution has produced organisms suited
to

a wide range of environments. Other differences are the product of
evolutionary

history, and are not necessarily adaptive.

Some basic attributes of organisms that differ from one species to the

next:

The presence of a nucleus-Prokaryotes, the most primitive, widespread, and

metabolically diverse organisms on the planet, lack an organized nucleus.
These

include bacteria and archaeans. Eukaryotes have an organized nucleus

Organisms can be multicellular, single-celled, or colonial (multicellular,
but

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
is the

timing of devleopment and reproduction based upon the probability of dying
at a

given age, and the method of reproduction. In addition, many organisms go

through different stages in their life cycle. These stages may differ in
their

number of chromosomes, the resources they exploit, and the places they
inhabit.

Many organisms also have resistant, resting stages able to survive tough

conditions.


.

Relevant Pages

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