How do we define
What must living things be able to do? They must be self-replicating
and able to extract energy from the environment and use it for
growth and biological maintenance. Our first life forms will
consist of very simple units, single cells, as on earth, (or
perhaps some other invented form -- if you can think of something
that might work --). Chemicals are easily stirred in liquid,
and who knows what happened in the primordial soup?
Life needs to attend
to two concerns: the passing on of genetic information and the
capture of energy. Both require having some control over the
environment -- and that is why life forms are made up of cells.
A cell is a tiny package that keeps the genetic information (the
chromosomes) together and stores and transforms chemicals that
provide energy. A cell regulates what comes into
out of itself by regulating what passes through the cell wall: it
controls its interior environment. How did cells come into being?
We do not know. We know that there are very simple cells
cells in which the genetic material floats in the other cell
contents. More complex cells
(eukaroytes), cells in which the
nucleus is inside its own membrane in the cell, developed from
these simpler forms.
For the purposes
of this course we will divide life forms into
can make their own food from materials in the environment,
cells that consume organic matter
that has been assembled by other cells
detritovores, special kinds of
heterotrophs that eat waste
material, dead plant parts
and animals, old wood and
roots, and so on.
Some single celled organisms have chloroplasts in their
cytoplasm (the fluid cell contents) and are able to synthesize
that they need to live and
grow by using an outside
energy source, sunlight.
photosynthesizing cells were
bacteria, and they used a
type of chlorophyll to
capture solar energy
Even on the single cell level, some cells engulf
and eat other cells.
They also capture organic molecules that
may be floating in the water around
them. They extract energy from
A third group is the
detritovores, who extract energy as
they break down dead organic materials.
Single celled detritovores
break down the chemicals in
dead organic matter so that
the chemicals can be
recycled through the food
chain. Early in our
planet's history they would
absorb organic molecules:
today they also digest fallen leaves,
rotten wood, and animal
bodies. Many are
still single celled
bacteria, but decomposers
also include fungi.
Often we don't notice the decomposers, but they perform very important
These methods of obtaining food
have serious consequences for life
strategies. We shall see
this as our life forms evolve.
Organisms with chlorophyll need to
find water and light, but do not produce enough energy to be
able to move around very much.
On earth they do their moving
by slowly growing in
the direction that they want
to go, or, in the
reproductive stage, by
producing spores or seeds
that are blown or carried to new locations.
on the other hand, take in a concentrated form of energy, which
enables them to move around and find more
organic matter to eat.
between autotrophs and the
predator / prey
relationships between the
heterotrophs is described
as a food chain.
Decomposers are often not
mentioned in a food chain,
but without their work the
community would not cycle
idea that will guide us through all our environments is the idea
of an ecological community. Just as a body works with the cooperation
of all the different organs, so living creatures live interactively
with other organisms. For example, the rabbit eats the grass,
and the coyote eats the rabbit. If there are too few coyotes,
the rabbits will eat almost all the grass and many will starve;
if there are too many coyotes they will catch too many rabbits,
and then some of the coyotes will go hungry. But the picture
is more complex than this. The coyote eats frogs, too, and prairie
dogs (useful if the rabbits get a plague and many die). The tunnels
of the prairie dogs help to aerate the soil, which could not
maintain fertility without the presence of earthworms, nematodes,
and hosts of tiny invisible creatures that turn dead animal and
vegetable matter into food for the grass. The rabbits eat wild
flowers, too, may of which are pollinated by insects. In an ecological
community a number of different animals and plants live interactively
together. This makes for a flexible community that can deal with
adverse weather conditions or environmental changes.
Even though you are working
on unicellular organisms today, your group should start thinking
immediately about interactions between life forms -- what is
there to eat on your planet, and who will eat it? On earth there
is a flow of (usually) solar energy through a
community: energy is first captured by plants and then turned into flesh by grazers.
We will learn more about ecological communities as this course