Encyclopedia of Evolution.pdf - Online Reading Center

Since organisms must bring molecules in from, and eject
molecules out into, their environments through surfaces
large organisms must have additional surface area to
compensate for this relatively lesser surface area. Small
animals can absorb the oxygen that they need through
their external surfaces, but large animals need additional
surface areas (either gills on the outside, or lungs on the
inside) that absorb oxygen. The surface area of human
lungs, convoluted into millions of tiny sacs, is as great as
that of a tennis court. In small organisms, molecules can
diffuse and flow everywhere that is necessary, since they
do not need to go very far; larger organisms need circulatory
systems.
The following is a brief outline of some of the areas of
study within biology:
I. Autecological context. This is the ecological interaction of
an individual organism with its nonliving environment (aut-
comes from the Greek for self). Organisms must interact with
the energy and matter of their environments. This results in
the flow of energy and the cycling of matter.
A. Energy flows from the Sun, through the systems of
the Earth, and then is lost in outer space. Some of this energy
empowers climate and weather and keeps organisms warm.
Photosynthetic organisms (see photosynthesis, evolution
of) absorb a small amount of the energy and store it in
sugar and other complex organic molecules. Other organisms
obtain energy by eating photosynthetic organisms, or one
another. In this way, energy passes through the food web of
organisms. Eventually all organisms die, and the decomposers
release the energy into the environment, where it eventually
goes into outer space.
B. Matter cycles over and over on the Earth. Photosynthetic
organisms obtain small molecules, such as carbon
dioxide from the air and nitrates from the soil, from which
they make complex organic molecules. Other organisms
obtain molecules by eating photosynthetic organisms, or one
another. In this way, atoms pass through the food web of
organisms. Eventually all organisms die and the decomposers
release the atoms into the environment, where they are used
again by photosynthetic organisms.
C. Energy can flow, and matter can cycle, on a dead
planet, but on the living Earth, these processes are almost
completely different than they would be on a lifeless planet.
Two examples are oxygen and water. Photosynthesis produces
oxygen gas, which is highly reactive. An atmosphere
contains oxygen gas only if it is continually replenished.
Therefore the presence of oxygen gas in a planetary atmosphere
is evidence that there is life on the planet. Water cycles
endlessly, through evaporation from oceans, condensation in
clouds, and precipitation onto the ground. Forests slow down
the rain and allow it to percolate into the soil. In this way,
forests prevent the floods and mudslides that would occur on
a bare hillside and recharge the groundwater. Trees release
water vapor into the air, creating more clouds than would
form over a lifeless landscape.
biology
II. Chemistry of life. Organisms consist largely of carbon,
hydrogen, oxygen, nitrogen, phosphorus, and a few other
kinds of atoms. Most of the other chemical elements play no
part in organisms except as contaminants. Carbon, hydrogen,
oxygen, nitrogen, and phosphorus are the principal components
of biological molecules, which include: carbohydrates
and fats, which often store chemical energy; nucleic acids,
which store genetic information; and proteins, which often
control the chemical reactions of organisms. Proteins release
genetic information from nucleic acids, allowing that information
to determine the chemical reactions (metabolism) of
the organism.
III. Cells and tissues. All life processes occur within cells. Tissues
are groups of similar cells. Cells can replicate their genetic
information, which allows one cell to become two (cell
division). Cell division allows three things:
A. Old or damaged cells can be replaced by new ones
(maintenance).
B. Cells or organisms can produce new cells or organisms
(reproduction).
C. A single cell can grow into an embryo, which grows
into a new organism (development). The use of genetic information
changes during development, which allows a small
mass of similar cells (the embryo) to develop into a large
mass of many different kinds of cells (the juvenile and adult).
IV. Organs. Large organisms (mostly plants and animals)
need organs to carry out basic processes necessary to their
survival. Plants grow by continually adding new organs (new
leaves, stems, and roots) and shedding some old organs (dead
leaves). Plants can lose organs and keep on living. Animal
growth, however, involves the growth of each organ, the loss
of any of which may be fatal to the animal.
A. Exchange. Both plants and animals have surfaces
through which food molecules enter and waste molecules
leave the organism. In plants, most of these surfaces are external
(thin leaves, fine roots), while in animals they are internal
(in the intestines, lungs, and kidneys).
B. Internal movement. Both plants and animals have
internal passageways that allow molecules to move from one
place to another within the body. In plants this movement is
mostly one direction at a time (water from the roots to the
leaves, sugar from the leaves to the roots), while large animals
have internal circulation of blood.
C. Internal coordination. Both plants and animals have
structures that support them and functions that allow their
organs to work together. Hormones carry messages from
one part of a plant to another, allowing it to coordinate its
growth. The responses to hormones are mostly on the cellular
level in plants. Hormones also carry messages from one
part of an animal to another, but in addition animals have
nerves that allow internal coordination, for example, maintaining
homeostasis of body temperature and balance during
movement. The responses to nerves and to hormones in animals
can be on the cellular level or involve the movements of
muscles and bones.