Hierarchical Organization of Living Systems
Life’s organisms interact with each other at many levels, in ways
simple and complex. A key factor organizing these interactions is
their degree of complexity. The organization of the biological
world is hierarchical—that is, each level builds on the level below
it, from the very simplest level of individual atoms to the vastly
complex level of interacting ecosystems.
1.The Cellular Level
At the cellular level,atoms 1,
fundamental elements of matter, are joined together by chemical bonds into stable assemblies called
molecules 2.large and complex molecules are called macromolecules 3.
DNA, which stores the hereditary
informations , is a macromolecule. Complex biological
molecules are assembled into tiny structures called
Organelles 4 , within which cellular activities are
organized. A mitochondrion is an organelle within
which the cell extracts energy from food molecules.
Membrane-bounded units called cells 5 are the basic
units of life. Bacteria are composed of single cells.
Animals, plants, and many other organisms are
multicellular—composed of many cells.
2.The Organismal Level
Cells of multicellular organisms
exhibit three levels of organization. The most basic
level is that of tissues 6 , which are groups of similar
cells that act as a functional unit. Nerve tissue is one
kind of tissue, specialized to carry electrical signals.
Tissues, in turn, are grouped into organs 7 —body
structures composed of several different tissues that act
as a structural and functional unit. Your brain is an
organ composed of nerve cells and cells that nourish
and support them, as well as a variety of associated
connective tissues that form both protective coverings
and a network of blood vessels to bring oxygen and
nutrients to the brain. At the third level of organization,
organs are grouped into organ systemS 8
. The nervous
system, for example, consists of sensory organs, the brain
and spinal cord, and a network of neurons that convey
signals between the brain and the other organs and tissues
of the body.
3.The Population Level
Individual organisms 9
several hierarchical levels within the living world. The
most basic of these is the population 10—a group of
organisms of the same species living in the same place.
All populations of a particular kind of organism together
form a species 11 , its members similar in appearance
and able to interbreed. At a higher level of biological
organization, a biological community 12 consists of
all the populations of different species living together in
one place.
4.The Ecosystem Level
.
At the highest tier of biological
organization, a biological community and the physical
habitat (soil composition, available water, temperature
range, wind, and a host of other environmental
influences) within which it lives together constitute an
ecological system, or ecosystem 13 . The entire planet
can be thought of as a global ecosystem we call the
bioshere 14.
Hierarchical organization of living systems.
Life is highly organized, from the simplest atoms to complex ,
multicellular organisms. Along this hierarchy of structure, atoms
form molecules, which are used to form organelles, which in turn
form the functional subsystems within cells. Cells are organized
into tissues, then into organs and organ systems such as the
nervous system. This organization extends beyond individual
organisms to populations, communities, ecosystems, and finally
the entire biosphere.
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