Organisms have several levels of organization. Biological organization is hierarchical:a complex organism is
organized at the chemical, cellular, tissue, organ and organ system levels. All these levels together work in
harmony. As a result, the organism performs its life activities.
Chemical level.
The basic level of organization includes atoms and molecule. The atom, such as carbon, hydrogen, and
oxygen atoms, is the smallest unit of chemical elements. Atoms combine chemically with each other to form
molecules. For example carbon, hydrogen and oxygen atoms combine to form sugar molecules.
6 Carbon + 12 Hydrogen +6 Oxygen = Glucose (С
6
H
12
O
6
)
Cellular level.
At the cellular level, many atoms and molecules come together in order to form cells. Cells are the basic unit
of life. They are very small but have extraordinary functions.
Tissue level
In complex organisms, many cells which have the same function and structure come together to form tissue,
such as bone cells forming bone tissue, and muscle cells forming muscle tissue.
Organ level
In the most complex organisms, tissue is organized into functional structures called organs, such as the
heart, lung and kidney (animal organs). Leaves, roots and flowers are plant organs.
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