The body of living organisms is organised, i.e., several
components and sub-components cooperate with each
other for the functioning of whole body.
Physical and Biological Hierarchies
There is a physical (non-living) hierarchy and biological
(living) hierarchy in the organisation of living body.
In physical hierarchy, various non-living components
(atoms and molecules) aggregate to form compounds, which
finally enter the living world in the form of cells. Each cell is
made up of many compartmentalised cell organelles, each of
which has specific intracellular function. These cells organise
to form tissues, that form organs and several organs
combine to form organ systems. Finally, many organ
systems organise and form a living organism.
The properties of tissues are not present in the constituent
cells, but arise as a result of interactions among the
constituent cells. For example, bone is a hard tissue, which
provides framework to the body, but the cells present inside
it do not have this property. This phenomenon of
interactions between various components of the body results
in the hierarchy of organisation.
The various life processes are the result of this interaction
and coordination. The complexity in organisation enables
living organisms to self-replicate, evolve, self-regulate and
respond to external stimuli. All living organisms along
with their ancestors and descendants are linked to one
another by sharing of common genetic material in the
form of DNA in varying degrees. This DNA is responsible
for the expression of specific traits in organisms.