1. LEVELS OF STRUCTURAL ORGANIZATION
The human body contains multiple levels of structural organization: chemical, cellular,
tissue, organ, organ system and organism levels.
The simplest level of structural organization of living organisms is the chemical level.
ATOMS of various elements combine to form larger, more complex structures termed
MOLECULES. Organic molecules aggregate to form cellular ORGANELLES responsible
for specific cell functions such as cell membranes, that regulate the movement of chemical
substances into and out of the cell, mitochondria, that convert energy of organic nutrients
into ATP, and ribosomes, that are involved in the production of protein molecules. (See
notes on Chemical Composition of the Body.)
CELLS are the structural and functional units of life. Each organism begins life as a single
cell formed when a sperm injects the male genetic material (paternal chromosomes) into
the ovum, forming a ZYGOTE, or fertilized ovum. Growth and development ofa zygote
through the stages of embryo, fetus, new born, child and adult requires two fundamental
processes: mitosis and cellular differentiat on. MITOSIS is a cell replication process - a
i
type of cell xeroxing that results in an increase in the number of somatic cells in an
organism. During mitosis, all the chromosomes and other cellular components in the parent
cell are replicated so that each daughter cell receives a full set of 46 chromosomes and all
essential cell components. Each somatic cell is formed from an existing cell by the process
of MITOSIS. The process of CELLULAR DIFFERENTIATION allows cells, by
SELECTIVE EXPRESSION OF GENES, to become anatomically distinct and functionally
specialized. Although all cells contain 46 chromosomes and hence the complete human
genome, cells generally only express about 15% of the genes they contain.
Cells of similar structure and function congregate to form TISSUES. There are 4 principle
types of tissues in the human organism: epithelial tissue, connective tissue, muscular tissue
and neural tissue. (See notes on Histology : Study of Tissues.)
Two or more types of tissues are combined within the same structure to form an ORGAN
such as the heart, liver, kidney or brain. Most organs contain all 4 of the principle types of
tissues.
Individual organs that function cooperatively to accomplish a common purpose are
grouped together into ORGAN SYSTEMS. Examples include the cardiovascular system
that functions to circulate the blood, the gastrointestinal system that functions to digest and
absorb nutrients and the urinary system that removes metabolic wastes andexcess minerals
and water from the blood. There are eleven organ systems in the human body that are
combined to form the complete human organism. See the text, pages 4-11 for the
introduction to organ systems.