Section II. CELLS
The cell is the basic structural, functional, and developmental unit of the body.
The human body contains hundreds of trillions of cells, each of which is capable of
performing the necessary functions of life.
There is no typical cell that can be examined to determine the functions of the
various parts. However, most cells have certain basic common components and it is,
therefore, possible to visualize and describe a composite cell that combines the
observed features of numerous cell types. Such a composite cell would be made up of
the following components (figure 1-8).
a. Cell Membrane. Each somatic (body) cell is surrounded by a semi
permeable membrane that controls the exchange of nutrients and waste between the
cell and its environment.
b. Nucleus. A cell usually contains a nucleus surrounded by some form of
protoplasm and enclosed by a semi permeable membrane. The nucleus has been
called the control center of the cell. One important known function of the nucleus is the
transmission of hereditary traits. This is actually carried out by the chromosomes or
chromatin material that is found in the nucleus of the cell. Chromosomes are made up
of DNA (deoxyribonucleic acid), which has been called the "super molecule." The cell
nucleus also contains a thick fluid called karyoplasms in which are suspended small
bodies known as nucleoli.
c. Cytoplasm. Cytoplasm is the fluid substance surrounding the nucleus and
surrounded by the cell membrane. Suspended within the semi liquid cytoplasm are
living bodies (organelles) and nonliving substances such as proteins, fat droplets,
pigments, and various crystals. Within the cytoplasm are found the following:
(1) Centrosome. Usually located near the cell nucleus and believed to
initiate cell division.
(2) Mitochondria. Minute fluid bodies in which numerous chemical reactions
occur from which the cell's energy is derived. Mitochondria are sometimes called the
power plants of the cells.
(3) Lysosomes. Digestive centers which break down large molecules and
which aid in destroying the cell at the end of its useful life.
Ribosomes. Believed to synthesize protein.