materials between the cell and its environment by
physical and chemical means. Solids and gases,
such as oxygen, proteins, carbohydrates, and
mineral salts, pass through the plasma membrane
by a process known as DIFFUSION.
The NUCLEUS is a small, dense, usually
spherical body that controls the chemical reactions
occurring in the cell. The substance contained in
the nucleus is called NUCLEOPLASM. It is also
important in the cells reproduction, since genetic
information for the cell is stored there. Every
human cell contains 46 chromosomes, and each
chromosome has thousands of genes that deter-
mine the cells function.
The CYTOPLASM is a water-to-gelatinous
substance surrounding the nucleus and is con-
tained by the plasma membrane. The cytoplasm
is all of the cell protoplasm except the nucleus.
The simplest living organism consists of a
single cell. The amoeba is a unicellular animal.
The single cell of such a one-celled organism must
be able to carry on all processes necessary for life.
This cell is called a SIMPLE or UNDIFFEREN-
In multicellular organisms, cells vary in size,
shape, and number of nuclei. When stained, the
various cell structures can be more readily
recognized under a microscope. Other differences
such as the number and type of cells can be seen
with the aid of a microscope. Many cells are highly
specialized. SPECIALIZED CELLS perform
special functions, such as muscle, which contracts,
or epithelial cells of the skin, which protect.
Tissues are groups of specialized cells similar
in structure and function. They are classified into
five main groups: epithelial, connective, muscular,
liquid, and nervous.
1. EPITHELIAL. The lining tissue of the
body is called epitheliums. It forms the
outer covering of the body known as the
free surface of the skin. It also forms the
lining of the digestive, respiratory, and
urinary tracts; blood and lymph vessels;
serous cavities; and tubules of certain
secretory glands, such as the liver and
kidneys. This tissue has little intercellular
fluid and may be further subdivided into
a. Columnar. The chief functions of this
tissue are to secrete digestive fluids and
absorb digested foods and fluids. It
consists of long narrow cells set close
together, resembling a palisade-type
fence (fig. 3-3). In certain areas, such
as the nostrils, bronchial tubes, and tra-
chea, this tissue has a crown of micro-
scopic hairlike processes known as cilia.
These cilia provide motion to move
secretions and other matter along the
surfaces from which they extend. They
also act as a barrier, preventing foreign
matter from entering these cavities.
Squamous. This is the main protective
tissue of the body. It is composed of
thin platelike or scalelike cells forming
a mosaic pattern (fig. 3-4). This tissue
Figure 3-3.Columnar epithelial tissue
Figure 3-4.Squamous epithelial tissue.