PHYSIOLOGY OF CELLS AND
Section I. CELLS
2-1. THE CELLULAR LEVEL
a. The individual cell is the unit of structure of all living things. An entire
organism may consist of a single cell (unicellular) or many cells (multicellular).
b. In human beings and other multicellular organisms, the cells tend to be
organized in specific ways. A group of like cells performing a particular function is
referred to as a tissue. An organ is a discrete structure composed of several different
tissues together. An organ system is a group of organs together performing an overall
function. (An example of an organ system is the digestive system.) The individual
organism is the combination of all of these things as a discrete and separate entity.
c. Although all living matter is composed of cells, animal cells and plant cells are
significantly different from each other. Not only do plant cells contain chlorophyll, a
green coloring matter; plant cells also have a cell wall around them which is made up of
a very complex carbohydrate known as cellulose. Neither chlorophyll nor a cell wall is
present in connection with animal cells.
2-2. THE MAJOR COMPONENTS OF A "TYPICAL" ANIMAL CELL
A "typical" animal cell is illustrated in Figure 2-1.
a. Cell Membrane. As its outer boundary, the animal cell has a special structure
called the cell or plasma membrane. All of the substances that enter or leave the cell
must in some way pass through this membrane.
b. Protoplasm. The major substance of the cell is known as protoplasm. It is a
combination of water and a variety of materials dissolved in the water. Outside the cell
nucleus (see below), protoplasm is called cytoplasm. Inside the cell nucleus,
protoplasm is called nucleoplasm.
c. Organelles. Within the cytoplasm, certain structures are called organelles.
These organelles include structures such as the endoplasmic reticulum, ribosomes,
various kinds of vacuoles, the Golgi apparatus, mitochondria, and centrioles.
(1) The endoplasmic reticulum resembles a circulatory system for the
individual cell. It is a network composed of unit (single-thickness) membranes.