TYPES OF CELLS
a. General. The cell is the basic unit, or building block, of all living matter--plant
or animal. A microorganism may consist of a single cell (unicellular) or, as in the higher
forms of life, it may consist of a number of cells arranged to perform specific functions
(multicellular). The cell determines not only the structure of organism, but also its
function. The cell itself is made up of components that have specific functions, just as
the organs comprising the body have specific functions. Modern classification systems
are based largely on differences in cell types. The more primitive cells are known as
prokaryotes, whereas higher forms are known as eucaryotes. Certain microorganisms
exhibit prokaryotic cell types while other microbes are of the eucaryotic cell type.
b. Eucaryotic Cell Components. Eucaryotic cell organization is common to
fungi and protozoa as well as to all higher multicellular organisms, including humans.
Since every organism has a distinctive type of cell, there is no such thing as a "typical"
cell. However, most eucaryotic cells consist of the following parts (figure 2-1).
(1) Cytoplasm. The cell is filled with a jelly-like fluid called the cytoplasm.
Proteins and dissolved nutrients that are suspended in this fluid are used by the cell for
metabolism and to build new cell structures. The cytoplasm contains organelles,
intracellular structures bounded by membranes that separate their contents from the
cytoplasm. Some organelles are described below.
(2) Nucleus. The organelle that contains the genetic material
(Chromosomes) and nucleolus.
(3) Genetic material, chromosomes. The genetic material contains the
hereditary characteristics of the cell. It is made up op several long strands of DNA
(deoxyribonucleic acid) that are called chromosomes.
(4) Nucleolus. An area of the nucleus that manufactures structural
components used in protein synthesis.
(5) Nuclear membrane. Separates the nucleus from the cytoplasm,
although pores permit the entrance and exit of certain chemicals.
(6) Mitochondria. Site where metabolic enzymes perform respiration
(chemical oxidation) within the cell (conversion of food energy.)
(7) Cell membrane. Encloses the cell and governs the exchange of food
material and release of waste products between the cell and its surroundings. In certain
types of organisms, the cell membrane is enclosed within a cell wall that provides
strength and rigidity, and is common to all classes of microorganisms.