the pores. This is important, since the cell must obtain the nutrients for its growth from
the extracellular fluid (fluid outside the cell) and discard waste products back into the
b. Cytoplasm (Figure 2-2). Cytoplasm is the fluid or semifluid contained inside
the cell membrane, but outside the nucleus. The cytoplasm functions as a medium to
contain many substances, such as fats, glucose, proteins, water, and electrolytes. The
clear portion of the cytoplasm is called hyaloplasm. Located within the cytoplasm are
the organelles that perform highly specialized functions in the cell.
c. Nucleus (Figure 2-2). The nucleus is the control center for the cell. It
controls the reproduction of the cell as well as the chemical reactions that occur within
the cell. The nucleus contains large amounts of deoxyribonucleic acid (DNA). The
DNA is responsible for controlling the characteristics of the protein enzymes of the
cytoplasm, and thus, it controls cytoplasmic activities. The DNA is also responsible for
controlling the hereditary characteristics of individuals.
d. Mitochondria (Figure 2-2). The mitochondria may be called the power
house" of the cell. The mitochondria are the site of cell respiratory activity. The
mitochondria are found in the cytoplasm. They are usually located near energy
requiring structures (that is, nodes of nerves, contracting ligaments of muscles, active
transport mechanisms in membranes and ribosomes). Their numbers depend on the
amount of energy required by the cell to perform its function. Several infoldings of the
inner unit membrane form shelves on which practically all of the oxidative enzymes of
the cell are said to be absorbed. When nutrients and oxygen meet these enzymes, they
combine to form carbon dioxide, water, and energy. The liberated energy is used to
synthesize ATP (adenosine triphosphate). This ATP then diffuses throughout the cell
and releases its energy whenever it is needed for cellular functions.
e. Lysosomes (Figure 2-2). Lysosomes may be called the digestive organs of
the cell. Lysosomes are surrounded by a membrane and contain digestive (hydrolytic)
enzymes. When this membrane ruptures, it releases the digestive enzymes that will
break down particles or molecules located near the ruptured area. For example, they
surround pinocyticle vesicles containing food particles and digest them. If a sufficient
number of lysosomes rupture, the entire cell may be digested. When the lysosomes
function properly, products of digestion can be used by the cell.
f. Nucleoli (Figure 2-2). In the nucleus of many cells, there may be one or
more structures called nucleoli. The nucleoli do not have a limiting membrane, as do
most organelles. These structures are primarily aggregate of loosely bound granules
composed mainly of ribonucleic acid (RNA). Hereditary units called genes are thought
to synthesize and store in the nucleolus. This stored RNA diffuses into the cytoplasm
where it controls cytoplasmic function. Therefore, the main functions of the nucleolus
are the synthesis of RNA and the storage of RNA.