These minute canals and vesicles (sacs) function as a kind of circulatory system to
transfer materials throughout a cell. The endoplasmic reticulum functions in protein
synthesis by serving as a site of attachment for the ribosomes, which in turn are the site
of protein synthesis.
d. Golgi Apparatus. The golgi apparatus is similar to the endoplasmic
reticulum in that it is a canalicular system. The surface membranes of the canals and
vesicles, however, are always smooth. This greatly contrasts with the outer surfaces of
the endoplasmic reticulum, which are frequently encrusted with rough granules. The
function of the golgi apparatus is subject to debate, however, it is frequently associated
with temporary storage of secretory materials and preparation of these for final
secretion. The golgi apparatus may also synthesize certain compounds, for enzymes
have been extracted from membranes of the complex.
e. Cytoplasm. The 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, proteins, glucose, water, and electrolytes. The clear
fluid portion of the cytoplasm is called hyaloplasm. Located within the cytoplasm are
the organelles that perform highly specific functions in the cell.
f. Ribosomes. Ribosomes are the site of protein synthesis and are referred to
as "protein factories" of the cell. They are found either attached to the endoplasmic
reticulum or in small groups in the cytoplasm called polyribosomes. A ribosome is
composed mainly of RNA.
g. Mitochondria. Mitochondria are very dynamic and complex cellular
structures. In living preparations, mitochondria are in constant motion and either vibrate
in place or migrate to other portions of the cell. Mitochondria possess double
membranes; the inner membrane has tiny projections called cristae, which extend into
the lumen (hollow tube) of the mitochondria. Cristae greatly increase surface area within
the organelle. Mitochondria are found in areas of greatest cellular activity and are
commonly referred to as "powerhouses" of the cell. Almost every oxidative enzyme of
the cell is present on the inner surfaces of the mitochondrion. Here nutrients and
oxygen react to provide energy in the form of a substance called adenosine
triphosphate (ATP). ATP molecules then diffuse throughout the cell and provide energy
wherever it is needed for cellular functions. This process is known as cellular
h. Lysosomes. Lysosomes are small bodies containing hydrolytic enzymes. If
lysosomes should suddenly rupture, autolysis or selfdestruction of the cell occurs.
However, when lysosomes function properly, they play an important role in intracellular
digestion. For example, a lysosome can fuse with a food vacuole, and digestion occurs
within the composite structure formed. Products of digestion can then be utilized by the