52
Chapter 3
or to the plasma membrane, where the protein contents of the
vesicle are released to the outside of the cell. Vesicles containing
proteins to be secreted from the cell are known as
secretory
vesicles.
Such vesicles are found, for example, in certain endo-
crine gland cells, where protein hormones are released into the
extracellular fl uid to modify the activities of other cells.
Endosomes
A number of membrane-bound vesicular and tubular struc-
tures called
endosomes
lie between the plasma membrane
and the Golgi apparatus. Certain types of vesicles that pinch
off the plasma membrane travel to and fuse with endosomes.
In turn, the endosome can pinch off vesicles that then move
to other cell organelles or return to the plasma membrane.
Like the Golgi apparatus, endosomes are involved in sorting,
modifying, and directing vesicular traffi c in cells.
Mitochondria
Mitochondria
(singular,
mitochondrion
) participate in the
chemical processes that transfer energy from the chemical
bonds of nutrient molecules to newly created adenosine tri-
phosphate (ATP) molecules, which are then made available to
cells. Most of the ATP that cells use is formed in the mito-
chondria by a process called cellular respiration, which con-
sumes oxygen and produces carbon dioxide, heat, and water.
Mitochondria are spherical or elongated, rodlike struc-
tures surrounded by an inner and an outer membrane (
Figure
3–14
). The outer membrane is smooth, whereas the inner
membrane is folded into sheets or tubules known as
cristae,
which extend into the inner mitochondrial compartment, the
matrix.
Mitochondria are found throughout the cytoplasm.
Large numbers of them, as many as 1000, are present in cells
that utilize large amounts of energy, whereas less active cells
contain fewer.
In addition to providing most of the energy needed to
power physiological events such as muscle contraction, mito-
chondria also play a role in the synthesis of certain lipids, such
as the hormones estrogen and testosterone (Chapter 11).
Lysosomes
Lysosomes
are spherical or oval organelles surrounded by a
single membrane (see Figure 3–4). A typical cell may con-
tain several hundred lysosomes. The fl uid within a lysosome is
acidic and contains a variety of digestive enzymes. Lysosomes
act as “cellular stomachs,” breaking down bacteria and the
debris from dead cells that have been engulfed by a cell. They
Function:
Concentrates,
modifies, and sorts proteins
arriving from the rough
endoplasmic reticulum prior
to their distribution, by way
of the Golgi vesicles, to other
organelles or to secretion
from the cell.
Structure:
Series of cup-shaped,
closely apposed, flattened,
membranous sacs; associated
with numerous vesicles.
Generally, a single Golgi
apparatus is located in the central
portion of a cell near its nucleus.
Golgi apparatus
Golgi apparatus
Membrane-enclosed
vesicle
Figure 3–13
Golgi apparatus.
Electron micrograph from W. Bloom and D. W. Fawcett,
Textbook of Histology,
9th ed. W. B. Saunders Company, Philadelphia, 1968.
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