38 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Figure 2.9:(Transmission electron micrograph.) Fusion of synaptic vesicles with the nerve cell membrane (upper
solid line) at the junction, or synapse, between a neuron (above) and a muscle fiber (below). A vesicle at the left has
arrived but not yet fused; two in the center are in the process of fusion, releasing their contents; one on the right is
almost completely incorporated into the cell membrane. Vesicle fusion is the key event in the transmission of nerve
impulses from one neuron to the next; see Chapter 12. [Digital image kindly supplied by J. Heuser.] [Copyrighted
figure; permission pending.]
obtain high-quality energy by capturing sunlight.- The cells of fungi such as yeast, as well as those of plants, also contain internal storage
areas called vacuoles (Figure 2.6). Like the cell itself, vacuoles also maintain electrical
potential drops across their bounding membranes (see Problem 11.3).
The part of the cytoplasm not contained in any membrane-bounded organelle is collectively called
the cell’scytosol.
In addition, cells create a variety ofvesicles(small bags). Vesicles can form by “endocytosis,”
as a part of the cell’s outer membrane engulfs some exterior object or fluid, then pinches off to
form an internal compartment. The resulting vesicle then fuses with internal vesicles containing
digestive enzymes, which break down its contents. Another class of vesicles are the “secretory
vesicles,” bags containing products destined for delivery outside the cell. A particularly important
class of secretory vesicles are thesynaptic vesicles,which hold neurotransmitters at the ends of
nerve cells. When triggered by an arriving electrical impulse, the synaptic vesicles fuse with the
outer membrane of the nerve cell (Figure 2.9), release their contents, and thus stimulate the next
cell in a neural pathway (see Chapter 12).
Other elements In addition to the membrane-bounded structures listed above, cells construct
various other structures visible with the light microscope. For example, during mitosis the chro-
mosomes condense into individual objects, each with a characteristic shape and size (Figure 2.10).
Another class of structures, the cytoskeletal elements, will appear in Section 2.2.4.
2.1.2 External gross anatomy
Though many cells have simple spherical or brick-shaped forms, still others can have a much richer
external anatomy. For example, the fantastically complex, branched form of nerve cells (Figure 2.22
on page 51) allows them to connect to their neighbors in a correspondingly complex way. Each