E ndocytosis and exocytosis • Figure 3.17
Endocytosis brings material inside the cell, while exocytosis secretes material from the cell.
Both types of transport use vesicles that travel between the plasma membrane and organ-
elles, such as lysosomes, ER, and the Golgi complex. These processes recycle segments of
the plasma membrane.
Nucleus
Nuclear envelope.
Rough endoplasmic
reticulum
Cytosol
Secretory
vesicle
Golgi
.
complex
*
Secondary
A
lysosome /
e
Primary
lysosome
Proteins for use
inside cell
Plasma
membrane
Extracellular fluid
V
Endocytosis
ft
^
"
Exocytosis
In contrast, cells often release substances by a process
called exocytosis (eks'-o-si-TO-sis;
exo-
= out of) (Figure
3.17). Proteins or other secretory substances are made
within the ER and transported within vesicles to the Gol-
gi complex. In the Golgi complex, the vesicle contents are
processed and sorted to vesicles, which move to and fuse
with the plasma membrane. Exocytosis is the fusion of
these vesicles with the plasma membrane and the release
of their contents outside the cell. Many substances (such as
neurotransmitters, endocrine hormones, and digestive en-
zymes) are secreted via exocytosis. The same process allows
proteins, such as hormone receptors and enzymes, to be in-
serted into the plasma membrane; these proteins do not go
all the way through the membranes of the vesicles but rather
are embedded within those membranes. When the vesicles
fuse with the plasma membrane and dump their contents,
the membrane of a vesicle gets incorporated into the plasma
membrane, where the embedded proteins remain.
In addition to their roles in membrane transport and
the structure of the plasma membrane, proteins carry out
numerous functions in the cell. As mentioned in Chapter
2 and earlier in this chapter, the instructions to build pro-
teins are contained in the DNA located in the nucleus on
DNA segments called genes. But how are these instruc-
tions carried out? The following section provides a simple
analogy to help you understand protein synthesis.
Proteins Are Made
in a Complex Process
Building a protein is much like building a house. To build
a house, an architectural firm creates a set of plans. Those
plans are copied or transcribed into blueprints, which are
taken to the home site. Workers at the site bring the build-
ing materials to a scaffold at the home’s foundation. The
workers assemble the materials according to the blue-
prints, essentially translating the information contained
in the blueprint into the final product, the house.
Now let’s apply this analogy to protein synthesis.
When building a protein, the following events occur:
64 CHAPTER 3
Cells and Tissues
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