Essential Cell Biology 5th edition

The Eukaryotic Cell
21
lysosome
nuclear
envelope
mitochondrion
peroxisome
Golgi
apparatus
cytosol
Figure 1–24 Membrane-enclosed
organelles are distributed throughout the
eukaryotic cell cytoplasm. (A) The various
types of membrane-enclosed organelles,
shown in different colors, are each
specialized to perform a different function.
(B) The cytoplasm that fills the space outside
of these organelles is called the cytosol
(colored blue).
transport
vesicle
(A)
endoplasmic
reticulum
(B)
plasma
membrane
which modifies and packages molecules made in the ER that are destined
to be either secreted from the cell or transported to another cell compartment.
Lysosomes are small, irregularly shaped organelles in which
intracellular digestion occurs,
ECB5
releasing
e1.23/1.24
nutrients from ingested food particles
into the cytosol and breaking down unwanted molecules for either
recycling within the cell or excretion from the cell. Indeed, many of the
large and small molecules within the cell are constantly being broken
down and remade. Peroxisomes are small, membrane-enclosed vesicles
that provide a sequestered environment for a variety of reactions in which
hydrogen peroxide is used to inactivate toxic molecules. Membranes also
form many types of small transport vesicles that ferry materials between
one membrane-enclosed organelle and another. All of these membraneenclosed
organelles are highlighted in Figure 1–24A.
A continual exchange of materials takes place between the endoplasmic
reticulum, the Golgi apparatus, the lysosomes, the plasma membrane,
and the outside of the cell. The exchange is mediated by transport vesicles
that pinch off from the membrane of one organelle and fuse with
another, like tiny soap bubbles that bud from and combine with other
bubbles. At the surface of the cell, for example, portions of the plasma
membrane tuck inward and pinch off to form vesicles that carry material
captured from the external medium into the cell—a process called endocytosis
(Figure 1–25). Animal cells can engulf very large particles, or even
entire foreign cells, by endocytosis. In the reverse process, called exocytosis,
vesicles from inside the cell fuse with the plasma membrane and
release their contents into the external medium (see Figure 1–25); most
of the hormones and signal molecules that allow cells to communicate
with one another are secreted from cells by exocytosis. How membraneenclosed
organelles move proteins and other molecules from place to
place inside the eukaryotic cell is discussed in detail in Chapter 15.
IMPORT BY ENDOCYTOSIS
endosome
plasma
membrane
Golgi
apparatus
The Cytosol Is a Concentrated Aqueous Gel of Large
and Small Molecules
If we were to strip the plasma membrane from a eukaryotic cell and
remove all of its membrane-enclosed organelles—including the nucleus,
endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, and
so on—we would be left with the cytosol (Figure 1−24B). In other words,
the cytosol is the part of the cytoplasm that is not contained within
intracellular membranes. In most cells, the cytosol is the largest single
compartment. It contains a host of large and small molecules, crowded
together so closely that it behaves more like a water-based gel than a
EXPORT BY EXOCYTOSIS
Figure 1–25 Eukaryotic cells engage in
continual endocytosis and exocytosis
across their plasma membrane. They
import ECB5 extracellular e1.24-1.25 materials by endocytosis
and secrete intracellular materials by
exocytosis. Endocytosed material is first
delivered to membrane-enclosed organelles
called endosomes (discussed in Chapter 15).