Essential Cell Biology 5th edition

498 CHAPTER 15 Intracellular Compartments and Protein Transport
The Golgi apparatus, which is usually situated near the nucleus, receives
proteins and lipids from the ER, modifies them, and then dispatches them
to other destinations in the cell. Small sacs of digestive enzymes called
lysosomes degrade worn-out organelles, as well as macromolecules and
particles taken into the cell by endocytosis. On their way to lysosomes,
endocytosed materials must first pass through a series of compartments
called endosomes, which sort the ingested molecules and recycle some
of them back to the plasma membrane. Peroxisomes are small organelles
that contain enzymes that break down lipids and destroy toxic molecules,
producing hydrogen peroxide. Mitochondria and (in plant cells) chloroplasts
are each surrounded by a double membrane and are the sites of
oxidative phosphorylation and photosynthesis, respectively (discussed in
Chapter 14); both contain internal membranes that are highly specialized
for the production of ATP.
Many of the membrane-enclosed organelles, including the ER, Golgi
apparatus, mitochondria, and chloroplasts, are positioned in the cell by
attachment to the cytoskeleton, especially to microtubules. Cytoskeletal
filaments provide tracks for moving the organelles around and for directing
the traffic of vesicles between one organelle and another. These
movements are driven by motor proteins that use the energy of ATP
hydrolysis to propel the organelles and vesicles along the filaments, as
discussed in Chapter 17.
On average, the membrane-enclosed organelles together occupy nearly
half the volume of a eukaryotic cell (Table 15–2), and the total amount of
membrane associated with them is enormous. In a typical mammalian
cell, for example, the area of the endoplasmic reticulum membrane is
20–30 times greater than that of the plasma membrane. In terms of its
area and mass, the plasma membrane is only a minor membrane in most
eukaryotic cells.
Much can be learned about the composition and function of an organelle
once it has been isolated from other cell structures. For the most part,
organelles are far too small to be isolated by hand, but it is possible to
separate one type of organelle from another by differential centrifugation
(described in Panel 4–3, pp. 164–165). Once a purified sample of one type
of organelle has been obtained, the organelle’s proteins can be identified.
In many cases, the organelle itself can be incubated in a test tube under
conditions that allow its functions to be studied. Isolated mitochondria,
for example, can produce ATP from the oxidation of pyruvate to CO 2 and
water, provided they are adequately supplied with ADP, inorganic phosphate,
and O 2 .
TABLE 15–2 THE RELATIVE VOLUMES AND NUMBERS OF THE MAJOR
MEMBRANE-ENCLOSED ORGANELLES IN A LIVER CELL (HEPATOCYTE)
Intracellular Compartment
Percentage of
Total Cell Volume
Approximate
Number per Cell
Cytosol 54 1
Mitochondria 22 1700
Endoplasmic reticulum 12 1
Nucleus 6 1
Golgi apparatus 3 1
Peroxisomes 1 400
Lysosomes 1 300
Endosomes 1 200