Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

641
Intracellular Compartments
and Protein Sorting
chapter
12
Unlike a bacterium, which generally consists of a single intracellular compartment
surrounded by a plasma membrane, a eukaryotic cell is elaborately subdivided
into functionally distinct, membrane-enclosed compartments. Each
compartment, or organelle, contains its own characteristic set of enzymes and
other specialized molecules, and complex distribution systems transport specific
products from one compartment to another. To understand the eukaryotic cell, it
is essential to know how the cell creates and maintains these compartments, what
occurs in each of them, and how molecules move between them.
Proteins confer upon each compartment its characteristic structural and
functional properties. They catalyze the reactions that occur there and selectively
transport small molecules into and out of the compartment. For membrane-enclosed
organelles in the cytoplasm, proteins also serve as organelle-specific surface
markers that direct new deliveries of proteins and lipids to the appropriate
organelle.
An animal cell contains about 10 billion (10 10 ) protein molecules of perhaps
10,000 kinds, and the synthesis of almost all of them begins in the cytosol, the
space of the cytoplasm outside the membrane-enclosed organelles. Each newly
synthesized protein is then delivered specifically to the organelle that requires it.
The intracellular transport of proteins is the central theme of both this chapter
and the next. By tracing the protein traffic from one compartment to another, one
can begin to make sense of the otherwise bewildering maze of intracellular membranes.
In This Chapter
The Compartmentalization
of Cells
THE TRANSPORT OF
MOLECULES BETWEEN THE
NUCLEUS AND THE CYTOSOL
THE TRANSPORT OF PROTEINS
INTO MITOCHONDRIA AND
CHLOROPLASTS
PEROXISOMES
THE ENDOPLASMIC RETICULUM
The Compartmentalization of Cells
In this brief overview of the compartments of the cell and the relationships
between them, we organize the organelles conceptually into a small number of
discrete families, discuss how proteins are directed to specific organelles, and
explain how proteins cross organelle membranes.
All Eukaryotic Cells Have the Same Basic Set of Membraneenclosed
Organelles
Many vital biochemical processes take place in membranes or on their surfaces.
Membrane-bound enzymes, for example, catalyze lipid metabolism; and oxidative
phosphorylation and photosynthesis both require a membrane to couple the
transport of H + to the synthesis of ATP. In addition to providing increased membrane
area to host biochemical reactions, intracellular membrane systems form
enclosed compartments that are separate from the cytosol, thus creating functionally
specialized aqueous spaces within the cell. In these spaces, subsets of molecules
(proteins, reactants, ions) are concentrated to optimize the biochemical
reactions in which they participate. Because the lipid bilayer of cell membranes is
impermeable to most hydrophilic molecules, the membrane of an organelle must
contain membrane transport proteins to import and export specific metabolites.
Each organelle membrane must also have a mechanism for importing, and incorporating
into the organelle, the specific proteins that make the organelle unique.