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4.5
THE ENDOMEMBRANE SYSTEM
The endomembrane system is a series of functionally
connected organelles in which lipids are assembled and
new polypeptide chains are modified. Its products are
sorted and shipped to different destinations. Figure 4.13
shows how its organelles—the ER, Golgi bodies, and
vesicles—interconnect with one another.
Endoplasmic Reticulum
The functions of the endomembrane system begin with
endoplasmic reticulum, or ER. In animal cells, the ER is
continuous with the nuclear envelope, and it extends
through the cytoplasm. Its membranes appear rough or
smooth, depending on whether ribosomes are attached
to the membrane facing the cytoplasm.
5 Vesicles budding from the
Golgi membrane transport
finished products to the plasma
membrane. The products are
released by exocytosis.
4 Proteins and lipids take on
final form in the space inside
the Golgi body. Different
modifications allow them to be
sorted out and shipped to their
proper destinations.
assorted
vesicles
Golgi
body
smooth ER
3 Vesicles bud from the ER
membrane and then transport
unfinished proteins and lipids
to a Golgi body.
2 In the membrane of smooth
ER, lipids are assembled from
building blocks delivered earlier.
We typically observe rough ER arranged into stacks of
flattened sacs with many ribosomes attached. Every new
polypeptide chain is synthesized on ribosomes. But only
the newly forming chains having a built-in signal can
enter the space within rough ER or become incorporated
into ER membranes. (The signal is a sequence of fifteen to
twenty specific amino acids.) Once the chains are in
rough ER, enzymes may attach oligosaccharides and
other side chains to them. Many specialized cells secrete
the final proteins. Rough ER is abundant in such cells.
For example, in your pancreas, ER-rich gland cells make
and secrete enzymes that end up in the small intestine
and help digest your meals.
Smooth ER is free of ribosomes and curves through
cytoplasm like connecting pipes. Many cells assemble
most lipids inside the pipes. Smooth ER is well
developed in seeds. In liver cells,
certain drugs as well as toxic metabolic wastes are inactivated in it.
Sarcoplasmic reticulum, a type of
smooth ER in skeletal muscle cells,
functions in muscle contraction.
Some vesicles form
at the plasma
membrane, then
move into the
cytoplasm. These
endocytic vesicles
might fuse with the
membrane of other
organelles or may
remain intact, as
storage vesicles.
Other vesicles bud
from ER and Golgi
membranes, then
fuse with the plasma
membrane. The
contents of these
exocytic vesicles
are thereby
released from the
cell.
rough ER
1 Some polypeptide chains
enter the space inside rough
ER. Modifications begin that
will shape them into the final
protein form.
SECRETORY PATHWAY
DNA instructions for
building polypeptide
chains leave the nucleus
and enter the cytoplasm.
The chains (green) are
assembled on ribosomes
in the cytoplasm.
Golgi Bodies
In Golgi bodies, enzymes put the
finishing touches on proteins and
lipids, sort them out, then package
them inside vesicles for shipment to
specific locations. For example, an
enzyme in one Golgi region might
attach a phosphate group to a new
protein, thereby giving it a mailing
tag to its proper destination.
Commonly, a Golgi body looks
vaguely like a stack of pancakes; it is
composed of a series of flattened
membrane-bound sacs (Figure 4.14).
In functional terms, the last portion
of a Golgi body corresponds to the
top pancake. Here, vesicles form as
patches of the membrane bulge out,
then break away into the cytoplasm.
Figure 4.13 Endomembrane system, a
membrane system in the cytoplasm that
synthesizes, modifies, packages, and
ships proteins and lipids. Green arrows
highlight a secretory pathway by which
some proteins and lipids are packaged
and released from many types of cells,
including gland cells that secrete mucus,
sweat, and digestive enzymes.
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internal space
budding vesicle
0.25 μm
Figure 4.14 Sketch and micrograph of a
Golgi body from an animal cell.
A Variety of Vesicles
Vesicles are tiny, membranous sacs that move through
the cytoplasm or take up positions in it. The lysosome, a
common type, buds from Golgi membranes of animal
cells and some fungal cells. Lysosomes are organelles of
intracellular digestion. They hold enzymes that digest
complex carbohydrates, proteins, nucleic acids, and some
lipids. Often, they fuse with vesicles formed earlier from
patches of plasma membrane that surrounded bacteria,
molecules, and other items that docked at the membrane
receptors. Lysosomes also digest entire cells and cell
parts. For example, as a tadpole is developing into an
adult frog, its tail slowly disappears. Lysosomal enzymes
are responding to developmental signals and are helping
to destroy cells that make up the tail.
Peroxisomes, another type, are sacs of enzymes that
break down fatty acids and amino acids. A product of
the reactions, hydrogen peroxide, is toxic, as the Chapter
5 introduction describes. But enzyme action converts it to
water and oxygen or uses it to break down alcohol and
other toxins. Drink alcohol, and the peroxisomes of liver
and kidney cells will degrade nearly half of it.
In the ER and Golgi bodies of the endomembrane system,
many proteins take on final form and lipids are assembled.
Lipids, proteins (such as enzymes), and other items become
packaged in vesicles destined for export, storage, membrane
building, intracellular digestion, and other cell activities.
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