4.5

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.

THE ENDOMEMBRANE SYSTEM

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.

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 meta-bolic wastes are inactivated in it. Sarcoplasmic reticulum, a type of smooth ER in skeletal muscle cells, functions in muscle contraction.

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.

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.

assorted

vesicles

Golgi

body

smooth ER

rough ER

DNA instructions for

building polypeptide

chains leave the nucleus

and enter the cytoplasm.

The chains (green) are

assembled on ribosomes

in the cytoplasm.

SECRETORY PATHWAY

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.

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.

1 Some polypeptide chains

enter the space inside rough

ER. Modifications begin that

will shape them into the final

protein form.

Figure 7.4 Page 1

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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.

internal space budding vesicle

0.25 μm

Figure 4.14 Sketch and micrograph of a

Golgi body from an animal cell.

Figure 7.4 Page 2

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