The Endomembrane System
Much of the volume of some eukaryotic cells is taken up by an extensive endomembrane system. This system includes two main components, the endoplasmic reticulum and the Golgi apparatus. Continuities between the nuclear envelope and the endomembrane system are visible under the electron microscope. Tiny, membrane-surrounded droplets called vesiclesappear to shuttle between the various components of the endomembrane system. This system has various structures but all of them are essentially compartments, closed off by their membranes from the cytoplasm. In this section, we will examine the functional significance of these compartments and we will see how materials synthesized in one organelle, the endoplasmic reticulum, are transferred to another organelle, the Golgi apparatus, for further processing, storage or transport. We will also describe the role of the lysosome in cellular digestion.
The endoplasmic reticulum is a complex factory
Electron micrographs reveal a network of interconnected membranes branching throughout the cytoplasm of a eukaryotic cell, forming tubes and flattened sacs. These membranes are collectively called the endoplasmic reticulum, or ER. The interior compartment of the ER, referred to as the lumen, is separate and distinct from the surrounding cytoplasm. The ER can enclose up to 10 percent of the interior volume of the cell, and its foldings result in a surface area many times greater than that of the plasma membrane.
Parts of the ER are studded with ribosomes, which are temporarily attached to the outer faces of its flattened sacs. These regions are called rough endoplasmic reticulum, or RER.
As a compartment, it segregates certain newly synthesized proteins away from the cytoplasm and transports them to other locations in the cell. While inside the RER, proteins can be chemically modified so as to alter their function and eventual destination. The attached ribosomes are sites for the synthesis of proteins that function outside the cytosol—that is, proteins that are to be exported from the cell incorporated into membranes or moved into the organelles of the endomembrane system. These proteins enter the lumen of the ER as they are synthesized. Once in the lumen of the ER, these proteins undergo several changes, including the formation of disulfide bridges and folding into their tertiary structures.
Proteins gain carbohydrate groups in the RER, thus becoming glycoproteins. In the case of proteins directed to the lysosomes, the carbohydrate groups are part of an “addressing” system that ensures that the right proteins are directed to the organelle.
Smooth endoplasmic reticulumor SERis more tubular (less like flattened sacs) and lacks ribosomes.
Within the lumen of the SER, proteins that have been synthesized on the RER are chemically modified. In addition, the SER has three other important roles:
It is responsible for chemically modifying small molecules taken in by the cell. This is especially true for drugs and pesticides.
It is the site for the hydrolysis of glycogen in animal cells.
It is the site for the synthesis of lipids and steroids. Cells that synthesize a lot of protein for export are usually packed with endoplasmic reticulum. Examples include glandular cells that secrete digestive enzymes and plasma cells that secrete antibodies. In contrast, cells that carry out less protein synthesis (such as storage cells) contain less ER. Liver cells which modify molecules that enter the body from the digestive system, have abundant smooth ER.
The Golgi apparatus stores,modifies, and packages proteins
The exact appearance of the Golgi apparatus (named for its discoverer, Camillo Golgi) varies from species to species but it always consists of flattened membranous sacs called cisternae and small membrane-enclosed vesicles. The cisternae appear to be lying together like a stack of saucers. The entire apparatus is about 1 m long.
The Golgi apparatus has several roles:
It receives proteins from the ER and may further modify them.
It concentrates, packages, and sorts proteins before they are sent to their cellular or extracellular destinations.
It is where some polysaccharides for the plant cell wall are synthesized.
In the cells of plants, protists, fungi, and many invertebrate animals, the stacks of cisternae are individual units scattered throughout the cytoplasm. In vertebrate cells, a few such stacks usually form a larger, single, more complex Golgi apparatus. The Golgi apparatus appears to have three functionally distinct parts: a bottom, a middle, and a top. The bottom cisternae, constituting the cis region of the Golgi apparatus, lie nearest to the nucleus or a patch of RER. The top cisternae constituting the trans region, lie closest to the surface of the cell. The cisternae in the middle make up the medial region of the complex. These three parts of the Golgi apparatus contain different enzymes and perform different functions. The Golgi apparatus receives proteins from the ER, packages them, and sends them on their way. Since there is often no direct membrane continuity between ER and Golgi apparatus, how does a protein get from one organelle to the other? The protein could simply leave the ER, travel across the cytoplasm, and enter the Golgi apparatus. But that would expose the protein to interactions with other molecules in the cytoplasm. On the other hand, segregation from the cytoplasm could be maintained if a piece of the ER could “bud off,” forming a vesicle that contains the protein— and that is exactly what happens. The protein makes the passage from ER to Golgi apparatus safely enclosed in the vesicle. Once it arrives, the vesicle fuses with the membrane of the Golgi apparatus, releasing its cargo. Vesicles form from the rough ER, move through the cytoplasm and fuse with the cis region of the Golgi apparatus, releasing their contents into the lumen. If living cells are stained specifically for ER and Golgi apparatus, the Golgi apparatus can be seen moving rapidly along the ER, possibly picking up vesicles as they go. Other small vesicles may move between the cisternae, transporting proteins, and it appears that some proteins move from one cisterna to the next by tiny channels. Vesicles budding off from the trans region carry their contents away from the complex.
Lysosomes contain digestive enzymes
Originating in part from the Golgi apparatus are organelles called lysosomes. They contain digestive enzymes, and they are the sites where macromolecules— proteins, polysaccharides, nucleic acids, and lipids—are hydrolyzed into their monomers. Lysosomes are about 1 m in diameter, are surrounded by a single membrane and have a densely staining, featureless interior. There may be dozens of lysosomes in a cell, depending on its needs. Lysosomes are sites for the breakdown of food and foreign objects taken up by the cell. These materials get into the cell by a process called phagocytosis(phago-, “eating”; cytosis, “cellular”), in which a pocket forms in the plasma membrane and eventually deepens and encloses material from outside the cell. This pocket becomes a small vesicle that breaks free of the plasma membrane to move into the cytoplasm as a phagosome containing food or other material. The phagosome fuses with a primary lysosome, forming a secondary lysosome where digestion occurs. The effect of this fusion is rather like releasing hungry foxes into a chicken coop: The enzymes in the secondary lysosome quickly hydrolyze the food particles. The products of digestion exit through the membrane of the lysosome, providing fuel molecules and raw materials for other cell processes. The “used” secondary lysosome, now containing undigested particles, then moves to the plasma membrane, fuses with it and releases the undigested contents to the environment. Lysosomes are also where the cell digests its own material in a process called autophagy. Autophagy is an ongoing process in which organelles such as mitochondria are engulfed by lysosomes and hydrolyzed to monomers, which pass out of the lysosome through its membrane into the cytoplasm for reuse. The importance of lysosome function is indicated by a group of human diseases called lysosomal storage diseases. If a cell lacks the ability to hydrolyze one or more macromolecules, these substances pile up in lysosomes, with harmful consequences. An example is Tay-Sachs disease, in which a lipid accumulates in the lysosomes of brain cells, resulting in death in early childhood. Plant cells do not appear to contain lysosomes, but the central vacuole of a plant cell (which we will describe below) may function in an equivalent capacity because it, like lysosomes, contains many digestive enzymes.
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