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 vesicles appear 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 eu-karyotic 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 (Figure 4.11). 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. Because of their appearance under the electron microscope,

Cells Slice Electron Microscopy

Lumen

4.11 The Endoplasmic Reticulum The transmission electron micrograph on the left shows a two-dimensional slice through the three-dimensional structures depicted in the drawing. In normal living cells, membranes never have open ends; they define closed compartments set off from the surrounding cytoplasm.

Ribosomes of the rough endoplasmic reticulum are sites for protein synthesis. They produce its rough appearance.

Lumen

Smooth endoplasmic reticulum is a site for lipid synthesis and chemical modification of proteins.

4.11 The Endoplasmic Reticulum The transmission electron micrograph on the left shows a two-dimensional slice through the three-dimensional structures depicted in the drawing. In normal living cells, membranes never have open ends; they define closed compartments set off from the surrounding cytoplasm.

Ribosomes of the rough endoplasmic reticulum are sites for protein synthesis. They produce its rough appearance.

Smooth endoplasmic reticulum is a site for lipid synthesis and chemical modification of proteins.

Protein Synthesis Rer

Rough ER

Smooth ER

Rough ER

Smooth ER

0.5 |m these regions are called rough endoplasmic reticulum, or

RER. RER has two roles:

► 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 (see Figure 3.4).

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 reticulum or SER is more tubular (less like flattened sacs) and lacks ribosomes (see Figure 4.11). 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 cis-ternae and small membrane-enclosed vesicles. The cisternae appear to be lying together like a stack of saucers (Figure 4.12). The entire apparatus is about 1 |im 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.

4.12 The Golgi Apparatus

The Golgi apparatus modifies proteins from the ER and "targets" them to the correct addresses within or outside the cell.

Golgi Complex Packing And Transporting

Plasma membrane

Outside of cell

4.12 The Golgi Apparatus

The Golgi apparatus modifies proteins from the ER and "targets" them to the correct addresses within or outside the cell.

Plasma membrane

Outside of cell

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 (see Figure 4.12). 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. The vesicles may not have far to go: 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 (see Figure 4.12).

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 (see Figure 3.3). Lysosomes are about 1 |im in diameter, are surrounded by a single membrane, and have a densely staining, featureless interior (Figure 4.13). 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 (see Figure 4.13). The phagosome fuses with a primary lyso-some, 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. These reactions are enhanced by the mild acidity of the lysosome's interior, where the pH is lower than in the surrounding cytoplasm. 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

The primary lysosome is generated by the Golgi.

Phagocytose Lysosome

The primary lysosome is generated by the Golgi.

Primary lysosome

Plasma membrane

Outside of cell

Food particles taken in by phagocytosis

4) Undigested materials are released when the digestion vesicle fuses with the plasma membrane.

Primary lysosome

| The lysosome fuses with a phagosome.

| Small molecules generated by digestion diffuse into the cytoplasm.

Plasma membrane

Outside of cell

Food particles taken in by phagocytosis

4) Undigested materials are released when the digestion vesicle fuses with the plasma membrane.

4.13 Lysosomes Isolate Digestive Enzymes from the Cytoplasm

Lysosomes are sites for the hydrolysis of material taken into the cell by phagocytosis.

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 macromole-cules, 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 lyso-somes, contains many digestive enzymes.

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Essentials of Human Physiology

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Responses

  • ABAALOM
    What are the organelles of the endomembrane system?
    5 years ago
  • raakel
    What are the main components of the endomembrane system?
    5 years ago
  • david
    How protein get transferred to plasma membrane?
    5 years ago
  • michael herz
    How does a protein go into the endomembrane system?
    5 years ago
  • jessica
    How to tell if its rough or smooth ER in a transmission electron micrograph of a plasma cell?
    5 years ago
  • Folcard Proudfoot
    How do newly synthesized proteins enter the endomembrane system?
    5 years ago
  • klaudia brandt
    What are two cells in the endomembrane system?
    5 years ago
  • Liliana Udinese
    Does the golgi apparatus produce its own membrane?
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  • julia
    Is the golgi apparatus the major storage site for a cell's digestive enzymes?
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  • keith roberson
    Why ser rer and golgi aparatus are part of endomembrane system?
    5 years ago
  • Scolastica
    What are the steps for the endomembrane?
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  • Robert
    WHAT IS SMOOTH ER IN Organelles of the endomembrane system?
    5 years ago
  • Dominik Eisenhower
    Which part of the endomembrane system modify and sorts proteins?
    5 years ago
  • stefan schmidt
    Where does lipid synthesis occur in the endomembrane system?
    4 years ago
  • eyob
    How nucleus, endoplasmic reticulum, golgi apparatus, lysosome and plasma membrane work together?
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  • selamawit yusef
    What is the link between the endomembrane system and the creation of the plasma membrane?
    4 years ago
  • P
    What cell organelle is responsible for digestive system where macromolecules are hydrolized?
    4 years ago
  • james
    Why is the plasma membrane a part of the Indo membrane system?
    2 years ago
  • hailey
    How does the endomembrane system compare to the nucleus?
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  • william taylor
    Is plasma membrane part of the endomembrane system?
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    How do newly synthesized proteins enter the endo membrane system?
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    Why is the plasma membrane a member of the endomembrane system?
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  • katherine
    How are proteins transported out of the endoplasmic reticulum to the golgi apparatus?
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