3.05:_Endoplasmic_Reticulum

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<p class="lt-bio-3969" style="background-color: unset;">The <strong>endoplasmic reticulum</strong> (ER) is a system of membrane-enclosed sacs and tubules in the cell. Their lumens are probably all interconnected, and their membranes are continuous with the outer membrane of the nuclear envelope. All the materials within the system are separated from the cytosol by a membrane. The endoplasmic reticulum is the site where the cell manufactures most of the membranes of the cell (plasma membrane, Golgi apparatus, lysosomes, nuclear envelope), lipids (including lipids for membranes, e.g., of the mitochondria, that are not made by the ER), and transmembrane proteins and secreted proteins. The ER comes in two versions: the <strong>rough endoplasmic reticulum</strong> (RER) and the<strong> smooth endoplasmic reticulum</strong> (SER).</p>

<span id="Rough_Endoplasmic_Reticulum_(RER)"></span><span id="Rough_Endoplasmic_Reticulum_(RER)"></span><h2 style="background-color: unset;" class="lt-bio-3969">Rough Endoplasmic Reticulum (RER)</h2>

<p class="lt-bio-3969" style="background-color: unset;">The RER is typically arranged as interconnecting stacks of disc-like sacs. The cytosolic surface of the RER is studded with ribosomes engaged in protein synthesis. As the messenger RNA is translated by the ribosome, the growing polypeptide chain is inserted into the membrane of the RER. Proteins destined to be secreted by the cell or shipped into the lumen of certain other organelles like the Golgi apparatus and lysosomes pass all the way through into the lumen of the RER. Transmembrane proteins destined for the plasma membrane or the membrane of those organelles are retained within the membrane of the RER.</p>

<figure><img height="226" width="403" class="internal" alt="Black and white image showing a detailed texture of wavy, parallel lines resembling the pattern seen in a cross-section of muscle fibers." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5374/RER.jpg?revision=2" />
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data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>: This electron micrograph (courtesy of Keith Porter) shows the RER in a bat pancreas cell. The clearer areas are the lumens.</figcaption>
</figure>

<p class="lt-bio-3969" style="background-color: unset;">In either case, the portion of the protein within the lumen of the RER is subject to extensive <strong>glycosylation</strong> (primarily N-linked) in Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="97" style="font-size: 85%; position: relative;"><svg width="4.652ex" height="1.557ex" role="img" focusable="false" viewbox="0 -666 2056 688" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-98-NCM-N-33" d="M303 353C369 378 431 441 431 526C431 569 410 604 369 631C333 654 292 666 246 666C201 666 162 654 127 631C88 605 68 571 68 528C68 495 90 472 122 472C154 472 176 495 176 527C176 560 157 578 119 580C145 615 186 633 242 633C302 633 332 598 332 527C332 485 324 450 309 421C282 373 245 364 183 364C171 362 165 357 165 348C165 333 172 333 192 333L235 333C310 333 348 280 348 173C348 88 317 14 241 14C176 14 128 36 99 80C134 79 160 105 160 139C160 173 135 198 101 198C62 198 42 178 42 137C42 88 64 49 108 18C147-9 193-22 244-22C301-22 350-3 393 34C436 71 457 117 457 173C457 267 383 332 303 353Z"></path><path id="MJX-98-NCM-N-2E" d="M192 53C192 82 168 106 139 106C110 106 86 82 86 53C86 24 110 0 139 0C168 0 192 24 192 53Z"></path><path id="MJX-98-NCM-N-35" d="M118 315C123 315 129 319 134 326C164 371 205 393 257 393C292 393 319 373 337 332C348 305 354 264 354 209C354 146 346 102 331 76C306 35 272 14 229 14C162 14 109 62 91 114C94 113 96 114 100 114C130 114 155 137 155 167C155 198 130 219 100 219C65 219 50 200 50 163C50 63 131-22 231-22C292-22 344 0 386 44C428 88 449 141 449 202C449 260 432 310 398 353C361 400 315 423 259 423C212 423 171 408 138 378L138 556C165 548 191 544 218 544C264 544 304 555 338 578C369 597 390 616 402 634C408 642 411 648 411 651C411 661 406 666 396 666C341 645 294 634 256 634C211 634 168 643 127 662C122 664 118 665 114 665C105 665 100 656 100 637L100 345C100 324 100 315 118 315Z"></path><path id="MJX-98-NCM-N-31" 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data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>. The RER takes up a large proportion of the cytoplasm of cells specialized for protein synthesis such as cells secreting digestive enzymes (e.g. the pancreas cell above) and antibody-secreting plasma cells</p>

<span id="Smooth_Endoplasmic_Reticulum_(SER)"></span><span id="Smooth_Endoplasmic_Reticulum_(SER)"></span><h2 style="background-color: unset;" class="lt-bio-3969">Smooth Endoplasmic Reticulum (SER)</h2>

<p class="lt-bio-3969" style="background-color: unset;">The SER differs from the RER in lacking attached ribosomes and usually being tubular rather than disc-like. A major function of the SER is the synthesis of lipids from which various cell membranes are made and like steroids, they are secreted from the cell. The SER represents only a small portion of the ER is most cells, e.g. serving as transport vesicles for the transport of protein to the Golgi apparatus. However, it is a prominent constituent of some cells, especially the cells of the adrenal cortex (which secrete steroid hormones), the cells of the liver (hepatocytes) where it synthesis lipids for secretion of lipoproteins, and the sarcoplasmic reticulum of muscle cells is SER.</p>

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<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/03:_The_Cellular_Basis_of_Life/3.05:_Endoplasmic_Reticulum">3.5: Endoplasmic Reticulum</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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