3.24:_Endocytosis

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<p class="lt-bio-4071" style="background-color: unset;">In endocytosis, the cell engulfs some of its extracellular fluid (ECF) including material dissolved or suspended in it. A portion of the plasma membrane is invaginated, coated with molecules of the protein clathrin, and pinched off forming a membrane-bounded vesicle called an endosome (Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="96" style="font-size: 85%; position: relative;" data-semantic-locale="en" data-semantic-domain="clearspeak" data-semantic-style="default" data-semantic-domain2style="mathspeak:default,clearspeak:default" data-semantic-collapsible="collapsible" data-semantic-expandable="expandable" data-semantic-level="Level" data-semantic-speech-none="3 period 24 period 1" data-semantic-speech="&lt;mark name=&quot;0&quot;/&gt; 3 &lt;mark name=&quot;1&quot;/&gt; period &lt;mark name=&quot;2&quot;/&gt; 24 &lt;mark name=&quot;3&quot;/&gt; period &lt;mark name=&quot;4&quot;/&gt; 1" data-speech-attached="true" data-semantic-braille="&#10300;&#10258;&#10280;&#10256;&#10300;&#10246;&#10290;&#10280;&#10256;&#10300;&#10242;" data-braille-attached="true" has-speech="true"><svg width="5.783ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2556 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-97-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-97-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-97-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path><path id="MJX-97-NCM-N-34" d="M353 677C344 677 336 672 330 663L28 199L28 163L289 163L289 81C289 63 285 51 278 46C271 41 252 39 219 39L194 39L194 0C223 2 269 3 331 3C393 3 439 2 468 0L468 39L443 39C410 39 391 41 384 46C377 51 373 63 373 81L373 163L471 163L471 202L373 202L373 660C373 670 366 677 353 677M295 553L295 202L67 202Z"></path><path id="MJX-97-NCM-N-31" d="M269 666C228 624 168 603 89 603L89 564C141 564 184 572 217 588L217 82C217 64 213 52 204 47C195 42 170 39 130 39L95 39L95 0C120 2 174 3 257 3C340 3 394 2 419 0L419 39L384 39C343 39 318 42 310 47C302 52 297 64 297 82L297 636C297 660 295 666 269 666Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="scale(1,-1)"><g data-mml-node="math" data-latex="\PageIndex{1}" data-semantic-structure="(5 0 1 2 3 4)"><g data-mml-node="TeXAtom" data-mjx-texclass="ORD" data-latex="1}" data-semantic-type="punctuated" data-semantic-role="sequence" data-semantic-annotation="depth:1" data-semantic-id="5" data-semantic-children="0,1,2,3,4" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{1};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true" data-semantic-speech-none="3 period 24 period 1" data-semantic-summary-none="period separated list of length 3" data-semantic-speech="&lt;mark name=&quot;0&quot;/&gt; 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<figure><img alt="Diagram illustrating endocytosis: phagocytosis with a food vacuole, pinocytosis with vesicles, and receptor-mediated endocytosis with receptors, coat protein, and coated vesicles." class="internal" loading="lazy" src="https://bio.libretexts.org/@api/deki/files/12467/Endocytosis_types.svg?revision=1" />
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It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. (Public Domain; Mariana Ruiz Villarreal <a title="User:LadyofHats" href="https://commons.wikimedia.org/wiki/User:LadyofHats" target="_blank" rel="external noopener nofollow" class="link-https">LadyofHats</a>)</figcaption>
</figure>

<span id="Phagocytosis"></span><span id="Phagocytosis"></span><h2 style="background-color: unset;" class="lt-bio-4071">Phagocytosis</h2>

<p class="lt-bio-4071" style="background-color: unset;">Phagocytosis (&quot;cell eating&quot;) results in the ingestion of particulate matter (e.g., bacteria) from the ECF. The endosome is so large that it is called a phagosome or vacuole. Phagocytosis occurs only in certain specialized cells (e.g., neutrophils, macrophages, the amoeba) and occurs sporadically.</p>

<figure><img class="internal" alt="Microscopic view of oval-shaped bacterial cells with textured surfaces, clustered together. The cells are dark against a lighter background." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6122/North.gif?revision=1" />
<figcaption>Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="98" style="font-size: 85%; position: relative;" data-semantic-locale="en" data-semantic-domain="clearspeak" data-semantic-style="default" data-semantic-domain2style="mathspeak:default,clearspeak:default" data-semantic-collapsible="collapsible" data-semantic-expandable="expandable" data-semantic-level="Level" data-semantic-speech-none="3 period 24 period 2" data-semantic-speech="&lt;mark name=&quot;0&quot;/&gt; 3 &lt;mark name=&quot;1&quot;/&gt; period &lt;mark name=&quot;2&quot;/&gt; 24 &lt;mark name=&quot;3&quot;/&gt; period &lt;mark name=&quot;4&quot;/&gt; 2" data-speech-attached="true" data-semantic-braille="&#10300;&#10258;&#10280;&#10256;&#10300;&#10246;&#10290;&#10280;&#10256;&#10300;&#10246;" data-braille-attached="true" has-speech="true"><svg width="5.783ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2556 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-99-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-99-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-99-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path><path id="MJX-99-NCM-N-34" d="M353 677C344 677 336 672 330 663L28 199L28 163L289 163L289 81C289 63 285 51 278 46C271 41 252 39 219 39L194 39L194 0C223 2 269 3 331 3C393 3 439 2 468 0L468 39L443 39C410 39 391 41 384 46C377 51 373 63 373 81L373 163L471 163L471 202L373 202L373 660C373 670 366 677 353 677M295 553L295 202L67 202Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="scale(1,-1)"><g data-mml-node="math" data-latex="\PageIndex{2}" data-semantic-structure="(5 0 1 2 3 4)"><g data-mml-node="TeXAtom" data-mjx-texclass="ORD" data-latex="2}" data-semantic-type="punctuated" data-semantic-role="sequence" data-semantic-annotation="depth:1" data-semantic-id="5" data-semantic-children="0,1,2,3,4" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{2};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true" data-semantic-speech-none="3 period 24 period 2" data-semantic-summary-none="period separated list of length 3" data-semantic-speech="&lt;mark name=&quot;0&quot;/&gt; 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North</figcaption>
</figure>

<p class="lt-bio-4071" style="background-color: unset;">This electron micrograph shows a guinea pig phagocyte ingesting polystyrene beads. Several beads are already enclosed in phagosomes while the others are in the process of being engulfed. In due course, phagosomes deliver their contents to lysosomes. The membranes of the two organelles fuse. Once inside the lysosome, the contents of the phagosome, e.g. ingested bacteria, are destroyed by the degradative enzymes of the lysosome.</p>

<span id="Games_parasites_play"></span><span id="Games_parasites_play"></span><h2 style="background-color: unset;" class="lt-bio-4071">Games parasites play</h2>

<p class="lt-bio-4071" style="background-color: unset;">Phagocytic cells, like macrophages and neutrophils, are an early line of defense against invading bacteria. However, some bacteria have evolved mechanisms to avoid destruction even after they have been engulfed by phagocytes.</p>

<p class="lt-bio-4071" style="background-color: unset;"><strong>Examples</strong>:</p>

<ul>
    <li class="lt-bio-4071" style="background-color: unset;"><strong><em>Salmonella enterica</em></strong> is a bacterium that causes food poisoning in humans. Once engulfed by phagocytosis, it secretes a protein that prevents the fusion of its phagosome with a lysosome.</li>
    <li class="lt-bio-4071" style="background-color: unset;"><strong>Mycobacteria</strong> (e.g., the tubercle bacillus that causes tuberculosis) use a different trick.
    <ul>
        <li class="lt-bio-4071" style="background-color: unset;">When the phagosome is first pinched off from the plasma membrane, it is coated with a protein called &quot;TACO&quot; (for tryptophan-aspartate-containing coat protein).</li>
        <li class="lt-bio-4071" style="background-color: unset;">This must be removed before the phagosome can fuse with a lysosome.</li>
        <li class="lt-bio-4071" style="background-color: unset;">Mycobacteria taken into a phagosome are able, in some way, to keep the TACO coat from being removed.</li>
        <li class="lt-bio-4071" style="background-color: unset;">Thus there is no fusion with lysosomes and the mycobacteria can continue to live in this protected intracellular location.</li>
    </ul>
    </li>
</ul>

<p class="lt-bio-4071" style="background-color: unset;">Some <strong>intracellular</strong> parasites exploit receptor-mediated endocytosis to sneak their way into their host cell. They have evolved surface molecules that serve as decoy ligands for receptors on the target cell surface. Binding to these receptors tricks the cell into engulfing the parasite.</p>

<p class="lt-bio-4071" style="background-color: unset;"><strong>Examples</strong>:</p>

<ul>
    <li class="lt-bio-4071" style="background-color: unset;"><strong>Epstein-Barr Virus</strong> (<strong>EBV</strong>). This virus causes mononucleosis and is a contributing factor in the development of Burkitt's lymphoma, a cancer of B lymphocytes. It binds to a receptor present on the surface of B cells.</li>
    <li class="lt-bio-4071" style="background-color: unset;"><strong>Influenza virus</strong>. The <strong>hemagglutinin</strong> on the surface of the virus binds to carbohydrate on the surface of the target cell tricking the cell into engulfing it.</li>
    <li class="lt-bio-4071" style="background-color: unset;"><strong><em>Listeria monocytogenes</em></strong>. This food-borne bacterium can be dangerous to people with defective immune systems as well as to pregnant women and their newborn babies. It has two kinds of surface molecules each a ligand for a different receptor on the target cell surface.</li>
    <li class="lt-bio-4071" style="background-color: unset;"><strong><em>Streptococcus pneumoniae</em></strong>. Epithelial cells like those in the nasopharynx have receptors that are responsible for transporting IgA and IgM antibodies from the blood to the apical surface of the cell. The pneumococcus piggybacks on this receptor on its return trip into the cell. <strong>This is the organism that led to the discovery that genes are DNA.</strong></li>
</ul>

<span id="Pinocytosis"></span><span id="Pinocytosis"></span><h2 style="background-color: unset;" class="lt-bio-4071">Pinocytosis</h2>

<p class="lt-bio-4071" style="background-color: unset;">In pinocytosis (&quot;cell drinking&quot;), the drop engulfed is relatively small (Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="99" style="font-size: 85%; position: relative;"><svg width="5.783ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2556 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-100-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-100-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-100-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path><path id="MJX-100-NCM-N-34" d="M353 677C344 677 336 672 330 663L28 199L28 163L289 163L289 81C289 63 285 51 278 46C271 41 252 39 219 39L194 39L194 0C223 2 269 3 331 3C393 3 439 2 468 0L468 39L443 39C410 39 391 41 384 46C377 51 373 63 373 81L373 163L471 163L471 202L373 202L373 660C373 670 366 677 353 677M295 553L295 202L67 202Z"></path><path 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data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn 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>). Pinocytosis occurs in almost all cells and continuously.</p>

<figure><img class="internal" alt="A grayscale microscopic image showing the structure of a neuron. Labeled regions include vesicle, synaptic button, and neurotransmitter vesicles, with red arrows pointing to the vesicles." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6123/pinocytosis.jpg?revision=1" />
<figcaption>Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="100" style="font-size: 85%; position: relative;"><svg width="5.783ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2556 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-101-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-101-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-101-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path><path id="MJX-101-NCM-N-34" d="M353 677C344 677 336 672 330 663L28 199L28 163L289 163L289 81C289 63 285 51 278 46C271 41 252 39 219 39L194 39L194 0C223 2 269 3 331 3C393 3 439 2 468 0L468 39L443 39C410 39 391 41 384 46C377 51 373 63 373 81L373 163L471 163L471 202L373 202L373 660C373 670 366 677 353 677M295 553L295 202L67 202Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="scale(1,-1)"><g data-mml-node="math" 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data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-id="1" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true"><use data-c="2E" href="#MJX-101-NCM-N-2E"></use></g></g><g data-mml-node="mn" data-latex="24" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-id="2" data-semantic-parent="5" data-semantic-attributes="latex:24" data-semantic-level-number="1" data-speech-node="true" transform="translate(778,0)"><use data-c="32" href="#MJX-101-NCM-N-32"></use><use data-c="34" href="#MJX-101-NCM-N-34" transform="translate(500,0)"></use></g><g data-mml-node="TeXAtom" data-mjx-texclass="ORD" data-latex="{.}" transform="translate(1778,0)"><g data-mml-node="mo" data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-id="3" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true"><use data-c="2E" href="#MJX-101-NCM-N-2E"></use></g></g><g data-mml-node="mn" data-latex="3" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-id="4" data-semantic-parent="5" data-semantic-attributes="latex:3" data-semantic-level-number="1" data-speech-node="true" transform="translate(2056,0)"><use data-c="33" href="#MJX-101-NCM-N-33"></use></g></g></g></g></svg><mjx-assistive-mml unselectable="on" display="inline"><math data-latex="\PageIndex{3}" data-semantic-structure="(5 0 1 2 3 4)"><mrow data-mjx-texclass="ORD" data-latex="3}" data-semantic-type="punctuated" data-semantic-role="sequence" data-semantic-annotation="depth:1" data-semantic-="" data-semantic-children="0,1,2,3,4" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{3};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="3" 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:3" data-semantic-level-number="1" data-speech-node="true">3</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="24" 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:24" data-semantic-level-number="1" data-speech-node="true">24</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="3" 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:3" data-semantic-level-number="1" data-speech-node="true">3</mn></mrow></math></mjx-assistive-mml></mjx-container>: Pinocytosis courtesy Fawcett, The Cell: Its Organelles and Inclusions, W. B. Saunders Co., 1966</figcaption>
</figure>

<p class="lt-bio-4071" style="background-color: unset;">This electron micrograph shows a section of the wall of a capillary (the smallest of the blood vessels). On the right is the interior or lumen of the capillary. In the middle is the tissue space separating the capillary wall from a nearby muscle cell (left). The small inpocketings of the plasma membrane are clearly seen (arrows). Most of these are open to the tissue space but some can also be seen on the other side of the cell apparently engulfing fluid from within the capillary. Perhaps most of the vesicles facing the tissue space are not taking up material by endocytosis but are instead discharging material by exocytosis. If so, the pinocytic vesicles formed at one surface of the cell may, after being detached, move through the cell to the opposite surface and there discharge their contents. In this way materials can be moved efficiently through the capillary wall. The pinocytosis vesicles in this image represent a subtype called <strong>caveolae</strong>. In addition to their function in endo- and exocytosis, they also can serve as a reservoir of plasma membrane. When a cell expands (e.g., by osmotic swelling) or is stretched, the caveolae flatten out providing more plasma membrane.</p>

<p class="lt-bio-4071" style="background-color: unset;">A cell sipping away at the ECF by pinocytosis acquires a representative sample of the molecules and ions dissolved in the ECF. But cells also have a much more elegant method for picking up critical components of the ECF that may be in scant supply as we shall now see.</p>

<span id="Receptor-Mediated_Endocytosis"></span><span id="Receptor-Mediated_Endocytosis"></span><h2 style="background-color: unset;" class="lt-bio-4071">Receptor-Mediated Endocytosis</h2>

<p class="lt-bio-4071" style="background-color: unset;">Some of the integral membrane proteins that a cell displays at its surface are receptors for particular components of the ECF (Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="101" style="font-size: 85%; position: relative;"><svg width="6.762ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2989 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-102-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-102-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-102-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path><path id="MJX-102-NCM-N-34" d="M353 677C344 677 336 672 330 663L28 199L28 163L289 163L289 81C289 63 285 51 278 46C271 41 252 39 219 39L194 39L194 0C223 2 269 3 331 3C393 3 439 2 468 0L468 39L443 39C410 39 391 41 384 46C377 51 373 63 373 81L373 163L471 163L471 202L373 202L373 660C373 670 366 677 353 677M295 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2 3 (7 4 6 5))"><mrow data-mjx-texclass="ORD" data-latex="1c}" data-semantic-type="punctuated" data-semantic-role="sequence" data-semantic-annotation="depth:1" data-semantic-="" data-semantic-children="0,1,2,3,7" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{1c};texclass:ORD" data-semantic-owns="0 1 2 3 7" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="3" 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="8" data-semantic-attributes="latex:3" data-semantic-level-number="1" data-speech-node="true">3</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="8" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="24" 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="8" data-semantic-attributes="latex:24" data-semantic-level-number="1" data-speech-node="true">24</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="8" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mrow data-semantic-added="true" data-semantic-type="infixop" data-semantic-role="implicit" data-semantic-annotation="clearspeak:simple;clearspeak:unit;depth:2" data-semantic-="" data-semantic-children="4,5" data-semantic-content="6" data-semantic-parent="8" data-semantic-owns="4 6 5" data-semantic-level-number="1" data-speech-node="true"><mn data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:3" data-semantic-="" data-semantic-parent="7" data-semantic-attributes="latex:1" data-semantic-level-number="2" data-speech-node="true">1</mn><mo data-semantic-type="operator" data-semantic-role="multiplication" data-semantic-annotation="depth:3" data-semantic-="" data-semantic-parent="7" data-semantic-added="true" data-semantic-operator="infixop,&#8290;" data-semantic-level-number="2" data-speech-node="true">&#8290;</mo><mi data-latex="c" data-semantic-type="identifier" data-semantic-role="latinletter" data-semantic-font="italic" data-semantic-annotation="clearspeak:simple;depth:3" data-semantic-="" data-semantic-parent="7" data-semantic-attributes="latex:c" data-semantic-level-number="2" data-speech-node="true">c</mi></mrow></mrow></math></mjx-assistive-mml></mjx-container>). For example, iron is transported in the blood complexed to a protein called <strong>transferrin</strong>. Cells have receptors for transferrin on their surface. When these receptors encounter a molecule of transferrin, they bind tightly to it. The complex of transferrin and its receptor is then engulfed by endocytosis. Ultimately, the iron is released into the cytosol. The strong <strong>affinity</strong> of the transferrin receptor for transferrin (its <strong>ligand</strong>) ensures that the cell will get all the iron it needs even if transferrin represents only a small fraction of the protein molecules present in the ECF. Receptor-mediated endocytosis is many thousand times more efficient than simple pinocytosis in enabling the cell to acquire the macromolecules it needs.</p>

<span id="Low-Density_Lipoprotein_(LDL)_Receptor"></span><span id="Low-Density_Lipoprotein_(LDL)_Receptor"></span><h3 style="background-color: unset;" class="lt-bio-4071">Low-Density Lipoprotein (LDL) Receptor</h3>

<p class="lt-bio-4071" style="background-color: unset;">Cells take up cholesterol by receptor-mediated endocytosis. Cholesterol is an essential component of all cell membranes. Most cells can, as needed, either synthesize cholesterol or acquire it from the ECF. Human cells get much of their cholesterol from the liver and, if your diet is not strictly &quot;100% cholesterol-free&quot;, by absorption from the intestine.</p>

<p class="lt-bio-4071" style="background-color: unset;">Cholesterol is a hydrophobic molecule and quite insoluble in water. Thus it cannot pass from the liver and/or the intestine to the cells simply dissolved in blood and ECF. Instead it is carried in tiny droplets of lipoprotein. The most abundant cholesterol carriers in humans are the <strong>low-density lipoproteins</strong> or <strong>LDL</strong>s.</p>

<p class="lt-bio-4071" style="background-color: unset;">LDL particles are spheres covered with a single layer of phospholipid molecules with their hydrophilic heads exposed to the watery fluid (e.g., blood) and their hydrophobic tails directed into the interior. Some 1,500 molecules of cholesterol (each bound to a fatty acid) occupy the hydrophobic interior of LDL particles. One molecule of a protein called <strong>apolipoprotein B</strong> (apoB) is exposed at the surface of each LDL particle.</p>

<figure><img style="width: 284px; height: 432px;" class="internal" alt="Diagram of cholesterol uptake by cells. Shows LDL attaching to receptors on the cell membrane, entering via endocytosis, and breaking down into cholesterol and other components." loading="lazy" width="284px" height="432px" src="https://bio.libretexts.org/@api/deki/files/6124/LDLendocytosis.png?revision=1&amp;size=bestfit&amp;width=284&amp;height=432" />
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</figure>

<p class="lt-bio-4071" style="background-color: unset;">The first step in acquiring LDL particles is for them to bind to <strong>LDL receptors</strong> exposed at the cell surface. These transmembrane proteins have a site that recognizes and binds to the apolipoprotein B on the surface of the LDL. The portion of the plasma membrane with bound LDL is internalized by endocytosis. A drop in the pH (from ~7 to ~5) causes the LDL to separate from its receptor. The vesicle then pinches apart into two smaller vesicles: one containing free LDLs; the other containing now-empty receptors. The vesicle with the LDLs fuses with a lysosome to form a <strong>secondary lysosome</strong>. The enzymes of the lysosome then release free cholesterol into the cytosol. The vesicle with unoccupied receptors returns to and fuses with the plasma membrane, turning inside out as it does so (exocytosis). In this way the LDL receptors are returned to the cell surface for reuse.</p>

<p class="lt-bio-4071" style="background-color: unset;">People who inherit two defective (mutant) genes for the LDL receptor have receptors that function poorly or not at all. This creates excessively high levels of LDL in their blood and predisposes them to atherosclerosis and heart attacks. The ailment is called <strong>familial</strong> (because it is inherited) <strong>hypercholesterolemia</strong>.</p>

<p class="lt-bio-4071" style="background-color: unset;">Mutations in <strong><em>APOB</em></strong>, the apoB gene, cause another form of inherited hypercholesterolemia.</p>

<p class="lt-bio-4071" style="background-color: unset;">Other small hydrophobic molecules are also transported in the blood while bound to soluble proteins:</p>

<ul>
    <li class="lt-bio-4071" style="background-color: unset;">the retinoid vitamin A (retinol) bound to the retinol-binding protein</li>
    <li class="lt-bio-4071" style="background-color: unset;">the steroids
    <ul>
        <li class="lt-bio-4071" style="background-color: unset;">25[OH] vitamin D<sub><small><font size="1">3</font></small></sub> bound to the vitamin D binding protein</li>
        <li class="lt-bio-4071" style="background-color: unset;">cortisol bound to the corticosteroid binding globulin</li>
        <li class="lt-bio-4071" style="background-color: unset;">testosterone and estrogens bound to the sex hormone binding globulin</li>
    </ul>
    </li>
</ul>

<p class="lt-bio-4071" style="background-color: unset;">There is growing evidence that, like cholesterol, they are taken into the cell by receptor-mediated endocytosis.</p>

<p class="lt-bio-4071" style="background-color: unset;">Endocytosis removes portions of the plasma membrane and takes them inside the cell. To keep in balance, membrane must be returned to the plasma membrane. This occurs by exocytosis.</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/3.24:_Endocytosis">3.24: Endocytosis</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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