4.05:_Cellular_Respiration

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<p class="lt-bio-4631" style="background-color: unset;">Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water.</p>

<p class="lt-bio-4631" style="background-color: unset;">The energy released is trapped in the form of <strong>ATP</strong> for use by all the energy-consuming activities of the cell. The process occurs in two phases:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>glycolysis</strong>, the breakdown of glucose to pyruvic acid</li>
    <li class="lt-bio-4631" style="background-color: unset;">the complete <strong>oxidation of pyruvic acid</strong> to carbon dioxide and water</li>
</ul>

<p class="lt-bio-4631" style="background-color: unset;">In eukaryotes, glycolysis occurs in the cytosol and the remaining processes take place in <strong>mitochondria</strong>.</p>

<span id="Mitochondria"></span><span id="Mitochondria"></span><h2 style="background-color: unset;" class="lt-bio-4631">Mitochondria</h2>

<p class="lt-bio-4631" style="background-color: unset;">Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their number within the cell ranges from a few hundred to, in very active cells, thousands. Their main function is the conversion of the potential energy of food molecules into ATP.</p>

<figure><img class="internal" alt="Diagram of a mitochondrion showing labeled components: outer membrane, inner membrane, intermembrane space, matrix, and crista. The inner membrane is folded into cristae." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6291/Mitochondrion.gif?revision=1" /> <img class="internal" alt="Black and white electron microscope image of a mitochondrion. It shows the organelle&rsquo;s double membrane and internal folds, or cristae, which appear as lines across the oval structure." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6292/mitochondrion.jpg?revision=1" /><figcaption>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="1.581ex" role="img" focusable="false" viewbox="0 -677 2056 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-98-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-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" 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"><g data-mml-node="mn" data-latex="4" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-id="0" data-semantic-parent="5" data-semantic-attributes="latex:4" data-semantic-level-number="1" data-speech-node="true"><use data-c="34" href="#MJX-98-NCM-N-34"></use></g><g data-mml-node="TeXAtom" 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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{1};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="4" 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:4" data-semantic-level-number="1" data-speech-node="true">4</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="5" 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:5" data-semantic-level-number="1" data-speech-node="true">5</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="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>: (left) Mitochondria (right) Mitochondrion from bat pancreas cell courtesy Keith R. Porter</figcaption></figure>

<p class="lt-bio-4631" style="background-color: unset;">Mitochondria have:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">an <strong>outer membrane</strong> that encloses the entire structure</li>
    <li class="lt-bio-4631" style="background-color: unset;">an <strong>inner membrane</strong> that encloses a fluid-filled <strong>matrix</strong></li>
    <li class="lt-bio-4631" style="background-color: unset;">between the two is the <strong>intermembrane space</strong></li>
    <li class="lt-bio-4631" style="background-color: unset;">the inner membrane is elaborately folded with shelflike <strong>cristae</strong> projecting into the <strong>matrix</strong>.</li>
    <li class="lt-bio-4631" style="background-color: unset;">a small number (some 5&ndash;10) circular molecules of <strong>DNA</strong></li>
</ul>

<p class="lt-bio-4631" style="background-color: unset;">This electron micrograph in Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="98" style="font-size: 85%; position: relative;"><svg width="4.652ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2056 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><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><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-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-99-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}" 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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-99-NCM-N-2E"></use></g></g><g data-mml-node="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-id="4" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true" transform="translate(1556,0)"><use data-c="31" href="#MJX-99-NCM-N-31"></use></g></g></g></g></svg><mjx-assistive-mml unselectable="on" display="inline"><math data-latex="\PageIndex{1}" data-semantic-structure="(5 0 1 2 3 4)"><mrow data-mjx-texclass="ORD" data-latex="1}" 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{1};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="4" 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:4" data-semantic-level-number="1" data-speech-node="true">4</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="5" 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:5" data-semantic-level-number="1" data-speech-node="true">5</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="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>, shows a single mitochondrion from a bat pancreas cell. Note the double membrane and the way the inner membrane is folded into cristae. The dark, membrane-bounded objects above the mitochondrion are lysosomes. The number of mitochondria in a cell can increase either by their fission (e.g. following mitosis) or decrease by their fusing together. Defects in either process can produce serious, even fatal, illness.</p>

<span id="The_Outer_Membrane"></span><span id="The_Outer_Membrane"></span><h3 style="background-color: unset;" class="lt-bio-4631">The Outer Membrane</h3>

<p class="lt-bio-4631" style="background-color: unset;">The outer membrane contains many complexes of integral membrane proteins that form channels through which a variety of molecules and ions move in and out of the mitochondrion.</p>

<span id="The_Inner_Membrane"></span><span id="The_Inner_Membrane"></span><h3 style="background-color: unset;" class="lt-bio-4631">The Inner Membrane</h3>

<p class="lt-bio-4631" style="background-color: unset;">The inner membrane contains 5 complexes of integral membrane proteins:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>NADH dehydrogenase</strong> (Complex I)</li>
    <li class="lt-bio-4631" style="background-color: unset;">succinate dehydrogenase (Complex II)</li>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>cytochrome c reductase</strong> (Complex III; also known as the cytochrome b-c<sub><font size="2">1</font></sub> complex)</li>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>cytochrome c oxidase</strong> (Complex IV)</li>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>ATP synthase</strong> (Complex V)</li>
</ul>

<span id="The_Matrix"></span><span id="The_Matrix"></span><h2 style="background-color: unset;" class="lt-bio-4631">The Matrix</h2>

<p class="lt-bio-4631" style="background-color: unset;">The matrix contains a complex mixture of soluble enzymes that catalyze the respiration of pyruvic acid and other small organic molecules. Here pyruvic acid is</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">oxidized by NAD<sup><font size="2">+</font></sup> producing NADH + H<sup><font size="2">+</font></sup></li>
    <li class="lt-bio-4631" style="background-color: unset;">decarboxylated producing a molecule of
    <ul>
        <li class="lt-bio-4631" style="background-color: unset;">carbon dioxide (CO<sub><font size="2">2</font></sub>) and</li>
        <li class="lt-bio-4631" style="background-color: unset;">a 2-carbon fragment of acetate bound to <strong>coenzyme A</strong> forming acetyl-CoA</li>
    </ul>
    </li>
</ul>

<span id="The_Citric_Acid_Cycle"></span><span id="The_Citric_Acid_Cycle"></span><h2 style="background-color: unset;" class="lt-bio-4631">The Citric Acid Cycle</h2>

<p class="lt-bio-4631" style="background-color: unset;">This 2-carbon fragment is donated to a molecule of <strong>oxaloacetic acid</strong>. The resulting molecule of <strong>citric acid</strong> (which gives its name to the process) undergoes the series of enzymatic steps shown in the diagram. The final step regenerates a molecule of oxaloacetic acid and the cycle is ready to turn again.</p>

<figure><img class="internal" alt="Diagram illustrating the Citric Acid Cycle, showing transformations of molecules such as NAD+, NADH, and FAD. Key compounds include succinyl-CoA, oxaloacetic acid, and citric acid. Arrows indicate flow." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6293/CitricAcidCycle.gif?revision=1" /><figcaption>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="4.652ex" height="1.581ex" role="img" focusable="false" viewbox="0 -677 2056 699" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><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 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-35" d="M118 315C123 315 129 319 134 326C164 371 205 393 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transform="translate(1278,0)"><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-100-NCM-N-2E"></use></g></g><g data-mml-node="mn" data-latex="2" 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:2" data-semantic-level-number="1" data-speech-node="true" transform="translate(1556,0)"><use data-c="32" href="#MJX-100-NCM-N-32"></use></g></g></g></g></svg><mjx-assistive-mml unselectable="on" display="inline"><math data-latex="\PageIndex{2}" data-semantic-structure="(5 0 1 2 3 4)"><mrow data-mjx-texclass="ORD" data-latex="2}" 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{2};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="4" 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:4" data-semantic-level-number="1" data-speech-node="true">4</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="5" 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:5" data-semantic-level-number="1" data-speech-node="true">5</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="2" 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:2" data-semantic-level-number="1" data-speech-node="true">2</mn></mrow></math></mjx-assistive-mml></mjx-container>: The Citric Acid Cycle</figcaption></figure>

<p class="lt-bio-4631" style="background-color: unset;">A brief summary of the cycle is as follows:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion leaves as a molecule of carbon dioxide (<strong>CO<sub><font size="2">2</font></sub></strong>).</li>
    <li class="lt-bio-4631" style="background-color: unset;">At 4 steps, a pair of electrons (<strong>2e<sup><font size="2">-</font></sup></strong>) is removed and transferred to <strong>NAD<sup><font size="2">+</font></sup></strong> reducing it to <strong>NADH + H<sup><font size="2">+</font></sup></strong>.</li>
    <li class="lt-bio-4631" style="background-color: unset;">At one step, a pair of electrons is removed from succinic acid and reduces the prosthetic group flavin adenine dinucleotide (<strong>FAD</strong>) to <strong>FADH<sub><font size="2">2</font></sub></strong>.</li>
    <li class="lt-bio-4631" style="background-color: unset;">The electrons of NADH and FADH<sub><font size="2">2</font></sub> are transferred to the <strong>electron transport chain</strong>.</li>
</ul>

<span id="The_Electron_Transport_Chain"></span><span id="The_Electron_Transport_Chain"></span><h2 style="background-color: unset;" class="lt-bio-4631">The Electron Transport Chain</h2>

<p class="lt-bio-4631" style="background-color: unset;">The electron transport chain consists of 3 complexes of integral membrane proteins</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">the <strong>NADH dehydrogenase</strong> complex (I)</li>
    <li class="lt-bio-4631" style="background-color: unset;">the <strong>cytochrome c reductase</strong> complex (III)</li>
    <li class="lt-bio-4631" style="background-color: unset;">the <strong>cytochrome c oxidase</strong> complex (IV)</li>
</ul>

<p class="lt-bio-4631" style="background-color: unset;">and two freely-diffusible molecules <strong>ubiquinone, </strong><strong>cytochrome</strong><strong> c, </strong> that shuttle electrons from one complex to the next.</p>

<figure><img class="internal" alt="Flowchart of electron transfer in mitochondria shows complexes I-IV, CoQ, and cytochrome c, illustrating the process from NADH and FADH&#8322; to O&#8322;, resulting in ATP production." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6294/respiratory_chain_rev.gif?revision=1" /><figcaption>Figure 4.5.4 Electron Transport Chain</figcaption></figure>

<p class="lt-bio-4631" style="background-color: unset;">The electron transport chain accomplishes:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">The stepwise transfer of electrons from <strong>NADH</strong> (and <strong>FADH<sub><font size="2">2</font></sub></strong>) to <strong>oxygen</strong> molecules to form (with the aid of protons) water molecules (<strong>H<sub><font size="2">2</font></sub>O</strong>). Cytochrome c can only transfer one electron at a time, so cytochrome c oxidase must wait until it has accumulated 4 of them before it can react with oxygen.</li>
    <li class="lt-bio-4631" style="background-color: unset;">Harnessing the energy released by this transfer to the pumping of protons (<strong>H<sup><font size="2">+</font></sup></strong>) from the <strong>matrix</strong> to the <strong>intermembrane space</strong>.</li>
    <li class="lt-bio-4631" style="background-color: unset;">Approximately 20 protons are pumped into the intermembrane space as the 4 electrons needed to reduce oxygen to water pass through the respiratory chain.</li>
    <li class="lt-bio-4631" style="background-color: unset;">The gradient of protons formed across the inner membrane by this process of active transport forms a miniature battery.</li>
    <li class="lt-bio-4631" style="background-color: unset;">The protons can flow back down this gradient only by reentering the matrix through <strong>ATP synthase</strong>, another complex (complex V) of 16 integral membrane proteins in the inner membrane. The process is called chemiosmosis.</li>
</ul>

<span id="Chemiosmosis_in_mitochondria"></span><span id="Chemiosmosis_in_mitochondria"></span><h2 style="background-color: unset;" class="lt-bio-4631">Chemiosmosis in mitochondria</h2>

<p class="mt-align-center lt-bio-4631" style="background-color: unset;">Fig.4.5.5 Chemiosmosis in Mitochondria</p>

<p class="lt-bio-4631" style="background-color: unset;">The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by three enzyme complexes of the respiratory chain (I, III, and IV) to pump <strong>protons</strong> (<strong>H<sup><font size="2">+</font></sup></strong>) <strong>against</strong> their concentration gradient <strong>from the matrix</strong> of the mitochondrion <strong>into the intermembrane space.</strong></p>

<p class="lt-bio-4631" style="background-color: unset;">As their concentration increases there (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the <strong>ATP synthase</strong> complex. As in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of <strong>ATP</strong>. The process is called <strong>chemiosmosis</strong> and is an example of facilitated diffusion. One-half of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their discovery of how ATP synthase works.</p>

<p class="lt-bio-4631" style="background-color: unset;">It is tempting to try to view the synthesis of ATP as a simple matter of stoichiometry (the fixed ratios of reactants to products in a chemical reaction). But (with 3 exceptions) <strong>it is not</strong>. Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane. The number of protons pumped out as electrons drop from NADH through the respiratory chain to oxygen is theoretically large enough to generate, as they return through ATP synthase, 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH<sub><font size="2">2</font></sub>).</p>

<p class="lt-bio-4631" style="background-color: unset;">With 12 pairs of electrons removed from each glucose molecule,</p>

<p class="lt-bio-4631" style="background-color: unset;">this could generate 34 ATPs. Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38. However, the energy stored in the proton gradient is also used for the active transport of several molecules and ions through the inner mitochondrial membrane into the matrix. NADH is also used as reducing agent for many cellular reactions. So the actual yield of ATP as mitochondria respire varies with conditions and probably seldom exceeds <strong>30</strong>.</p>

<span id="The_three_exceptions"></span><span id="The_three_exceptions"></span><h3 style="background-color: unset;" class="lt-bio-4631">The three exceptions</h3>

<p class="lt-bio-4631" style="background-color: unset;">A stoichiometric production of ATP does occur at:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">one step in the citric acid cycle yielding 2 ATPs for each glucose molecule. This step is the conversion of alpha-ketoglutaric acid to succinic acid.</li>
    <li class="lt-bio-4631" style="background-color: unset;">at two steps in glycolysis yielding 2 ATPs for each glucose molecule.</li>
</ul>

<span id="Mitochondrial_DNA_(mtDNA)"></span><span id="Mitochondrial_DNA_(mtDNA)"></span><h2 style="background-color: unset;" class="lt-bio-4631">Mitochondrial DNA (mtDNA)</h2>

<p class="lt-bio-4631" style="background-color: unset;">The human mitochondrion contains 5&ndash;10 identical, circular molecules of DNA. Each consists of 16,569 base pairs carrying the information for <strong>37 genes</strong> which encode:</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">2 different molecules of <strong>ribosomal RNA</strong> (<strong>rRNA</strong>)</li>
    <li class="lt-bio-4631" style="background-color: unset;">22 different molecules of <strong>transfer RNA</strong> (<strong>tRNA</strong>) (at least one for each amino acid)</li>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>13 polypeptides</strong></li>
</ul>

<p class="lt-bio-4631" style="background-color: unset;">The <strong>rRNA</strong> and <strong>tRNA</strong> molecules are used in the machinery that synthesizes the 13 polypeptides.</p>

<p class="lt-bio-4631" style="background-color: unset;">The <strong>13 polypeptides</strong> participate in building several protein complexes embedded in the inner mitochondrial membrane.</p>

<ul>
    <li class="lt-bio-4631" style="background-color: unset;">7 subunits that make up the mitochondrial <strong>NADH dehydrogenase</strong> (complex I)</li>
    <li class="lt-bio-4631" style="background-color: unset;"><strong>cytochrome b</strong>, a subunit of cytochrome c reductase (complex III)</li>
    <li class="lt-bio-4631" style="background-color: unset;">3 subunits of <strong>cytochrome c oxidase</strong> (complex IV)</li>
    <li class="lt-bio-4631" style="background-color: unset;">2 subunits of <strong>ATP synthase</strong> (complex V)</li>
</ul>

<p class="lt-bio-4631" style="background-color: unset;">Each of these protein complexes also requires subunits that are encoded by nuclear genes, synthesized in the cytosol, and imported from the cytosol into the mitochondrion. Nuclear genes also encode ~1,000 other proteins that must be imported into the mitochondrion.</p>

<span id="Mutations_in_mtDNA_cause_human_diseases"></span><span id="Mutations_in_mtDNA_cause_human_diseases"></span><h3 style="background-color: unset;" class="lt-bio-4631">Mutations in mtDNA cause human diseases</h3>

<p class="lt-bio-4631" style="background-color: unset;">Mutations in 12 of the 13 polypeptide-encoding mitochondrial genes have been found to cause human disease. Although many different organs may be affected, disorders of the muscles and brain are the most common. Perhaps this reflects the great demand for energy of both these organs. (Although representing only ~2% of our body weight, the brain consumes ~20% of the energy produced when we are at rest.)</p>

<p class="lt-bio-4631" style="background-color: unset;">Some of these disorders are inherited in the germline. In every case, the mutant gene is received from the mother because none of the mitochondria in sperm survives in the fertilized egg. Other disorders are somatic; that is, the mutation occurs in the somatic tissues of the individual.</p>

<div class="box-example"><span id="Example:_Exercise_Intolerance"></span><span id="Example:_Exercise_Intolerance"></span><h5 class="box-legend"><span class="lt-icon-default">Example: Exercise Intolerance</span></h5>

<p class="lt-bio-4631" style="background-color: unset;">A number of humans who suffer from easily-fatigued muscles turn out to have a mutations in their <strong>cytochrome b</strong> gene. Curiously, only the mitochondria in their muscles have the mutation; the mtDNA of their other tissues is normal. Presumably, very early in their embryonic development, a mutation occurred in a cytochrome b gene in the mitochondrion of a cell destined to produce their muscles.</p>
</div>

<p class="lt-bio-4631" style="background-color: unset;">The severity of mitochondrial diseases varies greatly. The reason for this is probably the extensive mixing of mutant DNA and normal DNA in the mitochondria as they fuse with one another. A mixture of both is called <strong>heteroplasmy</strong>. The higher the ratio of mutant to normal, the greater the severity of the disease. In fact by chance alone, cells can on occasion end up with all their mitochondria carrying all-mutant genomes &mdash; a condition called <strong>homoplasmy</strong> (a phenomenon resembling genetic drift).</p>

<span id="Mitochondrial_Replacement_Techniques"></span><span id="Mitochondrial_Replacement_Techniques"></span><h3 style="background-color: unset;" class="lt-bio-4631">Mitochondrial Replacement Techniques</h3>

<p class="lt-bio-4631" style="background-color: unset;">As I noted above, only mothers can pass mutant mtDNA on to their offspring. Two techniques are under intense investigation, either of which could enable a mother to have children free of defective mitochondria. Mutations in some 228 nuclear genes have also been implicated in human mitochondrial diseases, but mitochondrial replacement techniques will not be able to help with these.</p>

<span id="Why_do_mitochondria_have_their_own_genome.3F"></span><span id="Why_do_mitochondria_have_their_own_genome.3F"></span><h3 style="background-color: unset;" class="lt-bio-4631">Why do mitochondria have their own genome?</h3>

<p class="lt-bio-4631" style="background-color: unset;">Many of the features of the mitochondrial genetic system resemble those found in bacteria. This has strengthened the theory that mitochondria are the evolutionary descendants of a bacterium that established an endosymbiotic relationship with the ancestors of eukaryotic cells early in the history of life on earth. However, many of the genes needed for mitochondrial function have since moved to the nuclear genome. The recent sequencing of the complete genome of <em>Rickettsia prowazekii</em> has revealed a number of genes closely related to those found in mitochondria. Perhaps rickettsias are the closest living descendants of the endosymbionts that became the mitochondria of eukaryotes.</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/04:_Cell_Metabolism/4.05:_Cellular_Respiration">4.5: Cellular Respiration</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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