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4.08:_Photosynthesis_-_The_Role_of_Light
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<p class="lt-bio-4634" style="background-color: unset;">The heart of photosynthesis as it occurs in most autotrophs consists of two key processes:</p> <ul> <li class="lt-bio-4634" style="background-color: unset;"><strong>the removal of hydrogen (H) atoms from water molecules</strong></li> <li class="lt-bio-4634" style="background-color: unset;"><strong>the reduction of carbon dioxide (CO<sub><font size="2">2</font></sub>) by these hydrogen atoms to form organic molecules</strong>.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">The second process involves a cyclic series of reactions named (after its discoverer) the <strong>Calvin Cycle</strong>.</p> <p class="lt-bio-4634" style="background-color: unset;">The electrons (e<sup><font size="2">−</font></sup>) and protons (H<sup><font size="2">+</font></sup>) that make up hydrogen atoms are stripped away separately from water molecules.</p> <p class="mt-align-center lt-bio-4634" style="background-color: 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data-latex="\mathrm{O}"><mrow data-semantic-added="true" data-semantic-type="subscript" data-semantic-role="identifier" data-semantic-annotation="collapsed:identifier;depth:4" data-semantic-="" data-semantic-children="27,29" data-semantic-parent="44" data-semantic-attributes="latex:{\vphantom{A}}_{\smash[t]{2}}" data-semantic-owns="27 29" data-semantic-level-number="3" data-speech-node="true"><mi mathvariant="normal" data-latex="O" data-semantic-type="identifier" data-semantic-role="latinletter" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;depth:5" data-semantic-="" data-semantic-parent="30" data-semantic-attributes="latex:\mathrm{O};texclass:ORD" data-semantic-level-number="4" data-speech-node="true">O</mi></mrow></mrow></mrow><msub data-latex="{\vphantom{A}}_{\smash[t]{2}}"><mrow data-mjx-texclass="ORD" data-latex="{\vphantom{A}}"><mrow data-mjx-texclass="ORD" data-latex="\vphantom{A}"><mpadded width="0"><mphantom><mi data-latex="A">A</mi></mphantom></mpadded></mrow></mrow><mrow data-mjx-texclass="ORD" data-latex="{\smash[t]{2}}"><mrow data-mjx-texclass="ORD" data-latex="\smash[t]{2}"><mpadded height="0"><mn data-latex="2" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;depth:5" data-semantic-="" data-semantic-parent="30" data-semantic-attributes="latex:{\smash[t]{2}};texclass:ORD" data-semantic-level-number="4" data-speech-node="true">2</mn></mpadded></mrow></mrow></msub></mrow></mtd></mlabeledtr></mtable></math></mjx-assistive-mml></mjx-container></p> <p class="lt-bio-4634" style="background-color: unset;">The electrons serve two functions:</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">They reduce NADP<sup><font size="2">+</font></sup> to <strong>NADPH</strong> for use in the Calvin Cycle.</li> <li class="lt-bio-4634" style="background-color: unset;">They set up an electrochemical charge that provides the energy for pumping protons from the <strong>stroma</strong> of the <strong>chloroplast</strong> into the interior of the thylakoid.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">The protons also serve two functions:</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">They participate in the reduction of NADP<sup><font size="2">+</font></sup> to NADPH.</li> <li class="lt-bio-4634" style="background-color: unset;">As they flow back out from the interior of the thylakoid (by facilitated diffusion), passing <strong>down</strong> their concentration gradient), the energy they give up is harnessed to the conversion of ADP to <strong>ATP</strong>.</li> <li class="lt-bio-4634" style="background-color: unset;">Because it is drive by light, this process is called <strong>photophosphorylation</strong>.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;"><mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" 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data-semantic-attributes="latex:\text{)}" data-semantic-operator="fenced" data-semantic-level-number="4" data-speech-node="true">)</mtext></mrow></mtd><mtd><mrow data-mjx-texclass="ORD" data-latex="{\mathrm{ADP}{}+{}\mathrm{P}{\vphantom{A}}_{\smash[t]{\mathrm{i}}}{}\mathrel{\mhchemlongrightarrow}{}\mathrm{ATP}}" data-semantic-type="infixop" data-semantic-role="unknown" data-semantic-annotation="depth:3" data-semantic-="" data-semantic-children="13,12" data-semantic-content="11" data-semantic-parent="16" data-semantic-attributes="latex:{\mathrm{ADP}{}+{}\mathrm{P}{\vphantom{A}}_{\smash[t]{\mathrm{i}}}{}\mathrel{\mhchemlongrightarrow}{}\mathrm{ATP}};texclass:ORD" data-semantic-owns="13 11 12" data-semantic-level-number="2" data-speech-node="true"><mrow data-semantic-added="true" data-semantic-type="infixop" data-semantic-role="addition" data-semantic-annotation="depth:4" data-semantic-="" data-semantic-children="5,10" data-semantic-content="6" data-semantic-parent="14" data-semantic-owns="5 6 10" data-semantic-level-number="3" data-speech-node="true"><mrow data-mjx-texclass="ORD" data-latex="\mathrm{ADP}"><mi data-mjx-auto-op="false" data-latex="ADP" data-semantic-type="identifier" data-semantic-role="unknown" data-semantic-font="normal" data-semantic-annotation="nemeth:number;depth:5" data-semantic-="" data-semantic-parent="13" data-semantic-attributes="latex:\mathrm{ADP};texclass:ORD" data-semantic-level-number="4" data-speech-node="true">ADP</mi></mrow><mrow data-mjx-texclass="ORD" data-latex="{}"></mrow><mo data-latex="+" data-semantic-type="operator" data-semantic-role="addition" data-semantic-annotation="depth:5" data-semantic-="" data-semantic-parent="13" data-semantic-attributes="latex:+" data-semantic-operator="infixop,+" data-semantic-level-number="4" data-speech-node="true">+</mo><mrow data-mjx-texclass="ORD" data-latex="{}"></mrow><mrow data-mjx-texclass="ORD" data-latex="\mathrm{P}"><mrow data-semantic-added="true" data-semantic-type="subscript" data-semantic-role="identifier" data-semantic-annotation="collapsed:identifier;depth:5" data-semantic-="" data-semantic-children="7,9" data-semantic-parent="13" data-semantic-attributes="latex:{\vphantom{A}}_{\smash[t]{\mathrm{i}}}" data-semantic-owns="7 9" data-semantic-level-number="4" data-speech-node="true"><mi mathvariant="normal" data-latex="P" data-semantic-type="identifier" data-semantic-role="latinletter" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;depth:6" data-semantic-="" data-semantic-parent="10" data-semantic-attributes="latex:\mathrm{P};texclass:ORD" data-semantic-level-number="5" data-speech-node="true">P</mi></mrow></mrow></mrow><msub data-latex="{\vphantom{A}}_{\smash[t]{\mathrm{i}}}"><mrow data-mjx-texclass="ORD" data-latex="{\vphantom{A}}"><mrow data-mjx-texclass="ORD" data-latex="\vphantom{A}"><mpadded width="0"><mphantom><mi data-latex="A">A</mi></mphantom></mpadded></mrow></mrow><mrow data-mjx-texclass="ORD" data-latex="{\smash[t]{\mathrm{i}}}"><mrow data-mjx-texclass="ORD" data-latex="\smash[t]{\mathrm{i}}"><mpadded height="0"><mrow data-mjx-texclass="ORD" data-latex="\mathrm{i}"><mi mathvariant="normal" data-latex="i" data-semantic-type="identifier" data-semantic-role="latinletter" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;depth:6" data-semantic-="" data-semantic-parent="10" data-semantic-attributes="latex:{\smash[t]{\mathrm{i}}};texclass:ORD" data-semantic-level-number="5" data-speech-node="true">i</mi></mrow></mpadded></mrow></mrow></msub><mrow data-mjx-texclass="ORD" data-latex="{}"></mrow><mrow data-mjx-texclass="REL" data-latex="\mathrel{\mhchemlongrightarrow}"><mo data-mjx-variant="-mhchem" data-mjx-texclass="REL" stretchy="true" data-latex="\mhchemlongrightarrow" data-semantic-type="operator" data-semantic-role="unknown" data-semantic-annotation="depth:4" data-semantic-="" data-semantic-parent="14" data-semantic-attributes="latex:\mathrel{\mhchemlongrightarrow};texclass:REL" data-semantic-operator="infixop," data-semantic-level-number="3" data-speech-node="true"></mo></mrow><mrow data-mjx-texclass="ORD" data-latex="{}"></mrow><mrow data-mjx-texclass="ORD" data-latex="\mathrm{ATP}"><mi data-mjx-auto-op="false" data-latex="ATP" data-semantic-type="identifier" data-semantic-role="unknown" data-semantic-font="normal" data-semantic-annotation="depth:4" data-semantic-="" data-semantic-parent="14" data-semantic-attributes="latex:\mathrm{ATP};texclass:ORD" data-semantic-level-number="3" data-speech-node="true">ATP</mi></mrow></mrow></mtd></mlabeledtr></mtable></math></mjx-assistive-mml></mjx-container></p> <p class="lt-bio-4634" style="background-color: unset;">The ATP provides the second essential ingredient for running the Calvin Cycle.</p> <p class="lt-bio-4634" style="background-color: unset;">The removal of electrons from water molecules and their transfer to NADP<sup><font size="2">+</font></sup> requires energy. The electrons are moving from a redox potential of about +0.82 volt in water to −0.32 volt in NADPH. Thus enough energy must be available to move them against a total potential of 1.14 volts. Where does the needed energy come from? The answer: <strong>Light</strong>.</p> <span id="The_Thylakoid_Membrane"></span><span id="The_Thylakoid_Membrane"></span><h2 style="background-color: unset;" class="lt-bio-4634">The Thylakoid Membrane</h2> <p class="lt-bio-4634" style="background-color: unset;">Chloroplasts contain a system of thylakoid membranes surrounded by a fluid <strong>stroma</strong>. Six different complexes of integral membrane proteins are embedded in the thylakoid membrane. The exact structure of these complexes differs from group to group (e.g., plant vs. alga) and even within a group (e.g., illuminated in air or underwater). They are as follows:</p> <span id="Photosystem_I"></span><span id="Photosystem_I"></span><h3 style="background-color: unset;" class="lt-bio-4634">Photosystem I</h3> <p class="lt-bio-4634" style="background-color: unset;">The structure of photosystem I in a cyanobacterium ("blue-green alga") has been completely worked out. It probably closely resembles that of plants as well. It is a homotrimer with each subunit in the trimer containing:</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">12 different protein molecules bound to</li> <li class="lt-bio-4634" style="background-color: unset;">96 molecules of <strong>chlorophyll a</strong> <ul> <li class="lt-bio-4634" style="background-color: unset;">2 molecules of the <strong>reaction center chlorophyll</strong> <strong>P<sub><font size="2">700</font></sub></strong></li> <li class="lt-bio-4634" style="background-color: unset;">4 accessory molecules closely associated with them</li> <li class="lt-bio-4634" style="background-color: unset;">90 molecules that serve as antenna pigments</li> </ul> </li> <li class="lt-bio-4634" style="background-color: unset;">22 <strong>carotenoid</strong> molecules</li> <li class="lt-bio-4634" style="background-color: unset;">4 lipid molecules</li> <li class="lt-bio-4634" style="background-color: unset;">3 clusters of Fe<sub><font size="2">4</font></sub>S<sub><font size="2">4</font></sub></li> <li class="lt-bio-4634" style="background-color: unset;">2 phylloquinones</li> </ul> <span id="Photosystem_II"></span><span id="Photosystem_II"></span><h3 style="background-color: unset;" class="lt-bio-4634">Photosystem II</h3> <p class="lt-bio-4634" style="background-color: unset;">Photosystem II is also a complex of</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">> 20 different protein molecules bound to</li> <li class="lt-bio-4634" style="background-color: unset;">50 or more <strong>chlorophyll a</strong> molecules <ul> <li class="lt-bio-4634" style="background-color: unset;">2 molecules of the <strong>reaction center chlorophyll</strong> <strong>P<sub><font size="2">680</font></sub></strong></li> <li class="lt-bio-4634" style="background-color: unset;">2 accessory molecules close to them</li> <li class="lt-bio-4634" style="background-color: unset;">2 molecules of pheophytin (chlorophyll without the Mg<sup><font size="2">++</font></sup>)</li> <li class="lt-bio-4634" style="background-color: unset;">the remaining molecules of <strong>chlorophyll a</strong> serve as <strong>antenna pigments</strong>.</li> </ul> </li> <li class="lt-bio-4634" style="background-color: unset;">some half dozen <strong>carotenoid</strong> molecules. These also serve as antenna pigments.</li> <li class="lt-bio-4634" style="background-color: unset;">2 molecules of <strong>plastoquinone</strong></li> </ul> <span id="Light-Harvesting_Complexes_(LHC)"></span><span id="Light-Harvesting_Complexes_(LHC)"></span><h3 style="background-color: unset;" class="lt-bio-4634">Light-Harvesting Complexes (LHC)</h3> <ul> <li class="lt-bio-4634" style="background-color: unset;">LHC-I associated with photosystem I</li> <li class="lt-bio-4634" style="background-color: unset;">LHC-II associated with photosystem II</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">These LHCs also act as <strong>antenna pigments</strong> harvesting light and passing its energy on to their respective photosystems.</p> <p class="lt-bio-4634" style="background-color: unset;">The LHC-II of spinach is a homotrimer, with each monomer containing</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">a single polypeptide</li> <li class="lt-bio-4634" style="background-color: unset;">8 molecules of chlorophyll a</li> <li class="lt-bio-4634" style="background-color: unset;">6 molecules of chlorophyll b</li> <li class="lt-bio-4634" style="background-color: unset;">4 carotenoid molecules</li> </ul> <span id="Cytochromes_b6_and_f"></span><span id="Cytochromes_b6_and_f"></span><h3 style="background-color: unset;" class="lt-bio-4634">Cytochromes b<sub><font size="3">6</font></sub> and f</h3> <span id="ATP_synthase"></span><span id="ATP_synthase"></span><h3 style="background-color: unset;" class="lt-bio-4634">ATP synthase</h3> <span id="How_the_System_Works"></span><span id="How_the_System_Works"></span><h2 style="background-color: unset;" class="lt-bio-4634">How the System Works</h2> <figure><img class="internal" alt="Diagram of photosynthesis electron transport chain showing Photosystem II, cytochrome b6f, Photosystem I, and ATP synthase. Arrows indicate electron flow and energy levels on the y-axis." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6301/Z-scheme-rev.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;"><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-38" d="M250 666C201 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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="8" 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:8" data-semantic-level-number="1" data-speech-node="true">8</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>: Photosystems</figcaption></figure> <ul> <li class="lt-bio-4634" style="background-color: unset;">Light is absorbed by the antenna pigments of <strong>photosystems II</strong> and <strong>I</strong>.</li> <li class="lt-bio-4634" style="background-color: unset;">The absorbed energy is transferred to the reaction center chlorophylls, <strong>P<sub><font size="2">680</font></sub></strong> in photosystem II, <strong>P<sub><font size="2">700</font></sub></strong> in photosystem I.</li> <li class="lt-bio-4634" style="background-color: unset;">Absorption of 1 photon of light by Photosystem <strong>II</strong> removes 1 electron from <strong>P<sub><font size="2">680</font></sub></strong>.</li> <li class="lt-bio-4634" style="background-color: unset;">With its resulting positive charge, P<sub><font size="2">680</font></sub> is sufficiently electronegative that it can remove 1 electron from a molecule of water.</li> <li class="lt-bio-4634" style="background-color: unset;">When these steps have occurred 4 times, requiring 2 molecules of water, 1 molecule of oxygen and 4 protons (H<sup><font size="2">+</font></sup>) are released</li> <li class="lt-bio-4634" style="background-color: unset;">The electrons are transferred (by way of <strong>plastoquinone</strong> — <strong>PQ</strong> in the figure) to the <strong>cytochrome b<sub><font size="2">6</font></sub>/f</strong> complex where they provide the energy for <strong>chemiosmosis</strong>.</li> <li class="lt-bio-4634" style="background-color: unset;">Activation of<strong> P<sub><font size="2">700</font></sub></strong> in photosystem <strong>I</strong> enables it to pick up electrons from the cytochrome b<sub><font size="2">6</font></sub>/f complex (by way of plastocyanin — <strong>PC</strong> in the figure) and raise them to a sufficiently high redox potential that, after passing through <strong>ferredoxin</strong> (<strong>Fd</strong> in the figure),</li> <li class="lt-bio-4634" style="background-color: unset;">they can reduce NADP<sup><font size="2">+</font></sup> to <strong>NADPH</strong>.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">The sawtooth shifts in redox potential as electrons pass from P<sub><font size="2">680</font></sub> to NADP<sup><font size="2">+</font></sup> have caused this system to be called the <strong>Z-Scheme</strong> (although as I have drawn the diagram, it looks more like an "N"). It is also called <strong>noncyclic photophosphorylation</strong> because it produces ATP in a one-way process (unlike cyclic photophosphorylation and pseudocyclic photophosphorylation described below).</p> <span id="Chemiosmosis_in_Chloroplasts"></span><span id="Chemiosmosis_in_Chloroplasts"></span><h2 style="background-color: unset;" class="lt-bio-4634">Chemiosmosis in Chloroplasts</h2> <p class="lt-bio-4634" style="background-color: unset;">The energy released as electrons pass down the gradient between photosystem II and plastocyanin (PC) is harnessed by the cytochrome b<sub><font size="2">6</font></sub>/f complex to pump <strong>protons</strong> (<strong>H<sup><font size="2">+</font></sup></strong>) <strong>against</strong> their concentration gradient from the stroma of the chloroplast into the interior of the thylakoid (an example of active transport). As their concentration increases inside (which is the same as saying that the pH of the interior decreases), a strong diffusion gradient is set up. The only exit for these protons is through the <strong>ATP synthase</strong> complex. As in mitochondria, 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.</p> <figure><img class="internal" alt="Diagram of photosynthesis: photosystems II and I, cytochrome b6f, and ATP synthase. Proton flow (H+) shown across thylakoid membrane, converting ADP to ATP, and forming NADPH." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6302/Chemiosmosis(chl)-rev3.png?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 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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{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="8" 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:8" data-semantic-level-number="1" data-speech-node="true">8</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>: Chemiosmosis in Chloroplasts</figcaption></figure> <span id="Cyclic_Photophosphorylation"></span><span id="Cyclic_Photophosphorylation"></span><h2 style="background-color: unset;" class="lt-bio-4634">Cyclic Photophosphorylation</h2> <ul> <li class="lt-bio-4634" style="background-color: unset;">Each CO<sub><font size="2">2</font></sub> taken up by the <a href="http://biology-pages.info/C/CalvinCycle.html" target="_blank" rel="external noopener nofollow" class="external">Calvin cycle</a>) requires 2 NADPH molecules and 3 ATP molecules</li> <li class="lt-bio-4634" style="background-color: unset;">Each molecule of oxygen released by the light reactions supplies the 4 electrons needed to make 2 NADPH molecules.</li> <li class="lt-bio-4634" style="background-color: unset;">The chemiosmosis driven by these 4 electrons as they pass through the cytochrome b<sub><font size="2">6</font></sub>/f complex liberates only enough energy to pump <strong>12</strong> protons into the interior of the thylakoid.</li> <li class="lt-bio-4634" style="background-color: unset;">But in order to make 3 molecules of ATP, the ATPase in chloroplasts appears to have <strong>14</strong> protons (H<sup><font size="2">+</font></sup>) pass through it.</li> <li class="lt-bio-4634" style="background-color: unset;">So there appears to be a deficit of 2 protons.</li> <li class="lt-bio-4634" style="background-color: unset;">How is this deficit to be made up?</li> <li class="lt-bio-4634" style="background-color: unset;">One likely answer: <strong>cyclic photophosphorylation</strong>.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">In cyclic photophosphorylation,</p> <ul> <li class="lt-bio-4634" style="background-color: unset;">the electrons expelled by the energy of light absorbed by photosystem I pass, as normal, to ferredoxin (Fd).</li> <li class="lt-bio-4634" style="background-color: unset;">But instead of going on to make NADPH,</li> <li class="lt-bio-4634" style="background-color: unset;">they pass to plastoquinone (PQ) and on back into the cytochrome b<sub><font size="2">6</font></sub>/f complex.</li> <li class="lt-bio-4634" style="background-color: unset;">Here the energy each electron liberates pumps 2 protons (H<sup><font size="2">+</font></sup>) into the interior of the</li> <li class="lt-bio-4634" style="background-color: unset;">thylakoid — enough to make up the deficit left by noncyclic photophosphorylation.</li> </ul> <p class="lt-bio-4634" style="background-color: unset;">This process is truly cyclic because no outside source of electrons is required. Like the photocell in a light meter, photosystem I is simply using light to create a flow of current. The only difference is that instead of using the current to move the needle on a light meter, the chloroplast uses the current to help synthesize ATP.</p> <span id="Pseudocyclic_Photophosphorylation"></span><span id="Pseudocyclic_Photophosphorylation"></span><h2 style="background-color: unset;" class="lt-bio-4634">Pseudocyclic Photophosphorylation</h2> <p class="lt-bio-4634" style="background-color: unset;">Another way to make up the deficit is by a process called pseudocyclic photophosphorylation in which some of the electrons passing to ferredoxin then reduce molecular oxygen back to H<sub><font size="2">2</font></sub>O instead of reducing NADP<sup><font size="2">+</font></sup> to NADPH.</p> <p class="lt-bio-4634" style="background-color: unset;">At first glance, this might seem a fruitless undoing of all the hard work of photosynthesis. But look again. Although the electrons cycle from water to ferredoxin and back again, part of their pathway is through the chemiosmosis-generating stem of cytochrome b<sub><font size="2">6</font></sub>/f. Here, then, is another way that simply by turning on a light, enough energy is imparted to electrons that they can bring about the synthesis of ATP.</p> <span id="Antenna_Pigments"></span><span id="Antenna_Pigments"></span><h2 style="background-color: unset;" class="lt-bio-4634">Antenna Pigments</h2> <p class="lt-bio-4634" style="background-color: unset;">Chlorophylls a and b differ slightly in the wavelengths of light that they absorb best (although both absorb red and blue much better than yellow and green). Carotenoids help fill in the gap by strongly absorbing green light. 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