4.07:_Photosynthesis_-_Pathway_of_Carbon_Fixation

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<p class="lt-bio-4633" style="background-color: unset;">Photosynthesis is the synthesis of organic molecules using the energy of light. For the sugar glucose (one of the most abundant products of photosynthesis) the equation is:</p>

<p class="lt-bio-4633" style="background-color: unset;">Light provides the energy to transfer electrons from water to nicotinamide adenine dinucleotide phosphate (<mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="97" style="font-size: 85%; position: relative;"><svg width="8.095ex" height="1.754ex" role="img" focusable="false" viewbox="0 -775.2 3578.1 775.2" aria-hidden="true" style="vertical-align: 0px;"><defs><path id="MJX-98-NCM-N-4E" d="M576 15C585.6 1 589 0 596 0C613 0 614 7 614 30L614 575C614 592 618 606 626 619C637 636 667 644 716 644L716 683L596 680L477 683L477 644C526 644 556 636 567 619C575 606 579 592 579 575L579 166L237 668C230 678 220 683 205 683L33 683L33 644L66 644C106 644 128 642 133 638C134 637 135 632 135 624L135 108C135 91 131 77 123 64C112 47 82 39 33 39L33 0L152 3L272 0L272 39C223 39 193 47 182 64C174 77 170 91 170 108L170 611Z"></path><path id="MJX-98-NCM-N-41" d="M583 3L717 0L717 39L695 39C670 39 653 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<span id="The_Steps_that_lead_to_Photosynthesis"></span><span id="The_Steps_that_lead_to_Photosynthesis"></span><h2 style="background-color: unset;" class="lt-bio-4633">The Steps that lead to Photosynthesis</h2>

<figure><img class="internal" alt="Flowchart of the Calvin Cycle showing chemical reactions and compounds, including ATP, NADPH, and RuBP, involved in photosynthesis. 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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-101-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="7" 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:7" data-semantic-level-number="1" data-speech-node="true">7</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>: The steps in the fixation of carbon dioxide during photosynthesis</figcaption></figure>

<ul>
    <li class="lt-bio-4633" style="background-color: unset;">CO<sub><font size="2">2</font></sub> combines with the phosphorylated 5-carbon sugar <strong>ribulose bisphosphate</strong>.</li>
    <li class="lt-bio-4633" style="background-color: unset;">This reaction is catalyzed by the enzyme <strong>ribulose bisphosphate carboxylase oxygenase</strong> (<strong>RUBISCO</strong>)(an enzyme which can fairly claim to be the most abundant protein on earth).</li>
    <li class="lt-bio-4633" style="background-color: unset;">The resulting 6-carbon compound breaks down into two molecules of <strong>3-phosphoglyceric acid</strong> (<strong>PGA</strong>).</li>
    <li class="lt-bio-4633" style="background-color: unset;">The PGA molecules are further phosphorylated (by <strong>ATP</strong>) and are reduced (by <strong>NADPH</strong>) to form <strong>phosphoglyceraldehyde</strong> (<strong>PGAL</strong>).</li>
    <li class="lt-bio-4633" style="background-color: unset;">Phosphoglyceraldehyde serves as the starting material for the synthesis of <strong>glucose</strong> and <strong>fructose</strong>.</li>
    <li class="lt-bio-4633" style="background-color: unset;">Glucose and fructose make the disaccharide <strong>sucrose</strong>, which travels in solution to other parts of the plant (e.g., fruit, roots).</li>
    <li class="lt-bio-4633" style="background-color: unset;">Glucose is also the monomer used in the synthesis of the <strong>polysaccharides</strong> <strong>starch</strong> and <strong>cellulose</strong>.</li>
</ul>

<p class="lt-bio-4633" style="background-color: unset;">The Figure 4.7.1 shows the steps in the fixation of carbon dioxide during photosynthesis. All of these reactions occur in the <strong>stroma</strong> of the chloroplast. These steps were worked out by Melvin Calvin and his colleagues at the University of California and, for this reason, are named the <strong>Calvin cycle</strong>.</p>

<span id="Calvin's_Experiment"></span><span id="Calvin's_Experiment"></span><h2 style="background-color: unset;" class="lt-bio-4633">Calvin's Experiment</h2>

<figure><img class="internal" alt="A black and white image of a laboratory setup with mirrors, glass equipment, and a round-bottom flask. Various light sources and optical devices are positioned around the experimental apparatus." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6298/Bassham1.jpg?revision=1" /><figcaption>Figure 4.7.2 Apparatus used for Calvin's experiment courtesy of Dr. James A. Bassham</figcaption></figure>

<p class="lt-bio-4633" style="background-color: unset;">The experimental apparatus is shown above. After various intervals of illumination, a suspension of unicellular algae is inactivated and the contents of the cells extracted. The compounds in a drop of the extract are then separated by paper chromatography.</p>

<p class="lt-bio-4633" style="background-color: unset;">The identity of each substance may be determined simply by comparing its position with the positions occupied by known substances under the same conditions. Or, a fragment containing the spot can be cut from the sheet and chemically analyzed.</p>

<p class="lt-bio-4633" style="background-color: unset;">To determine which, if any, of the substances separated on the chromatogram are radioactive, a sheet of X-ray film is placed next to the chromatogram. If dark spots appear on the film (because of radiation emitted by the <sup><small><font size="1">14</font></small></sup>C atoms), their position can be correlated with the positions of the chemicals in the chromatogram. Using this technique of <strong>autoradiography</strong>, Calvin found that <sup><small><font size="1">14</font></small></sup>C turned up in glucose molecules within 30 seconds after the start of photosynthesis. When he permitted photosynthesis to proceed for only 5 seconds, however, the radioactivity was concentrated in several other, smaller, molecules.</p>

<figure><img class="internal" alt="A scientific image with labeled spots showing substances: glucose, glycogen, trehalose, ATP, fumaric, pyruvic, fumaric-P, triose-P, glucose-6-P, and gluconic. Text indicates exposed 10 sec." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6299/Bassham2.jpg?revision=1" /> <img class="internal" alt="A blurry image showing various labeled chemical compounds in red text on a gradient background. The labels include Glucose, Pyruvate, and others. Some labels are partially obscured." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6300/Bassham3.jpg?revision=1" /><figcaption>Figure 4.7.3 Chromatogram of Calvin's experiment courtesy of Dr. James A. Bassham</figcaption></figure>

<p class="lt-bio-4633" style="background-color: unset;">The dark spots show the radioactive compounds produced after 10 secs (left) and 2 minutes (right) of photosynthesis by the green alga Scenedesmus. The alga was supplied with carbon dioxide labeled with <sup><small><font size="1">14</font></small></sup>C, a radioactive isotope of carbon. At 10 seconds, most of the radioactivity is found in 3-phosphoglyceric acid (&quot;P-Glyceric&quot;). At 2 minutes, phosphorylated 6-carbon sugars (glucose and fructose) have been synthesized as well as a number of amino acids. The small rectangle and circle (lower right-hand corners) mark the spots where the cell extract was applied.</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/4.07:_Photosynthesis_-_Pathway_of_Carbon_Fixation">4.7: Photosynthesis - Pathway of Carbon Fixation</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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