1.08:_pH

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<p class="lt-bio-3748" style="background-color: unset;">pH is a measure of the concentration of <strong>hydrogens ions</strong> (= <strong>H<sup><font size="2">+</font></sup></strong>) (= <strong>protons</strong>) in a solution. Numerically it is the <strong>negative</strong> logarithm of that concentration expressed in moles per liter (<strong>M</strong>).</p>

<p class="lt-bio-3748" style="background-color: unset;">Pure water spontaneously dissociates into ions, forming a 10<sup><font size="2">-7</font></sup> M solution of H<sup><font size="2">+</font></sup> (and OH<sup><font size="2">-</font></sup>). The negative of this logarithm is 7, so the pH of pure water is 7.</p>

<p class="lt-bio-3748" style="background-color: unset;">Solutions with a higher concentration of H<sup><font size="2">+</font></sup> than occurs in pure water have pH values below 7 and are <strong>acidic</strong>. Solutions containing molecules or ions that reduce the concentration of H<sup><font size="2">+</font></sup> below that of pure water have pH values above 7 and are <strong>basic</strong> or <strong>alkaline</strong>.</p>

<p class="lt-bio-3748" style="background-color: unset;">Is pH important? Yes!</p>

<p class="lt-bio-3748" style="background-color: unset;">The properties of most proteins, enzymes for example, are sensitive to pH.</p>

<p class="lt-bio-3748" style="background-color: unset;"><strong>As the pH drops</strong>,</p>

<ul>
    <li class="lt-bio-3748" style="background-color: unset;">H<sup><font size="2">+</font></sup> bind to the carboxyl groups (COO<sup><font size="2">-</font></sup>) of aspartic acid (Asp) and glutamic acid (Glu), neutralizing their negative charge, and</li>
    <li class="lt-bio-3748" style="background-color: unset;">H<sup><font size="2">+</font></sup> bind to the unoccupied pair of electrons on the N atom of the amino (NH<sub><font size="2">2</font></sub> ) groups of lysine (Lys) and arginine (Arg) giving them a positive charge.</li>
</ul>

<p class="lt-bio-3748" style="background-color: unset;"><strong>The result:</strong> Not only does the net charge on the molecule change (it becomes more positive) but many of the opportunities that its R groups have for ionic interactions with other molecules and ions are altered.</p>

<p class="lt-bio-3748" style="background-color: unset;"><strong>As the pH rises</strong>,</p>

<ul>
    <li class="lt-bio-3748" style="background-color: unset;">H<sup><font size="2">+</font></sup> are removed from the COOH groups of Asp and Glu, giving them a negative charge (COO<sup><font size="2">-</font></sup>), and</li>
    <li class="lt-bio-3748" style="background-color: unset;">H<sup><font size="2">+</font></sup> are removed from the NH<sub><font size="2">3</font></sub><sup><font size="2">+</font></sup> groups of Lys and Arg removing their positive charge.</li>
</ul>

<p class="lt-bio-3748" style="background-color: unset;"><strong>The result:</strong> Again the net charge on the molecule changes (it becomes more negative) and, again, many of the opportunities its R groups have for ionic interactions with other molecules or ions are altered.</p>

<p class="lt-bio-3748" style="background-color: unset;">The pH of the <strong>cytosol</strong> within a human cell is about 7.4. BUT, this value masks the pH differences that are found in various compartments within the cell. For example,</p>

<ul>
    <li class="lt-bio-3748" style="background-color: unset;">The interior of <strong>lysosomes</strong> is much more acidic (as low as pH 4) than the cytosol, and the enzymes within work best at these low pH values.</li>
    <li class="lt-bio-3748" style="background-color: unset;">The pH differential created within <strong>chloroplasts</strong> by the energy of the sun is harnessed to synthesize ATP which, in turn, powers the synthesis of food.</li>
    <li class="lt-bio-3748" style="background-color: unset;">The pH differential created within <strong>mitochondria</strong> during the respiration of food is harnessed to the synthesis of ATP which, in turn, powers most of the energy-consuming activities of the cell such as locomotion and biosynthesis of cell components.</li>
</ul>

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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/01:_The_Chemical_Basis_of_Life/1.08:_pH">1.8: pH</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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