<h2>Transport of Substances Into and Out of the Leaf</h2>
<p>The leaf is designed to <a data-lesson="secondary-active-transport" data-subject="biology" data-tutorial="cell-transport" href="https://app.ulearngo.com/biology/cell-transport/secondary-active-transport">transport</a> water, sugars, <a data-lesson="carbon-cycle" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/carbon-cycle">carbon dioxide</a> and <a data-lesson="transport-of-oxygen-in-the-blood" data-subject="biology" data-tutorial="respiratory-system" href="https://app.ulearngo.com/biology/respiratory-system/transport-of-oxygen-in-the-blood">oxygen</a> across its surface. Each of these involves separate processes and cells which we will discuss below.</p>
<h3><strong>Movement of Oxygen and Carbon Dioxide</strong></h3>
<p>Stomata are the site of gaseous exchange in the leaf. There are two major metabolic processes that take place in plants that involve the exchange of oxygen and carbon dioxide:</p>
<ul>
<li>
<p><strong>Photosynthesis:</strong> takes place during the day when the chloroplasts can absorb radiant <a data-lesson="biology-203-glossary-9" data-subject="biology" data-tutorial="cell-metabolism" href="https://app.ulearngo.com/biology/cell-metabolism/biology-203-glossary-9">energy</a> from the sun. Photosynthesis requires carbon dioxide and releases oxygen as a by-product. Therefore, during daylight hours, the <a data-lesson="basic-principles-of-gas-exchange" data-subject="biology" data-tutorial="respiratory-system" href="https://app.ulearngo.com/biology/respiratory-system/basic-principles-of-gas-exchange">concentration</a> of carbon dioxide is low in the leaf and the concentration of oxygen is high. As a result, during the day, carbon dioxide enters the leaf and oxygen is released.</p>
</li>
<li>
<p><strong>Cellular Respiration:</strong> occurs continuously throughout the day and night. Cellular respiration requires oxygen and releases carbon dioxide as a waste product. During the day, the plant can use some of the oxygen from <a data-lesson="photosynthesis" data-subject="biology" data-tutorial="cell-biology" href="https://app.ulearngo.com/biology/cell-biology/photosynthesis">photosynthesis</a> for cellular respiration. During the night, when photosynthesis stops, the concentration of oxygen in the plant drops and the concentration gradient switches: the concentration of carbon dioxide is high and the concentration of oxygen is low. Therefore at night time, oxygen enters the leaves, and carbon dioxide is released.</p>
</li>
</ul>
<h3><strong>Movement of Water into Leaf</strong></h3>
<p>Water is constantly being lost by the leaf through transpiration. This results in the cells in the mesophyll having a lower water concentration than the vascular bundles. Water thus moves down a concentration gradient from the <a data-lesson="vascular-tissue-xylem-and-phloem" data-subject="biology" data-tutorial="seedless-plants" href="https://app.ulearngo.com/biology/seedless-plants/vascular-tissue-xylem-and-phloem">xylem</a> vessel into the living cells of the mesophyll layer and to the surface of the mesophyll cell walls. This causes water to move up from the stem by means of transpirational pull. The movement of water is maintained because water <a data-lesson="first-organic-molecules" data-subject="biology" data-tutorial="evolution-overview" href="https://app.ulearngo.com/biology/evolution-overview/first-organic-molecules">molecules</a> constantly evaporate into leaf inter-cellular air space out of the stomatal pore and into the atmosphere.</p>
<div class="ns-school-defn2">
<h3>Note:</h3>
<p>You will learn more about the transport processes in plants in another tutorial.</p>
</div>
<h3><strong>Movement of Sugars</strong></h3>
<p>Chloroplasts found in the palisade layer capture radiant energy from the sun to make <a data-lesson="connections-of-other-sugars-to-glucose-metabolism" data-subject="biology" data-tutorial="cellular-respiration" href="https://app.ulearngo.com/biology/cellular-respiration/connections-of-other-sugars-to-glucose-metabolism">glucose</a> via photosynthesis. This glucose is used to make the simple sugar sucrose. Sucrose is transported to the rest of the plant through the phloem vessels present in the vascular tissue in the leaf. Plants convert sugars to starch for long-term storage.</p>
<p><strong>Opening and closing of stomata</strong>: The opening and closing of the stomata is important for gaseous exchange, <a data-lesson="transpiration" data-subject="biology" data-tutorial="plant-systems-intro" href="https://app.ulearngo.com/biology/plant-systems-intro/transpiration">transpiration</a> and the movement of sugars. Stomata open when it is bright and when there is high humidity. When water concentration in the <a data-lesson="summarizing-the-soil" data-subject="biology" data-tutorial="soil-plant-nutrition" href="https://app.ulearngo.com/biology/soil-plant-nutrition/summarizing-the-soil">soil</a> is low, indicating that the plant is dry, chemical changes in the plant result in the closing of the stomata.</p>
<div class="wp-caption alignnone" style="width: 648px"><img alt="" height="647" src="https://data.ulearngo.com/assets/08894997-eb3f-4298-bde2-f269f962b2b4" width="648"/><p class="wp-caption-text">Confocal microscope image of guard cells and stoma. The red-staining region is chlorophyll.</p></div>
<div class="ns-school-emp">
<h2>Investigation: Examining Leaf Structure Under a Microscope</h2>
<div class="ns-school-defn">
<h3>Aim</h3>
<p>To identify different <a data-lesson="complex-tissue-structure" data-subject="biology" data-tutorial="introducing-animal-diversity" href="https://app.ulearngo.com/biology/introducing-animal-diversity/complex-tissue-structure">tissues</a> found in plant leaf.</p>
<h3>Questions:</h3>
<p>Study the image shown and answer the questions given below.</p>
<div>
<table>
<tbody>
<tr>
<td>
<p><strong>Figure A</strong></p>
</td>
<td>
<p><strong>Figure B</strong></p>
</td>
</tr>
<tr>
<td><img alt="" src="https://data.ulearngo.com/assets/418f24a7-5b2c-45e6-b8a2-49983d2905aa"/></td>
<td><img alt="" src="https://data.ulearngo.com/assets/b147af5d-1716-41d0-be8a-f17a0602d378"/></td>
</tr>
</tbody>
</table>
</div>
<ol>
<li>Compare Figures A and B. Which of the numbered structures shown in B can you identify from Figure A?</li>
<li>Which of the numbered structures shown in B are absent in A?</li>
<li>The image given in Figure A is of a Spiderworts leaf. They grow in a part of Canada where the sun shines in the morning and it is cloudy in the afternoon. Describe what changes you would expect to see to the structures in the plant leaf during the day. How would these changes compare to a plant that grows during hot, sunny days and cold, dry nights?</li>
</ol>
<section class="activity"></section>
</div>
</div>

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Leaves and Substance Transport

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Leaves and Substance Transport

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