<h2 class="section-header"><strong class="title">Movement Across Membranes</strong></h2>
<p class="para">Movement of substances across cell membranes is necessary as it allows cells to acquire oxygen and nutrients, excrete waste products and control the concentration of required substances in the cell (e.g oxygen, water, hormones, ions, etc). The key processes through which such movement occurs include <em class="emphasis">diffusion, osmosis, facilitated diffusion</em> and <em class="emphasis">active transport.</em></p>
<h2>Diffusion</h2>
<p>Diffusion is the movement of substances from a region of high concentration to low concentration. It is therefore said to occur <i class="emphasis">down a concentration gradient</i>. The diagram below shows the movement of dissolved particles within a liquid until eventually becoming randomly distributed.</p>
<div class="wp-caption alignnone" id="attachment_100081" style="width: 382px"><a href="https://data.ulearngo.com/assets/9056bd8a-c528-49e6-972c-ac5a4caffc4a"><img alt="diffusion" class="size-full wp-image-100081" height="233" src="https://data.ulearngo.com/assets/9056bd8a-c528-49e6-972c-ac5a4caffc4a" width="382"/></a><p class="wp-caption-text">Image credit: Siyavula</p></div>
<p>Diffusion is a <i class="emphasis">passive process</i> which means it does not require any energy input. It can occur across a living or non-living membrane and can occur in a liquid or gas medium. Due to the fact that diffusion occurs across a concentration gradient it can result in the movement of substances into or out of the cell. Examples of substances moved by diffusion include carbon dioxide, oxygen, water and other small <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> that are able to dissolve within the lipid bilayer.</p>
<div class="ns-school-emp">
<h2 class="example-header"><span class="cnx_label"> </span><strong class="title">Investigation: Observing Diffusion</strong></h2>
<div class="ns-school-defn">
<h3 class="para"><b class="emphasis">Aim</b></h3>
<p class="para">To observe diffusion</p>
<h3 class="para"><b class="emphasis">Apparatus</b></h3>
<div class="list">
<ul class="bullet">
<li class="item">1 x 500 ml beaker</li>
<li class="item">large funnel</li>
<li class="item">plastic straw</li>
<li class="item">potassium permanganate crystals</li>
</ul>
</div>
<h3 class="para"><b class="emphasis">Method</b></h3>
<div class="list">
<ol class="arabic">
<li class="item">Fill a beaker with water and allow it to stand for a few minutes so that water movement stops.</li>
<li class="item">Place a large funnel into the water so that it touches the bottom of the beaker. Drop a few small potassium permanganate crystals through the straw. Remove the funnel carefully and slowly.</li>
<li class="item">Observe the size of the area that is coloured by the potassium permanganate at the beginning of the experiment, after 5 minutes and then after 20 minutes.</li>
</ol>
</div>
<h3 class="para"><b class="emphasis">Questions</b></h3>
<div class="list">
<ol class="arabic">
<li class="item">What do you observe happening in the beaker?</li>
<li class="item">What can you conclude based on your observations?</li>
<li class="item">Explain how using hot water would affect the results of this <a href="https://app.ulearngo.com/tutorials/introducing-study-of-life/experimenting-and-forming-theories">experiment</a> (remember that when you explain you need to give a reason for your answer).</li>
</ol>
</div>
</div>
</div>
<h2>Video: Diffusion Animation</h2>
<p>You will see a brief animation about diffusion in the video below by <a href="https://www.youtube.com/channel/UCobgOt3poNKlml3Y5TnVU8Q">Smart Learning for All</a>. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Consider a container in which there are some air particles on one side. These molecules possess energy. They will start moving randomly, bouncing and bumping into each other. Eventually these molecules will start spreading out and fill the entire container. This is the concept of diffusion.</p>
<p><iframe allowfullscreen="allowfullscreen" frameborder="0" height="360" src="https://www.youtube.com/embed/T7rbEJrlyvk?rel=0&amp;modestbranding=1&amp;cc_lang_pref=en&amp;cc_load_policy=1" width="640"></iframe></p>

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Diffusion

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Diffusion

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