<h2>Characteristics of Life</h2>
<p>Sometimes it is difficult to tell the difference between living and nonliving things. At times, nonliving things have one or more of the characteristics of life, but it is necessary to have all of the <a data-lesson="characteristics-of-life" data-subject="biology" data-tutorial="biology-foundations" href="https://app.ulearngo.com/biology/biology-foundations/characteristics-of-life">characteristics of life</a> to be considered living. <em>Things that have all of the characteristics of life</em> are known as <strong>organisms</strong>.</p>
<div class="paragraph" data-perseus-paragraph-index="2" data-reactid=".fc94f4u0w.2.0.4.0.$0.$0.$2">
<div class="wp-caption alignnone" id="attachment_47382" style="width: 800px"><a href="https://data.ulearngo.com/assets/4d47d128-ab80-4a34-9d3a-532af6dc500c"><img alt="life-forms" class="size-full wp-image-47382" height="479" src="https://data.ulearngo.com/assets/4d47d128-ab80-4a34-9d3a-532af6dc500c" width="800"/></a><p class="wp-caption-text">Humans, animals and plants are all forms of life.</p></div>
<p class="paragraph" data-reactid=".fc94f4u0w.2.0.4.0.$0.$0.$2.$2">All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, <a data-lesson="population-growth-patterns" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/population-growth-patterns">growth</a> and development, regulation, homeostasis, <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> processing, and evolution. Generally, when viewed together, these characteristics serve to define life.</p>
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<h3 data-type="title">Order</h3>
<div class="wp-caption alignnone" id="attachment_68754" style="width: 544px"><a href="https://data.ulearngo.com/assets/16fd12e6-fa07-4880-b996-759b656ee7f8"><img alt="toad" class="size-full wp-image-68754" height="354" src="https://data.ulearngo.com/assets/16fd12e6-fa07-4880-b996-759b656ee7f8" width="544"/></a><p class="wp-caption-text">A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. Image Attribution: Ivengo/Wikimedia Commons</p></div>
<p>Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, <a data-lesson="chemical-reactions-molecules" data-subject="biology" data-tutorial="chemical-foundation-of-life" href="https://app.ulearngo.com/biology/chemical-foundation-of-life/chemical-reactions-molecules">atoms</a> make up molecules. These in turn make up cell organelles and other cellular inclusions.</p>
<p>In <a data-lesson="cell-organization" data-subject="biology" data-tutorial="cell-biology" href="https://app.ulearngo.com/biology/cell-biology/cell-organization">multicellular</a> organisms (<a href="#attachment_68754">see image above</a>), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.</p>
<h3 data-type="title">Sensitivity or Response to Stimuli</h3>
<div class="wp-caption alignnone" id="attachment_68757" style="width: 530px"><a href="https://data.ulearngo.com/assets/7419b762-5605-4238-a6a2-d785b801b0e7"><img alt="mimosa-pudica" class="size-full wp-image-68757" height="398" src="https://data.ulearngo.com/assets/7419b762-5605-4238-a6a2-d785b801b0e7" width="530"/></a><p class="wp-caption-text">The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. Image Attribution: Alex Lomas.</p></div>
<p>Organisms respond to diverse <strong>stimuli</strong>. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (<a href="#attachment_68757">see image above</a>). Even tiny bacteria can move toward or away from chemicals (a process called <em data-effect="italics">chemotaxis</em>) or light (<em data-effect="italics">phototaxis</em>). Movement toward a <a data-lesson="reception-and-transduction-intro" data-subject="biology" data-tutorial="sensory-systems" href="https://app.ulearngo.com/biology/sensory-systems/reception-and-transduction-intro">stimulus</a> is considered a <strong>positive response</strong>, while movement away from a stimulus is considered a <strong>negative response</strong>.</p>
<h3 data-type="title">Reproduction</h3>
<p>Another important characteristic of life is reproduction. <strong>Reproduction</strong> is the ability of an organism to make more of the same type of organism. The new organisms that are made are called <em>offspring</em>. Although <a data-lesson="reproduction-3" data-subject="biology" data-tutorial="fungi" href="https://app.ulearngo.com/biology/fungi/reproduction-3">reproduction</a> is not needed for the survival of an individual organism, it must occur for the continuation of the organism’s species.</p>
<div class="wp-caption alignnone" id="attachment_47383" style="width: 800px"><a href="https://data.ulearngo.com/assets/a33af922-5780-4f0d-b83c-70baec6fe46b"><img alt="american-crocodile" class="size-full wp-image-47383" height="537" src="https://data.ulearngo.com/assets/a33af922-5780-4f0d-b83c-70baec6fe46b" width="800"/></a><p class="wp-caption-text">American Crocodile. Image Attribution: Animals, by National Geographic</p></div>
<p>A <em>species</em> consists of a <em>group of organisms</em> that can mate with each other and produce <a data-lesson="reproduction-methods" data-subject="biology" data-tutorial="reproductive-system" href="https://app.ulearngo.com/biology/reproductive-system/reproduction-methods">offspring</a> that are able to reproduce. For example, there are many species of crocodiles including the American crocodile (<a href="#attachment_47383">see image above</a>), the Australian freshwater crocodile, and the saltwater crocodile. American crocodiles reproduce only American crocodiles. In fact, without reproduction, the species would die out.</p>
<p>When reproduction occurs, organisms pass along genes containing <a href="https://app.ulearngo.com/tutorials/introducing-genetics/dna">DNA</a> to an their offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.</p>
<h3 data-type="title">Growth and Development</h3>
<div class="wp-caption alignnone" id="attachment_68759" style="width: 530px"><a href="https://app.ulearngo.com/assets/uploads/2017/01/cats.jpg"><img alt="kittens" class="size-full wp-image-68759" height="311" src="https://data.ulearngo.com/assets/f2b6822c-f833-4300-bb45-247fc2a6c764" width="530"/></a><p class="wp-caption-text">Although no two look alike, these kittens have inherited genes from both parents and share many of the same characteristics. (credit: Rocky Mountain Feline Rescue)</p></div>
<p>Another characteristic of life is that growth and development must take place. An organism begins life as a single cell. As time passes, it grows and develops. As growth and development take place, the organism takes on the characteristics of its species. <strong>Growth</strong> results in the formation of new structures and an increase in the amount of living material. <strong>Development</strong> refers to the changes that occur in each organism’s life.</p>

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Properties of Life

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Properties of Life

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