{"id":66,"date":"2019-06-17T17:46:01","date_gmt":"2019-06-17T21:46:01","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/2-2-shared-traits-of-all-living-things\/"},"modified":"2022-01-19T15:39:43","modified_gmt":"2022-01-19T20:39:43","slug":"2-2-shared-traits-of-all-living-things","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/2-2-shared-traits-of-all-living-things\/","title":{"raw":"2.2\u00a0Shared Traits of All Living Things","rendered":"2.2\u00a0Shared Traits of All Living Things"},"content":{"raw":"Created by CK-12\/Adapted by Christine Miller\n

The Thinker<\/h1>\n[caption id=\"attachment_65\" align=\"alignright\" width=\"188\"]\"The Figure 2.2.1 The Thinker by Auguste Rodin.<\/em>[\/caption]You've probably seen this famous statue created by the French sculptor Auguste Rodin<\/a>. Rodin's skill as a sculptor is especially evident here because the statue \u2014 which is made of bronze \u2014 looks so lifelike. How does a bronze statue\u00a0differ from a living,\u00a0breathing\u00a0human being or other living organism? What is life? What does it mean to be alive? Science has answers to these questions.\n
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Characteristics of Living Things<\/h1>\n<\/div>\nTo be classified as a living thing, most scientists agree that an object must have all seven of the traits listed below. Humans share these characteristics with other living things.\n
    \n \t
  1. Homeostasis<\/li>\n \t
  2. Organization<\/li>\n \t
  3. Metabolism<\/li>\n \t
  4. Growth<\/li>\n \t
  5. Adaptation<\/li>\n \t
  6. Response to stimuli<\/li>\n \t
  7. Reproduction<\/li>\n<\/ol>\n

    Homeostasis<\/h2>\nAll living things are able to maintain a more-or-less constant internal environment. Regardless of the conditions around them, they can keep things relatively stable on the inside. The condition in which a system is maintained in a more-or-less steady state is called [pb_glossary id=\"1346\"]homeostasis<\/strong>[\/pb_glossary].\u00a0Human beings, for example, maintain a stable internal body\u00a0temperature. If you go outside when the air temperature is below\u00a0freezing, your body doesn't freeze. Instead, by shivering and other means, it maintains a stable internal temperature.\n\n[caption id=\"attachment_65\" align=\"aligncenter\" width=\"534\"]\"\" Figure 2.2.2 Homeostasis of body temperature.<\/em>[\/caption]\n\n
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    Organization<\/span><\/h2>\n<\/div>\nLiving things have multiple levels of organization. Their molecules are organized into one or more\u00a0cells. A\u00a0[pb_glossary id=\"1298\"]cell<\/strong>[\/pb_glossary] is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in Figures 2.2.3 and 2.2.4. What similarities do you see?\n\n \n\n[caption id=\"attachment_65\" align=\"aligncenter\" width=\"296\"]\"Shows Figure 2.2.3 Human cheek cells.<\/em>[\/caption]\n
    [caption id=\"attachment_63\" align=\"aligncenter\" width=\"297\"]\"Shows Figure 2.2.4 Onion cells.<\/em>[\/caption]<\/div>\n

    Metabolism<\/h2>\nAll living things can use\u00a0energy. They\u00a0require\u00a0energy\u00a0to maintain internal conditions (homeostasis), to grow, and to execute other processes.\u00a0Living\u00a0cells\u00a0use\u00a0the \"machinery\" of\u00a0[pb_glossary id=\"1352\"]metabolism<\/strong>[\/pb_glossary],\u00a0which is the building up and breaking down of chemical compounds.\u00a0Living things can transform\u00a0energy\u00a0by building up large molecules from smaller ones. This form of metabolism is called\u00a0[pb_glossary id=\"1320\"]anabolism<\/strong>.[\/pb_glossary] Living things can also break down, or decompose, large organic molecules into smaller ones. This form of metabolism is called\u00a0[pb_glossary id=\"1326\"]catabolism<\/strong>[\/pb_glossary].\n\nConsider\u00a0weight\u00a0lifters\u00a0who\u00a0eat high-protein\u00a0diets. A\u00a0protein\u00a0is a large molecule made up of several small\u00a0amino acids. When we eat\u00a0proteins, our\u00a0digestive system\u00a0breaks\u00a0them down\u00a0into\u00a0amino acids\u00a0(catabolism), so that they are small enough to be absorbed\u00a0by\u00a0the\u00a0digestive system\u00a0and into the\u00a0blood. From there,\u00a0amino acids\u00a0are transported to\u00a0muscles, where they are converted back to\u00a0proteins\u00a0(anabolism).\n\n[caption id=\"attachment_65\" align=\"alignright\" width=\"300\"]\"Image Figure 2.2.5 Humans grow and develop.<\/em>[\/caption]\n

    Growth<\/h2>\nAll living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of [pb_glossary id=\"1320\"]anabolism<\/strong>[\/pb_glossary] than [pb_glossary id=\"1326\"]catabolism<\/strong>[\/pb_glossary]. A human infant, for example,\u00a0has changed\u00a0dramatically in size by the time it reaches\u00a0adulthood, as is apparent from the image\u00a0below. In what other ways do we change as we grow from\u00a0infancy\u00a0to\u00a0adulthood?\n
    \n\nA human infant has a lot of growing to do before adulthood.\n\n<\/div>\n

    \u00a0Adaptations and Evolution<\/h2>\nAn\u00a0adaptation<\/a>\u00a0is a characteristic that helps living things survive and reproduce in a given environment. It comes about because living things have the ability to change over time in response to\u00a0the environment. A change in the characteristics of living things over time is called\u00a0evolution<\/a>.\u00a0It\u00a0develops in a\u00a0population\u00a0of organisms\u00a0through random genetic\u00a0mutations and\u00a0natural selection.\n

    Response to Stimuli<\/h2>\nAll living things detect changes in their environment and respond to them. These stimuli can be internal or external, and the response can take many forms, from the\u00a0movement\u00a0of a unicellular organism\u00a0in response to\u00a0external chemicals (called chemotaxis<\/a>) to complex reactions involving all the\u00a0senses\u00a0of a\u00a0multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the\u00a0sun\u00a0(called phototropism<\/a>).\n\nClick through the images below: the venus fly trap, the cat, and the flower are all showing response to a stimuli.\n\n[h5p id=\"7\"]\n\nFigure 2.2.6 Examples of responses to environmental stimuli.\u00a0<\/em>\n

    Reproduction<\/h2>\nAll living things are capable of\u00a0[pb_glossary id=\"1369\"]reproduction<\/strong>[\/pb_glossary], the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing to form two daughter cells, which is how\u00a0bacteria\u00a0reproduce. Reproduction in human beings and many other organisms, of course, is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.\n

    [h5p id=\"8\"]<\/h3>\n

    Feature: Myth vs. Reality<\/h1>\nMyth:<\/strong>\u00a0Viruses are living things.\n\nReality:<\/strong>\u00a0The traditional scientific view of viruses<\/a> is that they originate from bits of\u00a0DNA\u00a0or\u00a0RNA shed from the cells of living things, but that they are not living things\u00a0themselves<\/em>. Scientists have long argued that viruses are not living things because they do not\u00a0exhibit\u00a0most of the defining traits of living organisms. A single\u00a0virus, called a virion, consists of a set of genes (DNA\u00a0or RNA) inside a protective\u00a0protein\u00a0coat, called a capsid. Viruses have organization, but they are not cells, and they do not possess the cellular \"machinery\" that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow.\n\n[caption id=\"attachment_65\" align=\"alignleft\" width=\"256\"]\"Transmission Figure 2.2.7 Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000.<\/em>[\/caption]\n\nThey do not seem to respond to their environment, and they can reproduce only by invading and using \"tools\" inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them! That's why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.\n\nWithin the last decade, new discoveries in virology (the study of viruses) suggest that this traditional view about viruses may be incorrect, and that the \"myth\" that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms, such as\u00a0bacteria. Some of the genes code for\u00a0proteins\u00a0needed to build new viruses,\u00a0which suggests\u00a0that these giant viruses may be able\u00a0\u2014\u00a0or were once able\u00a0\u2014\u00a0to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells, but at some point they lost their cellular nature\u00a0and became\u00a0modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other\u00a0species require a host to complete their\u00a0life cycle.\n\n \n
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    2.2 Summary<\/span><\/h1>\n<\/header>\n
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