2.2 Shared Traits of All Living Things

Created by CK-12/Adapted by Christine Miller

The Thinker

The Thinker (French: Le Penseur) is a bronze sculpture by Auguste Rodin, usually placed on a stone pedestal. The work shows a nude male figure of over life-size sitting on a rock with his chin resting on one hand as though deep in thought, often used as an image to represent philosophy.
Figure 2.2.1 The Thinker by Auguste Rodin.

You’ve probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin’s skill as a sculptor is especially evident here because the statue — which is made of bronze — looks so lifelike. How does a bronze statue differ from a living, breathing human being or other living organism? What is life? What does it mean to be alive? Science has answers to these questions.

Characteristics of Living Things

To 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.

  1. Homeostasis
  2. Organization
  3. Metabolism
  4. Growth
  5. Adaptation
  6. Response to stimuli
  7. Reproduction

Homeostasis

All 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 homeostasis. Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

Figure 2.2.2 Homeostasis of body temperature.

Organization

Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell 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?

 

Shows the image through a microscope of human cheek cells. The cells are oval in shape and light blue, with a darker blue spot close to the centre. The light blue shows the cell membrane and cytoplasm and the darker blue shows the nucleus of the cell.
Figure 2.2.3 Human cheek cells.
Shows an image through a microscope of onion cells. The cells are packed together and are rectangular in shape. Their cell walls and nuclei are stained a darker blue and the cytoplasm is whitish.
Figure 2.2.4 Onion cells.

Metabolism

All living things can use energy. They require energy to maintain internal conditions (homeostasis), to grow, and to execute other processes. Living cells use the “machinery” of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by building up large molecules from smaller ones. This form of metabolism is called anabolism. Living things can also break down, or decompose, large organic molecules into smaller ones. This form of metabolism is called catabolism.

Consider weight lifters who eat high-protein diets. A protein is a large molecule made up of several small amino acids. When we eat proteins, our digestive system breaks them down into amino acids (catabolism), so that they are small enough to be absorbed by the digestive system and into the blood. From there, amino acids are transported to muscles, where they are converted back to proteins (anabolism).

Image shows a man and woman holding hands with a toddler between them. All three are walking down a grassy path in their bare feet.
Figure 2.2.5 Humans grow and develop.

Growth

All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. A human infant, for example, has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?

A human infant has a lot of growing to do before adulthood.

 Adaptations and Evolution

An adaptation is 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 the environment. A change in the characteristics of living things over time is called evolution. It develops in a population of organisms through random genetic mutations and natural selection.

Response to Stimuli

All 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 movement of a unicellular organism in response to external chemicals (called chemotaxis) to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).

Click through the images below: the venus fly trap, the cat, and the flower are all showing response to a stimuli.

Figure 2.2.6 Examples of responses to environmental stimuli. 

Reproduction

All living things are capable of reproduction, 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 bacteria reproduce. 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.

Feature: Myth vs. Reality

Myth: Viruses are living things.

Reality: The traditional scientific view of viruses is that they originate from bits of DNA or RNA shed from the cells of living things, but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not exhibit most of the defining traits of living organisms. A single virus, called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, 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.

Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000
Figure 2.2.7 Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000.

They 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.

Within 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 bacteria. Some of the genes code for proteins needed to build new viruses, which suggests that these giant viruses may be able — or were once able — to 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 and became modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.

 

2.2 Summary

  • To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics. Humans share these traits with all other living things.
  • All living things:
    • Can maintain a more-or-less constant internal environment, which is called homeostasis.
    • Have multiple levels of organization and consist of one or more cells.
    • Can use energy and are capable of metabolism.
    • Grow and develop.
    • Can evolve adaptations to their environment.
    • Can detect and respond to environmental stimuli.
    • Are capable of reproduction, which is the process by which living things give rise to offspring.

2.2 Review Questions

  1. Identify the seven traits that most scientists agree are shared by all living things.
  2. What is homeostasis? What is one way humans fulfill this criterion of living things?
  3. Define reproduction and describe two different examples.
  4. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you ethically determine if it is a living thing?
  5. Describe viruses and which traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
  6. People who are biologically unable to reproduce are certainly still considered alive. Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
  7. What are the two types of metabolism described here. What are their differences?
  8. What are some similarities between the cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
  9. What are two processes in a living thing that use energy?
  10. Give an example of a response to stimuli in humans.
  11. Do unicellular organisms (such as bacteria) have an internal environment that they maintain through homeostasis? Why or why not?
  12. Evolution occurs through natural ____________ .
  13. If alien life is found on other planets, do you think the aliens will have cells? Discuss your answer.
  14. Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .

2.2 Explore More

Characteristics of Life, Ameoba Sisters, 2017.

Attributions

Figure 2.2.1

The Thinker MET 131262, by Auguste Rodin, 1910, from the Metropolitan Museum of Art, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.2.2

Homeostasis: Figure 4, by OpenStax College, Biology is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license. Download for free at http://cnx.org/contents/04fdb865-17a1-43d8-bb33-36f821ddd119@7.

Figure 2.2.3

Human cheek cells, by Joseph Elsbernd, 2012, on Flickr, is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 2.2.4

Onion cells 2, by Umberto Salvagnin, 2009, on Flickr, is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 2.2.5

Photo (family) by Jakob Owens on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 2.2.6

Figure 2.2.7

Bacteriophages, by Dr. Graham Beards, is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Ameoba Sisters. (2017, October 26). Characteristics of life. YouTube. https://www.youtube.com/watch?v=cQPVXrV0GNA&feature=youtu.be

OpenStax. (2016, March 23). Figure 4 The body is able to regulate temperature in response to signals from the nervous system. In OpenStax, Biology (Section 33.3). OpenStax CNX. http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8.

Wikipedia contributors. (2020, June 14). Adaptation. Wikipedia. https://en.wikipedia.org/w/index.php?title=Adaptation&oldid=962556016

Wikipedia contributors. (2020, June 21). Auguste Rodin. Wikipedia. https://en.wikipedia.org/w/index.php?title=Auguste_Rodin&oldid=963668399

Wikipedia contributors. (2020, June 22). Chemotaxis. Wikipedia. https://en.wikipedia.org/w/index.php?title=Chemotaxis&oldid=963884872

Wikipedia contributors. (2020, June 22). Evolution. Wikipedia. https://en.wikipedia.org/w/index.php?title=Evolution&oldid=963929880

Wikipedia contributors. (2020, June 20). Phototropism. Wikipedia. https://en.wikipedia.org/w/index.php?title=Phototropism&oldid=963567791

Wikipedia contributors. (2020, June 22). Virus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Virus&oldid=963829311

 

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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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