2.8: Case Study Conclusion: Our Invisible Inhabitants and Chapter Summary

Case Study Conclusion: Our Invisible Inhabitants

As you may recall, Wajiha’s strep throat was caused by Streptococcus pyogenes bacteria, the species shown in the photomicrograph above. Wajiha took antibiotics to kill the S. pyogenes, but this also killed her “good” bacteria, throwing off the balance of microorganisms living inside of her, which resulted in diarrhea and a yeast infection.

Figure \(\PageIndex{1}\): Streptococcus pyogenes

After reading this chapter, you should now know that microorganisms such as the bacteria and yeast that live in humans are also similar to us in many ways. They are living organisms and therefore share the traits of homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction with us. They, like us, contain genes, consist of cells, and have the ability to evolve. Wajiha’s beneficial gut bacteria help digest her food as part of her metabolic processes. Wajiha got a yeast infection likely because the growth and reproductive rates of the yeast living on her body were not held in check by beneficial bacteria after she took antibiotics. You can see that there are many ways in which an understanding of the basic characteristics of life can directly apply to your own.

You also learned how living organisms are classified, from bacteria that are in the Bacteria domain, to yeast (fungus kingdom) and humans (animal kingdom) that are both in the Eukarya domain. You probably now recognize that Streptococcus pyogenes is the binomial nomenclature for this species and the fact that Streptococcus refers to the genus name.

As Wajiha’s doctor told her, there are many different species of microorganisms living in the human digestive system. You should recognize this as a type of biodiversity called species diversity. This diversity is maintained in a balance, or homeostasis, that can be upset when one type of organism is killed — for instance, by antibiotics.

Wajiha’s doctor advised her to complete the entire course of antibiotics because stopping too early would kill the bacteria that are most susceptible to the antibiotic while leaving the bacteria that are more resistant to the antibiotic alive. This difference in susceptibility to antibiotics is an example of genetic diversity. Over time, the surviving antibiotic-resistant bacteria will have increased survival and reproductive rates compared to the more susceptible bacteria, and the trait of antibiotic resistance will become more common in the population. In this way, the bacteria can evolve and become better adapted to their environment — at a major cost to our health because our antibiotics will no longer be effective. This issue of improper use of antibiotics leading to increased antibiotic resistance is a major concern of public health experts.

After reading the last section of this chapter, you know how humans are classified and some characteristics of humans and our near relatives. Beyond our more obvious features of big brains, intelligence, and the ability to walk upright, we also serve as a home to many different organisms that may be invisible to the naked eye but play a big role in maintaining our health.

Chapter Summary

In this chapter, you learned about the basic principles of biology and how humans are situated among other living organisms. Specifically, you learned:

• To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics, including:
  • Maintaining a more-or-less constant internal environment, which is called homeostasis.
  • Having multiple levels of the organization and consisting of one or more cells.
  • Using energy and being capable of metabolism.
  • The ability to grow and develop.
  • The ability to evolve adaptations to the environment.
  • The ability to detect and respond to environmental stimuli.
  • The ability to reproduce, which is the process by which living things give rise to offspring.
• Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity within species, and ecosystem diversity.
• The formal biological definition of species is a group of actually or potentially interbreeding organisms. In reality, organisms are often classified into species on the basis of morphology.
• A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, called binomial nomenclature.
• The domain, a taxon higher than the kingdom, was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. Humans and other animal species are placed in the Eukarya domain.
• Modern systems of classification take into account phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams.
• The human species, Homo sapiens, is placed in the primate order of the class of mammals, which are chordates in the animal kingdom.
• Traits humans share with other primates include five digits with nails and opposable thumbs; an excellent sense of vision including the ability to see in color and stereoscopic vision; a large brain, high degree of intelligence, and complex behaviors. Like most other primates, we also live in social groups. Many of our primate traits are adaptations to life in the trees.
• Within the primate order, our species is placed in the hominid family, which also includes chimpanzees, gorillas, and orangutans.
• The genus Homo first evolved about 2.8 million years ago. Early Homo species were fully bipedal but had small brains. All are now extinct.
• During the last 800,000 years, Homo sapiens evolved, with smaller faces, jaws, and front teeth but much bigger brains than earlier Homo species.
• The human body consists of multiple parts that function together to maintain life. The biology of the human body incorporates the body’s structure, or anatomy, and the body’s functioning, or physiology.

• The organization of the human body is a hierarchy of increasing size and complexity, starting at the level of atoms and molecules and ending at the level of the entire organism.
• Cells are the level of organization above atoms and molecules, and they are the basic units of structure and function of the human body. Each cell carries out basic life functions as well as other specific roles. Cells of the human body show a lot of variation.
  • Variations in cell function are generally reflected in variations in cell structure.
  • Some cells are unattached to other cells and can move freely; others are attached to each other and cannot move freely. Some cells can divide readily and form new cells; others can divide only under exceptional circumstances. Many cells are specialized to produce and secrete particular substances.
  • All the different cell types within an individual have the same genes. Cells can vary because different genes are expressed depending on the cell type.
  • Many common types of human cells consist of several subtypes of cells, each of which has a special structure and function. For example, subtypes of bone cells include osteocytes, osteoblasts, osteogenic cells, and osteoclasts.
• A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues that make up all the organs of the human body: epithelial, muscle, nervous, and connective tissues.
  • Connective tissues, such as bone and blood, are made up of cells that are separated by non-living material, called the extracellular matrix.
  • Epithelial tissues, such as skin and mucous membranes, protect the body and its internal organs and secrete or absorb substances.
  • Muscle tissues are made up of cells that have the unique ability to contract. They include skeletal, smooth, and cardiac muscle tissues.
  • Nervous tissues are made up of neurons, which transmit electrical messages, and glial cells of various types, which play supporting roles. Types of nervous tissues include gray matter, white matter, nerves, and ganglia.
• An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples include the brain and heart.
  • Many organs are composed of a major tissue that performs the organ’s main function, as well as other tissues that play supporting roles.
  • The human body contains five organs that are considered vital for survival. They are the heart, brain, kidneys, liver, and lungs. If any of these five organs stops functioning, the death of the organism is imminent without medical intervention.
• An organ system is a group of organs that work together to carry out a complex overall function. For example, the skeletal system provides structure to the body and protects internal organs.
  • There are 11 major organ systems in the human organism. They are the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive systems. Only the reproductive system varies significantly between males and females.
• The human body is divided into a number of body cavities. A body cavity is a fluid-filled space in the body that holds and protects internal organs. The two largest human body cavities are the ventral cavity and the dorsal cavity.
  • The ventral cavity is at the anterior, or front, of the trunk. It is subdivided into the thoracic cavity and abdominopelvic cavity.
  • The dorsal cavity is at the posterior, or back, of the body, and includes the head and the back of the trunk. It is subdivided into the cranial cavity and spinal cavity.
• Organ systems of the human body must work together to keep the body alive and functioning normally. This requires communication among organ systems. This is controlled by the autonomic nervous system and endocrine system. The autonomic nervous controls involuntary body functions, such as heart rate and digestion. The endocrine system secretes hormones into the blood that travel to body cells and influence their activities.
  • Cellular respiration is a good example of organ system interactions because it is a basic life process that occurs in all living cells. It is the intracellular process that breaks down glucose with oxygen to produce carbon dioxide and energy. Cellular respiration requires the interaction of the digestive, cardiovascular, and respiratory systems.
  • The fight-or-flight response is a good example of how the nervous and endocrine systems control other organ system responses. It is triggered by a message from the brain to the endocrine system and prepares the body for flight or a fight. Many organ systems are stimulated to respond, including the cardiovascular, respiratory, and digestive systems.
  • Digesting food requires teamwork between the digestive system and several other organ systems, including the nervous, cardiovascular, and muscular systems.
  • Playing softball or doing other voluntary physical activities may involve the interaction of nervous, muscular, skeletal, respiratory, and cardiovascular systems.
• Homeostasis is the condition in which a system such as the human body is maintained in a more-or-less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain homeostasis.
  • For any given variable, such as body temperature, there is a particular set point that is the physiological optimum value. The spread of values around the setpoint that is considered insignificant is called the normal range.
  • Homeostasis is generally maintained by a negative feedback loop that includes a stimulus, sensor, control center, and effector. Negative feedback serves to reduce an excessive response and to keep a variable within the normal range. Negative feedback loops control body temperature and blood glucose level.
  • Sometimes homeostatic mechanisms fail, resulting in homeostatic imbalance. Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, making the elderly more susceptible to disease.
• Positive feedback loops are not common in biological systems. Positive feedback serves to intensify a response until an endpoint is reached. Positive feedback loops control blood clotting and childbirth.

Chapter Summary Review

1. What are the seven traits of life?
2. A scientist is exploring in a remote area with many unidentified species. They find an unknown object that does not appear to be living. What is one way they could tell whether it is a dead organism that was once alive, versus an inanimate object that was never living?
3. Cows are dependent on bacteria living in their digestive systems to help break down cellulose in the plant material that the cows eat. Explain what characteristics these bacteria must have to be considered living organisms themselves, and not just part of the cow.
4. What is the basic unit of structure and function in living things?
5. Give one example of homeostasis that occurs in humans.
6. Can a living thing exist without using energy? Why or why not?
7. True or False. Evolution is a change in the characteristics of living things over time.
8. True or False. Only some living things have genes.
9. Give an example of a response to stimuli that occurs in a unicellular organism.
10. A scientist discovers two types of similar-looking insects that have not been previously identified. Answer the following questions about this discovery.
    1. What is one way they can try to determine whether the two types are the same species?
    2. If they are not the same species, what are some ways they can try to determine how closely related they are to each other?
    3. What is the name for a type of diagram they can create to demonstrate their evolutionary relationship to each other and to other insects?
    4. If they determine that the two types are different species but the same genus, create your own names for them using binomial nomenclature. You can be creative and make up the genus and species names, but be sure to put them in the format of binomial nomenclature.
    5. If they are the same species but have different colors, what kind of biodiversity does this most likely reflect?
    6. If they are the same species, but one type of insect has a better sense of smell for their limited food source than the other type, what do you think will happen over time? Assume the insects will experience natural selection.
11. Put the following taxa in order from the most specific to the most inclusive: phylum; species; kingdom; genus; family; domain; class; order
12. Humans are in the which domain?
13. Monkeys, apes, and humans are all in the:
    1. Same genus
    2. Same order
    3. Same class
    4. Both B and C
14. Amphibians, such as frogs, have a backbone but no hair. What is the most specific taxon that they share with humans?
15. Arboreal means:
    1. Living on the ground
    2. Living in the ocean
    3. Living in trees
    4. Living on grasslands
16. What is one characteristic of extinct Homo species that was larger than that of modern humans?
17. What is one characteristic of modern humans that is larger than that of extinct Homospecies?
18. True or False. Most primates live in social groups.
19. True or False. Most other mammals have longer lifespans than primates.
20. True or False. Archaea are classified into the Bacteria domain.
21. How is the long period of dependency of infants on adults in primates related to learning?
22. Name one type of primate in the hominid family, other than humans.
23. Why do you think that scientists compare the bones of structures (such as the feet) of extinct Homo species to ours?
24. Some mammals other than primates also have their eyes placed in the front of their face, such as cats. How do you think the vision of a cat compares to that of a mouse, where the eyes are placed more at the sides?
25. Living sponges are animals. Are we in the same kingdom as sponges? Explain your answer.
26. Compare and contrast tissues and organs.
27. Osteocyte cells are part of which type of tissue and organ system?
28. Adipose tissue, or body fat, is the same general type of tissue as:
    1. mucous membranes
    2. gray matter
    3. skin
    4. blood
29. Which type of tissue lines the inner and outer surfaces of the body?
30. True or False. The extracellular matrix that surrounds cells is always solid.
31. True or False. Skin is an organ.
32. What is a vital organ? What happens if a vital organ stops working?
33. Name three organ systems that transport or remove wastes from the body.
34. Name two types of tissue in the digestive system.
35. For each of the following body functions, choose the organ system that is most associated with the function. Organ systems: integumentary; skeletal; muscular; nervous; endocrine; cardiovascular; lymphatic; respiratory; digestive; urinary; reproductive
    1. Processes sensory information
    2. Secretes hormones
    3. Releases carbon dioxide from the body to the outside world
    4. Produces gametes
    5. Controls water balance in the body
36. The spleen is part of which organ system?
    1. Digestive
    2. Lymphatic
    3. Integumentary
    4. Urinary
37. Describe one way in which the integumentary and cardiovascular systems work together to regulate homeostasis in the human body.
38. Name the two largest body cavities in humans and describe their general locations.
39. What are the names given to the three body cavity divisions where the reproductive organs are located?
40. True or False. There are two pleural cavities.
41. True or False. Body cavities are filled with air.
42. The pituitary gland is in which organ system? Describe how the pituitary gland increases metabolism.
43. When the level of thyroid hormone in the body gets too high, it acts on other cells to reduce the production of more thyroid hormone. What type of feedback loop does this represent?
44. Hypothetical organ A is the control center in a feedback loop that helps maintain homeostasis. It secretes molecule A1 which reaches organ B, causing organ B to secrete molecule B1. B1 negatively feeds back onto organ A, reducing the production of A1 when the level of B1 gets too high.
    1. What is the stimulus in this feedback loop?
    2. If the level of B1 falls significantly below the setpoint, what do you think happens to the production of A1? Why?
    3. What is the effector in this feedback loop?
    4. If organs A and B are part of the endocrine system, what type of molecules do you think A1 and B1 are likely to be?
45. What are the two main systems that allow various organ systems to communicate with each other?
46. The hypothalamus is part of the:
    1. spinal cord
    2. thoracic cavity
    3. kidneys
    4. brain
47. What are two functions of the hypothalamus that you learned about in this chapter?

Attributions

1. Streptococcus pyogenes by CDC, public domain via Wikimedia Commons