Maintaining a Dynamic Equilibrium. The Need for Homeostasis

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1 THEME III Maintaining a Dynamic Equilibrium 10 The Need for Homeostasis HOMEOSTASIS Organisms live in a world of changing conditions. But, to remain alive, every organism needs to keep the conditions inside of itself fairly constant. An organism must have ways to keep its internal conditions from changing as its external environment changes. This ability of all living things to detect deviations and to maintain a constant internal environment is known as homeostasis. An obvious change that has occurred in the course of evolution is the development of larger multicellular organisms from microscopic, single-celled ones. Is there an advantage to being multicellular? Being microscopic and single-celled makes it difficult for an organism to maintain homeostasis. Having a multicellular body makes possible many types of protection against changes in the environment. In other words, an organism with many cells is able to have structures and systems that protect its individual cells from external changes, thus helping it to stay alive. (See Figure 10-1.) To maintain homeostasis, organisms actually must make constant changes. That is why Surface of skin Blood vessels Hair Figure 10-1 Multicellular organisms have systems and structures that help them maintain homeostasis. For example, our skin has features that detect and respond to changes in external temperature. homeostasis is often referred to as maintaining a dynamic equilibrium. Dynamic means active, and equilibrium means balanced. Homeostasis requires active balancing. THE CELL AND ITS ENVIRONMENT One of the most fascinating facts about our bodies is that each of our many, many cells is 72

2 Chapter 10: The Need for Homeostasis 73 Capillary Lymph Red blood cells Skull Cerebrum Lymph vessel ICF Tissue cells Figure 10-2 All the cells in our body are surrounded by intercellular fluid (ICF). Materials are exchanged between the cells and the fluid, which helps to maintain stable conditions inside each of the cells. surrounded by liquid. The smallest blood vessels in our bodies, the capillaries, are close to every cell. There is a small amount of space between the capillaries and the body cells. This space is filled with fluid. The fluid that surrounds cells is made up mostly of water, with many substances dissolved in it. This intercellular fluid is important in helping to maintain stable conditions inside each of our cells. Many materials are exchanged between the cells and the fluid. In turn, materials may be exchanged between the fluid and the blood in the capillaries. All of this is done to make sure that each and every body cell is able to maintain homeostasis and remain healthy. (See Figure 10-2.) MAINTAINING HOMEOSTASIS WHEN WE EXERCISE Exercise involves increased muscle activity. This activity creates changes within the body. To maintain homeostasis, the body needs to be able to respond to these changes. An example of a change that occurs when we exercise is the increase in the body of carbon dioxide (CO 2 ), produced by muscle cells as a result of cellular respiration. The level of CO 2 increases in both the intercellular fluid and the blood. To maintain homeostasis, the body first must be able to detect this change and then respond to the change. A structure in the brain detects the increased CO 2 level in the blood passing through the brain and in the fluid around the brain cells. As a result, this part of the brain Medulla Spinal cord sends signals to the chest to increase the rate of breathing and the amount of air taken in on each breath. These changes in breathing increase the exchange of gases in the lungs, lowering the CO 2 levels in the body. These lower levels are then detected in the brain, which in turn sends a signal to reduce the breathing rate. This process is an example of a feedback mechanism. Feedback mechanisms are important in maintaining homeostasis. (See Figure 10-3.) FEEDBACK MECHANISMS Cerebellum Vertebrae Figure 10-3 A structure in the brain (the medulla) monitors the amount of CO 2 in the body, adjusting the breathing rate to maintain proper levels. Carbon dioxide levels in your body are regulated somewhat as a thermostat regulates the temperature of your house. A thermostat measures the temperature of the air in a room. When the air temperature in the house falls below a preset figure, the thermostat turns a furnace on. The furnace produces heat, and the temperature of the air in the house increases. When the temperature of the air rises above the preset temperature, the thermostat tells the furnace to shut down. The temperature in the house stops rising, the air begins to cool, and the thermostat continues the cycle of telling the furnace to produce heat or to shut down. (See Figure 10-4.)

3 74 Reviewing the Living Environment: Biology Thermostat Sensor Temperature sensors in brain Hypothalamus Body temperature control Skeletal muscles Stomate Guard cells Nucleus Chloroplasts Temperature sensors in skin Leaf cells Stomate closed Nucleus Stomate open Home heating system Sweat glands In this type of feedback mechanism, a change occurs that produces another change, which in turn reverses the first change. This is an important process in maintaining homeostasis. The following are parts of a feedback mechanism used in maintaining homeostasis: Sensor. Something must be able to detect a change. A thermometer attached to a thermostat is a sensor. In the body, structures in the brain detect changes in CO 2 levels. Control unit. Something must know what the correct level should be. A thermostat in a house is set to a particular comfort level. Information in the brain is preset at the correct CO 2 level. Effector. Something must take instructions from the control unit and make the necessary changes. In a house, the effector would be a furnace or an air conditioner. In the body, the effector for CO 2 levels would be the muscles in the chest that are used for breathing. MAINTAINING HOMEOSTASIS: WATER BALANCE IN PLANTS Superficial arteries Figure 10-4 Both CO 2 levels and body temperature are regulated by feedback mechanisms, much as a thermostat controls the temperature in a room. Maintaining water balance is a major concern for all living things. Plants as well as animals Figure 10-5 Special openings in the surface of a leaf function to maintain water balance in plants. must maintain water balance. Openings in the surface of a leaf are adapted to control the loss of water. Each opening is surrounded by two guard cells. These guard cells,like any cells, allow water to diffuse through their cell membrane. When water is abundant, it moves into the guard cells. The increased quantity of water increases the pressure within the cells. Guard cells are somewhat curved in shape; when they are filled with water, they become even more curved. The space between them expands, the opening widens, and excess water is allowed to evaporate out of the air spaces inside the leaf to the air that surrounds the plant. (See Figure 10-5.) When water becomes scarce, the guard cells become less curved in shape and the opening closes. Water loss is reduced and the plant is able to maintain its water balance. SYSTEMS FOR MAINTAINING HOMEOSTASIS Multicellular animals have evolved highly organized, complex organ systems especially suited to maintaining a relatively constant internal environment. These organ systems include the excretory system, which regulates the chemistry of the body s fluids while removing harmful wastes; the nervous system, which uses electrochemical impulses to regulate body functions; the endocrine system, which produces hormones chemical messengers essential in regulating the functions and behavior of the body; and, finally, the immune system, which uses a set of defenses to protect the body from dangerous substances and microorganisms that could upset the internal balance on which life itself depends.

4 Chapter 10: The Need for Homeostasis 75 Chapter 10 Review Part A Multiple Choice 1. Organisms undergo constant chemical changes as they maintain an internal balance known as 1 interdependence 2 synthesis 3 homeostasis 4 recombination 2. What characteristic has evolved that helps to maintain homeostasis? 1 taller bodies with larger cells 2 shorter bodies with fewer cells 3 multicellular bodies with many cells 4 multicellular bodies with fewer cells 3. Asystem in dynamic equilibrium 1 makes constant changes 2 changes in intervals or steps 3 changes very infrequently 4 never changes at all 4. Intercellular fluid is made up mostly of 1 water 2 blood 3 mineral salts 4 cytoplasm 5. Intercellular fluid is important for the exchange of materials between 1 body cells and arteries 2 body cells and veins 3 veins and capillaries 4 body cells and capillaries 6. As a result of exercise, CO 2 levels increase in the 1 blood only 2 intercellular fluid only 3 blood and intercellular fluid 4 muscles only 7. The brain sends a signal to increase the breathing rate when the CO 2 level has 1 not changed for a while 2 decreased 3 increased 4 increased, then decreased 8. The increased breathing rate signaled by the brain serves 1 to increase the CO 2 level in the body 2 to decrease the CO 2 level in the body 3 to decrease the O 2 level in the body 4 no function in changing O 2 and CO 2 levels 9. In adjusting the CO 2 level, the part of the body that acts like a thermostat in the home is the 1 brain 2 chest 3 lungs 4 muscle tissue 10. If an organism fails to maintain homeostasis, the result may be 1 disease only 2 death only 3 disease or death 4 none of the above 11. Achange in the body results in another change. This second change reverses the first change in order to maintain homeostasis. This describes a type of 1 control mechanism 2 feedback controller 3 feedback mechanism 4 effector mechanism 12. The effector for adjusting the CO 2 level in the body would be the 1 blood tissue 2 brain 3 lungs 4 chest muscles 13. Why might a blood clot be important to maintaining homeostasis? 1 It slows the flow of blood through the body. 2 It prevents the loss of blood from the body. 3 It increases the amount of water in the blood. 4 It adds more cells to the blood tissue. 14. The changing shape of a plant s guard cells helps to 1 allow the plant to grow stronger 2 prevent the plant from losing food 3 regulate the temperature of the plant 4 maintain the plant s water balance

5 76 Reviewing the Living Environment: Biology Base your answer to question 15 on the table below, which shows the rate of water loss in three different plants. Plant Liters of Water Lost Per Day Part B Analysis and Open Ended Base your answer to question 19 on the photograph below, which shows a microscopic view of the underside (lower surface) of a leaf. Cactus 0.02 Potato plant 1.00 Apple tree One reason each plant loses a different amount of water from the other plants is that each has 1 different guard cells that are adapted to maintain homeostasis 2 the same number of chloroplasts but different rates of photosynthesis 3 different types of insulin-secreting cells that regulate water levels 4 the same rate of photosynthesis but different numbers of chloroplasts 16. The nervous system helps to maintain homeostasis by 1 using electrochemical impulses to regulate functions 2 regulating the chemistry of the body s fluids 3 releasing hormones directly into the bloodstream 4 protecting the body from harmful bacteria 17. Which homeostatic adjustment does the human body make in response to an increase in environmental temperatures? 1 a decrease in glucose levels 2 an increase in perspiration 3 a decrease in fat storage 4 an increase in urine production 19. What is the main function of the cells indicated by the black pointer? 1 to regulate the rate of gas exchange 2 to store food for winter dormancy 3 to undergo mitotic cell division 4 to give support to the leaf s veins 20. How does being multicellular increase an organism s ability to maintain homeostasis and survive? 21. Write a brief essay comparing the life of a cell in your body with that of an ameba in the soil. Why is it more likely that the body cell will survive for a long time, but the ameba will not? Refer to the diagram below to answer questions 22 and Which situation is not an example of the maintenance of a dynamic equilibrium in an organism? 1 Guard cells contribute to the regulation of water content in a geranium plant. 2 The release of insulin lowers the blood sugar level in a human after eating a big meal. 3 Water passes into an animal cell, causing it to swell. 4 Arunner perspires while running a race on a hot summer day. Capillary Lymph vessel Lymph ICF Red blood cells Tissue cells

6 Chapter 10: The Need for Homeostasis Which analogy most accurately describes the location of the body s tissue cells? 1 cities within states 2 islands within oceans 3 chains of mountains 4 clouds in the air 23. Use your knowledge of biology and the diagram to explain the purpose of intercellular fluid (ICF). Why is it so important for homeostasis? Base your answer to question 24 on the information and diagrams below. To survive, an organism must maintain the health of its cells. The normal internal environment of a human s cells would include a temperature of 37 C, a ph of 7, and a water/salt balance of 0.1 percent. 37 C ph 6.1% 40 C ph 7.5% 24. Which of the cells shown above would belong to someone who is not maintaining homeostasis? 25. List, and describe the roles of, the three components of a homeostatic process. 26. Use the diagram below to explain how feedback mechanisms maintain homeostasis. Pancreas Insulin 37 C ph 7.1% Glucagon 36 C ph 7.2% A B C D 28. The best title for this concept map probably would be: 1 The Respiratory System and CO 2 Levels 2 The Circulatory System and CO 2 Levels 3 Feedback Mechanisms and CO 2 Levels 4 The Bloodstream and Its CO 2 Levels 29. Briefly explain the way our bodies adjust breathing rates in order to maintain homeostasis. Base your answer to question 30 on the data in the graph below. Body Temperature ( C) A B C 36 0 Initial change in body system is detected by Sensor that is sensitive to change, with control unit for correct level sends signals to Effector that makes necessary changes results in this Necessary change in body system D 12 Time (hours) E F 24 Range of Homeostasis Glucose Bloodstream Glucose Glucose 27. Use the following terms to replace the definitions given within the boxes in the following chart: Higher CO 2 levels; Lower CO 2 levels; Muscles in the chest; Structures in the brain (with preset information). 30. The graph shows evidence of disease in the human body. A disruption in the dynamic equilibrium is indicated by the temperature change that occurs between points 1 Aand B 2 B and C 3 C and D 4 E and F

7 78 Reviewing the Living Environment: Biology Study the following graph to answer questions 31 and 32. Temperature ( C) Internal Skin 31. The graph shows the effect of external (environmental) temperatures on a student s skin and internal temperatures. Which statement best describes what happens as the environmental temperature increases? 1 The skin temperature increases, then decreases to 20 C. 2 The internal temperature increases abruptly to about 30 C. 3 The skin temperature decreases, due to sweating, to 30 C. 4 The skin temperature increases, then levels off at about 36 C Environmental Temperature ( C) 32. What is the difference between the effects of rising external temperatures on the student s internal temperature and skin temperature? Explain how homeostatic processes are responsible for the effects seen in the graph. 33. In desert environments, organisms that cannot maintain a constant internal body temperature, such as snakes and lizards, rarely go out during the hottest daylight hours. Instead, they stay in the shade, under rocks, or in burrows. Explain how this behavior helps these organisms to maintain homeostasis. 34. Describe how plants maintain their water balance. Your answer should include the following: one reason why water balance is important to plants the structure that plants have to perform this function how this structure works to maintain water balance 35. In what way are the functions of the contractile vacuoles of an ameba and the guard cells of a plant similar? 36. Identify the four main organ systems that are involved in maintaining homeostasis. Briefly describe each of their roles in this process. Part C Reading Comprehension Base your answers to questions 37 to 40 on the information below and on your knowledge of biology. Use one or more complete sentences to answer each question. In 2002, flight engineers Carl Walz and Dan Bursch set the record for the longest United States space flight, with 196 days in space as members of Expedition 4 on the International Space Station (ISS). Typically, ISS crews have six or seven members who live on the station for 3 to 6 months. The crews live in a world of weightlessness the station has no up or down, so there are no real ceilings or floors. While the total inside space of the station is about equal to that of a jumbo jet, the individual spaces in which the astronauts actually live and work are relatively small, each about the size of a school bus s interior. Crews sleep standing up or camping out where they feel comfortable by attaching their sleep restraints to the wall with Velcro.

8 Chapter 10: The Need for Homeostasis 79 Biomedical researchers are interested in studying the effects of weightlessness on humans. Being weightless is a brand-new challenge never experienced before in the millions of years humans have lived on Earth. And yet, time and again, space travel has demonstrated the marvelous, and often subtle, abilities of the human body to adapt. The body s reactions to weightlessness are teaching us a great deal about its normal responses to gravity. Astronauts report that when they grab the wall of a spacecraft and move their bodies back and forth, they feel as if they are staying in one place and that the spacecraft is moving. Being free of gravity s effects makes us aware of new things. Humans have evolved many automatic reactions to deal with the constant pressure of living in a downward-pulling world. Until we leave that world, we are usually not aware of such reactions. These reactions include the use of signals from our eyes, from the fluid-filled tubes in our ears, from pressure receptors on the bottom of our feet, and from the distribution of liquids in our blood vessels. A sophisticated control system has evolved to keep gravity from pulling all the liquid in our body to our legs. Within minutes of being in a weightless environment, the veins in an astronaut s neck begin to bulge. The astronaut s face begins to fill out and become puffy. In this situation, the fluids in an astronaut s body are not being pulled down by gravity. The fluids spread throughout the body. Because the body seeks to maintain homeostasis, this new distribution of fluid causes other changes in the body in order to control fluid movement. Included in these are changes in hormone levels, kidney function, and red blood cell production. Keeping things stable even when conditions change that is, dynamic equilibrium is as necessary for life in space as it is on Earth. The unexpected result of living in space is a better understanding of how the body works here on Earth. 37. Describe three ways in which life on the ISS is very different from everyday life on Earth. 38. Why are the effects of weightlessness on humans of interest to researchers? 39. How do the body s responses to weightlessness help explain homeostasis? 40. Describe some adaptations of the body related to living in a world with gravity.