Homeostasis Across Body Systems Teacher -...

AP* BIOLOGY

HOMEOSTASIS ACROSS BODY SYSTEMS

Teacher Packet

AP* is a trademark of the College Entrance Examination Board. The College Entrance Examination Board was not involved in the production of this material.

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Homeostasis Across Body Systems

Copyright © 2009 Laying the Foundation®, Inc., Dallas, TX. All rights reserved. Visit: www.layingthefoundation.org

Objective To review the student on the concepts and processes necessary to successfully answer questions over homeostasis. Standards Homeostasis is addressed in the topic outline of the College Board AP Biology Course Description Guide as described below. AP Biology Exam Connections The principles of homeostasis are tested every year on the multiple choice section and are quite common on the free response section of the exam. For the purposes of this Prep packet, pure body system questions are treated separately. As with many AP Biology free response questions, these topics are often intertwined with other topics. The list below identifies free response questions that have been previously asked over this topic. Free response from 2004 form B and 2000 are especially pertinent to this packet. These questions (except 2000) are available from the College Board and can be downloaded free of charge from AP Central http://apcentral.collegeboard.com.

Free Response Questions 2006 Question #4 2004 B Question #3 2003 Question #2 2002 B Question #2 2002 Question #3 2001 Question #1 2000 Question #2

III. Organisms and Populations A. Diversity of Organsims Evolutionary patterns Survey of the diversity of life Phylogenetic classification Evolutionary relationships B. Structure and Function of Plants and Animals Reproduction, growth, and development Structural, physiological and behavioral adaptations Response to the environment



A word about body systems and the AP Biology exam

The AP exam rarely stresses anatomy of body systems unless it relates to adaptations/evolution (2 chamber fish hearts vs. 3 chamber amphibian hearts vs. 4 chamber mammalian hearts) or the actual, functional physiology of the structure. A student could very well be required to describe the significance of dendrites, myelin sheath, or Na+/K+ pumps while explaining how neurons carry impulses. A student would certainly not be asked to list the cranial nerves. In addition, the AP exam tends to blend body systems in the free response section. A student is more likely to see a question centered on a theme like “transport of amterials” via the digestive, respiratory, and circulatory systems than a pure question on the excretory system for example. This rule is not iron-clad as 2 pure immune questions have been asked recently (2007 form B and 2005). The AP exam tends to favor the following systems primarily: Nervous, Immune, Endocrine The AP exam tends to favor the following systems secondarily: Respiratory, Circulatory, Excretory The AP exam seems to nearly neglect the integumentary (except as it relates to nonspecific immunity) and skeletal (except as it relates to classification/evolution) systems. Note: The “homeostasis question” on which this prep packet is based was asked nearly identically in 2000 and 2004. If this trend were to persist, one would expect to see the homeostasis question soon.

Homeostasis Basics Homeostasis is a steady, yet dynamic state. The organism will attempt to maintain a steady state (such as body temperature in endotherms), but may have to change or make adjustments (dynamic) by shivering in order to do so. In addition, the organism may shift response entirely in specific situations (positive feedback below). Generally homeostatic systems consist of (a) receptor(s), a control center, and (an) effector(s.) There are 2 primary homeostatic systems:

Positive Feedback: An increase in product results in increasing the rate of the synthesis of that product. This is the “finish what you started” mechanism. Example: A B C D where the synthesis of D activates the enzyme “A B-ase” thereby increasing the rate of synthesis of D eventually.

Negative Feedback: An increase in product results in a decrease in synthesis of more product. This is the mechanism by which levels of a substance remain relatively constant continually. Example: A B C D where the synthesis of D acts as an allosteric inhibitor of the enzyme “A B-ase”



Examples of Positive Feedback: Childbirth & Blood clotting

Child Birth: Release of oxytocin causes the uterus to begin contracting. The contracting uterus pushes the

baby and increases pressure on the opening of the uterus (cervix). This increase in pressure results in release of additional oxytocin which will increase contraction strength and intensity…which will increase pressure on the cervix…which will increase oxytocin release…

Blood clotting, complement proteins in the immune system, and inflammation are additional examples of positive feedback loops.

Homeostasis: Hunger Stomach Fullness:

The hypothalamus monitors the fullness of volume of the stomach. A stomach with much empty space results in feelings of hunger. This explains why calorie poor foods such as lettuce or celery may curb the appetite somewhat initially.

Blood Sugar: When blood sugar is low between meals for

example, glucagon is released from the pancreas. The presence of glucagon results in the breakdown of glycogen (polymer of glucose stored in the liver). The glucose is dumped into the blood stream raising blood sugar levels.

When blood sugar is high after a meal for example, the pancreas will release insulin. Insulin will remove sugar from the blood and “feed” cells while additionally storing excess sugar as glycogen in the liver. The hypothalamus monitors blood sugar as well.



Homeostasis: Respiratory Rate & Heart Rate The exchange of O2 & CO2 occurs at alveoli.

Surface Area: The alveoli are sack-shaped to greatly increase the surface area across which O2 and CO2 can diffuse.

Thickness: The alveoli are but a single cell layer thick leading to more efficient diffusion of gases across such a thin membrane.

Exchange: Each alveolus is covered in a capillary mesh. The capillaries are also a single cell in thickness to facilitate diffusion.

The medulla oblongata controls respiratory rate and blood pressure.

During vigorous exercise an increase in CO2 production will occur as a result of increased cellular respiration. When CO2 is dissolved in H2O in the cytoplasm of the red blood cells it is converted into carbonic acid as seen in the equation below:

2 2 2 3 3H O CO H CO H HCO+ −+ ↔ ↔ + Carbonic acid dissociates into bicarbonate ions and hydrogen ions resulting in a drop in pH.

The medulla oblongata (m.o.) receives information from chemoreceptors responsible for monitoring pH. When blood pH drops due to an increase in H+ ions, the m.o. will respond by stimulating intercostals and the diaphragm for deeper and more frequent respirations.

The m.o. also gets feedback on blood pressure from baroreceptors. The m.o. can control vessel diameter to adjust for blood pressure changes.



Homeostasis: Red Blood Cells & Hemoglobin

Hemoglobin responds to heat and acidity. Like all proteins, hemoglobin (Hb) will begin

to change shape as it encounters warmer or more acidic environments. In this case however, Hb will undergo slight conformational changes as it encounters warmer, more acidic tissues. These fully reversible, slight changes in shape result in a decreased affinity for O2. Exercising muscles are warm (friction) and acidic (lactic acid). Hb will therefore “know” to drop off more O2 molecules at muscles that are contracting due to exercise.

Homeostasis: Temperature Regulation

Why does an organism typically have such a narrow temperature range in which it can live? Enzymatic reactions, protein shape, and other biochemical events all occur optimally in a specific

temperature range. An excessively high temperature may denature enzymes, while a sharp decrease in temperature may result in too few collisions of reactants. Endotherms & Ectotherms:

Endotherms (warm blooded) are able to maintain a steady body temperature regardless (to some extent) of environmental temperature.

Ectotherms (cold-blooded) are at the mercy of external temperature and must find other means to regulate body temperature.

Primary methods of temperature regulation…

Evaporative cooling: H2O has a high specific heat meaning that

it will take a great amount of energy to raise H2O 1o C due to the hydrogen bonding between H2O molecules. The evaporation of a thin layer of sweat from the skin of an animal will require an input of significant heat. That heat is provided by the animal resulting in cooling of the animal.

Controlling heat exchange:

Vasoconstriction of deeper vessels and dilation of surface vessels will aid in cooling. As blood passes nearer the surface of the body, it will receive some of the benefits of evaporative cooling. The reverse is true for maintaining warmth.

Increasing body temperature: The friction of skeletal muscle (shivering),

the flapping of wings, or disengaging the chemiosmotic process of cellular respiration will all create additional body heat.

Behavior:

This is a major method of temperature regulation in ectotherms. A lizard for example may sun itself on a rock or seek shade in order to maintain an acceptable temperature.

Acclimiation:

Some species are able to modify proteins so that they are able to withstand extreme cold. Some species even have antifreeze like components that aid in maintaining membrane fluidity.



Homeostasis: Maintaining H2O balance

Water regulation (osmoregulation) is based largely on shuffling solutes and H2O between internal and external fluids.

Additionally, organisms must rid themselves (excretion) of toxic, nitrogenous waste resulting from protein catabolism.

Excretion of nitrogenous waste Water dwelling organisms convert amino groups into ammonia (NH3) and may simply excreted

through diffusion via skin, gills, etc. Most land dwelling organisms are forced to store nitrogenous waste for extended periods of time

before excretion is possible. NH3 is extremely toxic. How might land dwellers adjust? Ammonia is modified in the liver and converted to urea. Urea is more than 1x105 times less toxic than NH3 and can be stored more safely. In addition, urea is extremely water soluble.

Uric acid is an insoluble form of modified NH3 (the white paste in bird excrement is actually uric acid). Conservation of water is the advantage of uric acid over urea in adults. In addition, “egg layers” such as birds and reptiles as a general rule utilize uric acid synthesis so that a developing organism is not saturated with urine during development.

Osmoregulation: Water dwelling organisms Saltwater organisms:

The priority is to rid the body of excess NaCl and maintain H2O. Bony fish for example are osmoregulators. They drink salt water and actively transport NaCl out

through the gills. Sharks are osmoconformers and actually maintain a significantly higher than normal level of urea

in the blood. Because their osmolarity is somewhat similar to the surrounding water, sharks lose little H2O to the environment.

Freshwater organisms: The priority is to maintain electrolytes while riding itself of H2O. A protist may simply use contractile vacuoles to “shoot” out excess water Most multicellular organisms will simply manufacture very dilute urine in order to deal with

excess water. Osmoregulation: Land dwelling organisms

The basics: Whether it is the metanephridia of an earthworm, the malpighian tubules of an insect, or the

nephron of a mammal. Most systems proceed as follows:

Filter Reabsorb what is valuable Secrete toxins Excrete Metanephridia

Malpighian Tubules



Mammalian Kidney: Physical filtration due to pressure

across fenestrated capillaries occurs at the glomerulus.

Filtrate proceeds into the Bowman’s capsule

The loop of Henle descends down and back up through the renal cortex and medulla. The deeper portions of the medulla are more concentrated in salt and urea resulting in various stages of osmosis, diffusion, and active transport as primarily H2O is reabsorbed (98-99% of all H2O in the filtrate is reabsorbed).

Excretion occurs when the “remnants” travel through the collecting duct and are eventually passed as urine.

Mammalian Kidney



Multiple Choice Questions 1-3 refer to the diagram below. 1. Which of the following best explains the role of the end product (isoleucine) in this diagram? Isoleucine

(A) acts as a coenzyme. (B) is an activator of the enzyme. (C) is a competitive inhibitor of the enzyme. (D) operates by binding to an allosteric site. (E) is helping to break down the initial substrate.

D The end product isoleucine acts through noncompetitive inhibition by binding to an allosteric site on

“Enzyme 1.” 2. All of the following statements are true EXCEPT:

(A) This is an example of negative feedback. (B) This system will maintain a relatively stable amount of end product. (C) In this system the existence of product decreases the rate of production of that product. (D) This system displays noncompetitive inhibition. (E) A lack of enzyme 4 will result in increased rates of inhibition of “Enzyme 1.”

E The lack of enzyme 4 will result in a lack of the end product isoleucine. The lack of isoleucine

represents the lack of an inhibitor to “Enzyme 1.” Based on this system in isolation, rates of inhibition will actually approach and eventually reach zero.



3. The system above shares much in common with all of the following EXCEPT:

(A) Blood sugar regulation (B) Temperature regulation (C) Child birthing (D) Maintenance of blood pressure (E) The trp operon

C Child birthing is an example of positive feedback. All other examples display negative feedback as the primary regulatory mechanism.

Questions 4-6 refer to the following structures

(A) Malpighian tubules (B) Alveoli (C) Nephrons (D) Medulla oblongata (E) Metanephridia

4. The primary site of gas exchange in the lungs

B Alveoli are the primary site of gas exchange in the lungs.

5. Responsible for osmoregulation in insects

A Malpighian tubules are responsible for osmoregulation in insects.

6. Responsible for osmoregulation in mammals

C The nephron is the functional portion of the kidney responsible for osmoregulation.

7. Which of the following correctly describes the order of events in the mammalian kidney?

(A) Filtration Reabsorption Excretion Secretion (B) Filtration Reabsorption Secretion Excretion (C) Reabsorption Filtration Secretion Excretion (D) Secretion Filtration Reabsorption Excretion (E) Secretion Reabsorption Filtration Excretion

B Kidneys physically filter the blood and then reabsorb water, ions, and nutrients. The kidneys secrete undesirable substances and then finally excrete urine.



8. Land dwelling mammals are able to regulate body temperature through all of the following

mechanisms EXCEPT:

(A) Sweating (B) Shivering (C) Metabolic adjustments (D) Dilation or constriction of vessels (E) Increasing specific heat of tissue

E

Because the composition of tissue is in the short term essentially unchangeable, the specific heat of a particular tissue cannot be changed. This term was simply meant as a distracter to students that associate heating and cooling with specific heat. All other options are possible means of temperature regulation.

9. Damage to the human medulla oblongata would most likely result in which of the following.

(A) Erratic heart and respiratory rate (B) Increased activity in the malpighian tubules (C) Inability to properly control blood sugar levels (D) Inability to properly control absorption in the kidney (E) Decrease in the secretion of hormones responsible for homeostasis of body temperature

A

The medulla is primarily responsible for, among other things, heart rate and respiratory rate. Malpighian tubules are not found in humans. Blood sugar is more likely associated with the hypothalamus and pancreas. Body temperature is controlled primarily by the hypothalamus.

10. Uric acid is the product of which of the following processes.

I. catabolism of carbohydrates

II. catabolism of amino acids III. fatty acid metabolism

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

B Urea and uric acid are biproducts resulting from the breakdown of the nitrogen containing amino groups of amino acids as well as some nucleotides.



Free Response 1. The circulatory system is instrumental in delivering oxygen to all cells of the body. In a controlled experiment, a scientist chose to test the relationship between heart rate and exercise on a group of individuals. The results are displayed below.

Exercise Intensity (arbitrary

units)

Heart rate participant #1 (bpm)

Heart rate participant #2 (bpm)

Heart rate participant #3

(bpm)

0 1 2 3 4 5 6 7 8

60 65 71 79 86 94 103 111 117

70 77 85 91 100 106 113 124 130

65 70 77 85 93 100 106 115 122

A. Construct a graph on the grid below.



1 point for each of the following (3 point maximum) _Graph: Title + labeled axes _Graph: Units on both axes + legend _Data plotted: correctly plotted data, may have one erroneously plotted point

B. Describe the structure of hemoglobin and describe how this structure aids in optimum oxygen delivery to working muscles.

1 pt for each of the following (3 point maximum) _Hemoglobin contains 4 heme groups OR 4 binding sites for O2. _Hemoglobin undergoes a conformational change increasing affinity for O2 as it drops off each O2. _Contracting muscles will increase in temperature due to friction. _Hemoglobin displays a decreased affinity for O2 in the presence of increasing temperature due to contracting muscles. _Contracting muscles will produce lactic acid resulting in a decrease in pH. _Hemoglobin displays a decreased affinity for O2 in the presence of a lower pH.

C. It is expected that the heart rate will increase in an exercising individual. Describe how the brain monitors respiratory and heart rate in order to maintain homeostasis. Describe the corresponding response to vigorous exercise.

1 pt for each of the following (5 point maximum) _The medulla oblongata (m.o.) is primarily responsible for monitoring and controlling respiratory rate and heart rate _Exercise increased cellular respiration increased CO2 production _Concept that CO2 is converted to an acid (carbonic acid) or to bicarbonate ions and H+ ions _Increased CO2 results in decrease in blood pH _Sensory neurons send pH information to the m.o. _When the pH drops, motor neurons from the m.o. will stimulate the diaphragm and heart _Heart rate and/or “depth” of contractions will increase while respiratory rate and/or “depth” will increase _In time, pH will normalize due to changes and the m.o. will require less vigorous responses from the circulatory and respiratory systems



Free Response 2. Feedback mechanisms are found throughout biological systems.

A. Describe in detail how a mammal maintains each of the following:

i. body temperature ii. blood sugar

1 point for each of the following (6 points maximum) Maintenance of body temperature in endotherms (maximum of 4 points) _Significance of shivering: Shivering increases friction among muscle fibers resulting in increased body heat. _Behavioral response: Mammals may seek shelter for warmth or from extreme heat. _Arrector pili muscles: Some mammals may raise the hairs creating a warm layer of air close to the skin. _Vessel constriction/dilation: External vessels may dilate to increase heat dissipation or constrict to conserve heat. The reverse is also true for internal vessels. _Significance of sweating: As water changes phase to vapor heat must be input. Sweating releases a thin layer of water that evaporates due to the input of heat from the skin. When heat is leaving an organism, that organism is cooling. _Negative feedback: As body temperature normalizes the actions taken above such as shivering will cease in order to maintain a relatively ideal temperature Maintenance of blood sugar (maximum of 3 points) _Excess glucose is stored in long chains of glycogen in the liver _Insulin will lead to a decrease in blood sugar as sugar is taken up by cells or the liver. _Glucagon will lead to an increase in blood sugar by removing glucose that has previously been stored in the liver. _Negative feedback: As blood sugar levels fall within normal ranges, the amount of insulin or glucagon secreted will decrease/increase appropriately.

B. Animals maintain water balance (osmolarity) through various means. Describe methods by which a saltwater and fresh water fish maintain osmolarity and explain the necessity of specific adaptations for the success of land dwelling animals with regard to water balance. Give examples where possible

1 point for each of the following (6 point maximum) the student must earn at least 1 point from each section to earn the maximum of 6 points. Fish & Osmolarity _Osmoregulators maintain different environments from the surroundings _Saltwater fish drink salt water and actively export salt out. _Freshwater fish do not drink, and excrete very dilute urine in order to get rid of excess H2O. _Osmoconformers conform to their environment _Sharks maintain an osmolarity similar to the surrounding salt water by maintaining very high urea concentrations in the blood.



Adaptations _Amino acid metabolism results in the formation of toxic NH3. _NH3 can be converted into urea which is less toxic and highly water soluble for organisms that store urine for extended periods of time. _Uric acid formation is more energy expensive than urea formation, but results in less loss of H2O. _Valid example of an organism that utilizes uric acid formation such as birds. _Valid example of an organism that utilizes urea formation such as a mammal. _Insects may close spiracles in order to avoid dehydration _Insects contain a relatively impermeable exoskeleton (waxy layer, chitin, etc.) _Malpighian tubules of insects reabsorb water _Other valid adaptions for water conservation in land dwelling animals

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