homeostasis across body systems...

Homeostasis Across Body Systems

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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”

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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.

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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.

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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.

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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

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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

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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.

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.”

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

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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

5. Responsible for osmoregulation in insects

6. Responsible for osmoregulation in mammals

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

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

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

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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

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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.

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B. Describe the structure of hemoglobin and describe how this structure aids in optimum

oxygen delivery to working muscles.

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.

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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

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

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