Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Homeostasis

I. Homeostasis - "Life Exhibits Homeostasis" - one of the general characteristics of life

A.  General
    An animal (or plant) is separated from its external environment by some sort of boundary.  Since external and internal conditions are usually different, it shows that an organism is able to exert control over its internal conditions. Further, the internal conditions are usually remarkably stable and predictable.  This stable state is termed the "set point."  For example, the set points for blood pH and body temperature are 7.4 and 37 C, respectively, and they vary little from these values.  The ability/result of an individual to maintain a constant internal environment within tolerable limits is termed homeostasis.   Or in other words, maintaining a "constant internal milieu" in light of environmental fluctuations.

    Homeostasis literally translates into "same standing."

    Homeostasis occurs at various levels of biological organization (e.g., cell, organismal).

B.  Control Mechanism
    The basic regulatory system that allows an animal (or plant) to respond to its environment can be diagrammed:        signal (or stimulus) → sensor (or receptor) → control center or integrator → effector → response

    To summarize, a signal of some sort is received by a sensor, which in turn, relays the message to a control center that selects an appropriate response that is carried out by the effector.

    Let’s use temperature as an example. Imagine the thermostat in your home that is set for a comfortable 21 C. A thermostat (sensor) monitors the temperature (signal). The thermostat has a mechanism (integrator = control center) for turning off/on the air conditioner or furnace (effectors) as needed.  Thus, if it is a hot day and the temperature in your home increases above the set point (i.e., 21 C), then the thermostat switches on the air conditioner (and the furnace off). Once the temperature drops, the thermostat turns off the air conditioner. This results in a relatively constant temperature that will fluctuate slightly around the set point since it takes the system a brief period to respond to the temperature changes.   see diagram in class

    This basic system is at the heart of many homeostatic regulatory systems in animals including those for temperature, weight regulation, and blood glucose.

II.  Glucose Homeostasis  (covered during digestion unit)

A.  [Glucose]
    Our body maintains a fasting blood glucose concentration (set point) of approximately 70-100 mg glucose 100 mL^-1.  This is measured by a fasting blood test.  If the level increases (hyperglycermia) or decreases (hypoglycemia) much below this level, regulatory mechanisms kick in to return the glucose level to the set point.

B.  Mechanism - in class we will diagram how this system works.  The basics:

1. Increased [glucose] → pancreas → insulin → receptors in liver, muscle, other cells → increase glucose uptake from blood into cells → increased production of glycogen (liver, muscle) or fat (adipose cells) → blood [glucose] decrease
    
2. decreased [glucose] → pancreas → glucagon → liver → (a) convert glycogen → glucose; (b) convert amino acids and other molecules → glucose (gluconeogenesis); (c) converts fats →  glycerol + 3 fatty acids →  released into blood → [glucose] increase

C.  When the system goes awry = Diabetes mellitus

• Diabetes mellitus - causes by increase blood [glucose]. 
• Word origin:  diabetes = increased urine output; mellitus = honey, sweet.  Thus, disease characterized by increased urine output with increased level of sugars.  Thirst is also a symptom
• Caused by:  (a) failure to produce insulin (Type I - insulin dependent), childhood; or (b) defective receptors to remove glucose (Type II - non-insulin dependent).
• Type II frequency increasing.  Associated with race (e.g., Pima's, Native Americans, African Americans), diet, and obesity

III.  Temperature Regulation  

A.  Life and Temperature

1. Life can only exist from about 0 - 100 C.  Below 0, reactions occur too slowly, water is frozen; above 100 C there is too much thermal energy which is typically destructive.  Even so, there are few organisms that can survive in conditions much warmer than 60 C (e.g., thermophilic bacteria in hot springs)
   
2. Organisms can tolerate a narrow range of temperature (due to enzymes, etc)
   
3. Organisms have evolved various adaptations for temperature (see below)
    
4. Conclusions: Temperature restricts the range and habitats in which organism can live; Even within suitable environments, organisms must be able to regulate temperature

B.  Thermoregulation

insert:  graphs of body temp vs. external temp; and  metabolic rate vs. extermal temp for mouse and lizard

Take Home lessons from the Graphs:

1. Mouse (etc) body temperature doesn't change with environmental temperature - maintain constant temperature; homeotherms
   
2. Lizards (etc) body temperature fluctuates with the environment; there body temp is a function of the environmental temperature; heterotherms (poikilotherms)
   
3. Lizards require an external heat source to raise body temperature above ambient = ectotherm
   
4. Lizard metabolic rate increases directly as a function of the environmental temperature
   
5. Mouse regulates temperature by internal methods; produce their own heat = endotherms
   
6. Metabolism is the source of heat.  Recall cellular respiration? (click here for a refresher) During respiration, glucose is converted about 38 ATP.  Now, let's calculate the efficiency of this process.  It equals the amount of energy recovered divided by the total amount.  Efficiency of respiration  = energy ATP / energy in glucose x 100  The energy content of glucose is 686 kcal/mol and ATP is 7.3 kcal/mol.  Substituting these in the equation:  (7.3 kcal/ATP x 38 ATP) / 686 kcal x 100 = 38%.  This means that cellular respiration is 38% efficient - or in other words, only 38% of the energy in glucose is recovered and trapped in ATP.  Where does the other 62% of the energy from glucose go? Heat!  
   
7. Mice are regulators; lizards are conformers
    
8. Lizard metabolic rate is directly related to the temperature of the environment.
    
9. The metabolic rate of the mouse is a direct function of the environmental temperature.
    
10. As the temperature of the environment increases, mouse MR decreases - requires less heat to keep warm and hence, less metabolic activity is necessary.
    
11. Mouse MR is relatively constant within a range of temperatures - thermoneutral zone.  The rate of metabolism at this point is the basal metabolic rate.  Assuming no other activities, this represents the basal amount of energy expended by an organism to sustain itself, without using extra energy to keep itself warm, digest food, reproduce, etc.
     
12. As the temperature of the environment increases above the thermoneutral zone, metabolic rate of the mouse increases because energy is required to cool off the mouse.

C.  Mechanism of Thermoregulation

1  Behavior
    Organisms modify behavior to cool/heat body as necessary (e.g., lizards bask in sun, hide in burrow; elephants shower with water, wallow in mud/water; bees huddle in cold)

2.  Body Morphology
    Environmental temperature has directed evolution of body shape/size.  Two major concerns:

• surface - to - volume ratios have a major effect of size, shape (flattening, stretching, cube vs. sphere vs. filament) - check out our first lecture.
• adaptations:  (1) insulation - fur, feathers, fat; (2) countercurrent exchange - to move something  from one pipe to another, the most efficient way is for the pipes to run in opposite directions because the diffusion gradient between the incoming and outgoing pipes never become equal.  If the pipes run in the same direction, then the become equal when the incoming pipe has released 50% of its load.

3.  Physiological Adaptations

• iguana - brings blood to surface, adjusts heart rate
• cold vs. hot fish - countercurrent mechanism
• increase respiratory activities (shivering, fluttering wings, muscles)
• hibernation/torpor
• brown fat - rich in blood vessels & mitochondria; thermogenin uncouples ATP production from electron transport - Gink & Go at the Ski Resort

D.  Vertebrate Temperature Regulation
    If we get too hot, blood temperature (signal) is monitored by the hypothalamus (receptor), which triggers the anterior pituitary (control center) to send a signal to the sweat glands (effector) to "chill out" (i.e., start sweating and dilate blood vessels).

    If we get too cold, blood temperature triggers the hypothalamus to send a signal to the nerves to tell the blood vessels to constrict, increase shivering, and increase the rate of metabolism (done through the pituitary and thyroid glands).

    Note, feedback mechanisms are important aspects of regulatory control. Feedback can be positive (increase in the signal causes an increased response) or negative (increase in the signal causes a decrease in response).