1. Temperature Dependence

• Chemical reactions have an exponential dependence on temperature. So, metabolic rate (R) also has an exponential dependence on temperature (T).
  R/M = k 10^{(b T)}

• Notice that we can change this into a straight line by taking the log (to the base 10) of both sides of the equation: (Please note that equation 17-5 in Eckert is in error!)
  log(R/M )= log(k) + b T

• If we plot R vs. T, we do not get a straight line. However, the R vs. T relationship often looks linear over the relatively small temperature range that an organism can live. We can describe the slope of this approximate straight line given by plotting R vs. T, and thus the temperature dependence of metabolism, by comparing the metabolic rates at ten degree (C) intervals. This leads to the concept of the Q₁₀:
  Q₁₀= R_(T+10)/R_(T)

• Here we see a plot of R/M for a tiger moth caterpillar (Eckert, Fig. 17-2a) and the log plot (Eckert, Fig. 17-2b)

2. Acclimation - Biochemistry and physiology can change in response to new thermal conditions.

• We see that frogs can change body chemistry when the temperature drops for long periods of time, such as from summer to winter. We will discuss some interesting topics such as muscle biochemistry changes from summer to winter and how temperature changes affect the ability of the frogs to see in dim light! (Eckert, Fig. 17-3a)
• How do we explain acclimation?

  • Changes in enzyme structure (Eckert, Fig. 17-13)
  • Homeoviscous membrane adaptation - example: Rainbow Trout (Eckert, Fig. 17-4a) In heat, more cholesterol and more saturated fatty acids - why?
  • Production of more "heat shock proteins", also known as "molecular chaparones" In some marine intertidal snails, heat shock proteins are produced in greater numbers when temperature is increased and the cells are moved out of sea water into the air for 2.5 hours of exposure. (Eckert, Fig. 17-5) Note that T. brunnea is an intertidal species and has the ability to make a lot more shock proteins, a characteristic particularly useful for its ecological niche.

3. Thermal Interaction with the environment

• Conduction and Convection (Eckert, Fig. 17-7a) (Eckert, Fig. 17-7b) - countercurrent heat exchange (Eckert, Fig. 17-24) In the context of this discussion, you should read about the "rete" of the bluefin tuna in the Eckert textbook.
• Radiation - Dragonflies and butterflies and basking reptiles. Plants orient their leaves too!
• Evaporation - evaporation uses a very large amount of heat. In other words, changing water from liquid to vapor requires a lot of energy (to get the molecules jumping around enough). Example of the efficiency of cooling in DRY climates - evaporative coolers on houses in the desert southwest USA. The waterproof frogs -ectothermy at work!
• Adaptations to cold: Some ectotherms, such as polar arthropods can avoid freezing by "osmotic depression of the freezing point". They can concentrate solutes such as sugars (glucose, fructose, trehalose) or sugar alcohols ( glycerol, sorbitol, mannitol) and depress freezing point by as much as 10 degrees C. Many fish (ectotherms!) use "antifreeze proteins" to lower body temperature. These proteins are glycoproteins, often with a repeating peptide of alanine-alanine-threonin linked to a version of the carbohydrate galactose or an ordinary protein, lacking a carbohydrate moiety. Antarctic fish, such as the icefish, Trematomus, commonly use antifreeze proteins.
• Adaptations to cold: Many animals can supercool - their temperature drops below normal freezing. If there is contact with ice crystals, such as would occur at the gills of the antarctic icefish, the ice crystals serve as sites of nucleation and promote freezing of the water in the gills. But, if there is no such contact with ice crystals, the animal's fluids can often remain liquid even at temperatures considerably below the osmotically depressed freezing point. Some fish in the fjords of Labrador supercool. Frogs can supercool and some spiders can supercool by as much as 20 degrees below zero (C).

4. Endothermy

• Homeothermic Endotherms (homeothermic = constant temperature, endotherm = temperature is regulated by mechanisms of heat production by the animal)
• Concept of the "thermal neutral zone" (Eckert, Fig. 17-21)
• Within thermal neutral zone, an endotherm is much like some ectotherms. Body temperature can be regulated by:
  • Vasomotor responses
  • Postural changes
  • Insulation adjustments. ("Goose bumps" of humans - our poor attempt to adjust our "fur")

Below the thermal neutral zone
• Thermogenesis:
  • Shivering
  • Brown fat (bats) (Eckert, Fig. 17-22). Some newborn and hibernating mammals have brown fat, but not in specialized organs. This organ uses mitochondrial uncoupling proteins to leak protons from the intermembrane space of mitochondria back to the matrix. Brown fat is brown in color because of the large number of mitochondria. Brown fat is also concentrated around the neck and large blood vessels of the thorax in human infants. Some hibernating mammals use brown fat to stay warm. Mitochondrial protein leak is one of several mechanisms that adult humans use for thermogenesis to maintain a constant body temperature. Other mechanisms include heat from Na+/K+ ion pumps.
  • In billfish (swordfish, etc), eye muscles are modified as heater tissue. ATP is burnt to heat (#3 in Eckert, Fig. 17-23) instead of being used to contract muscles.
• Countercurrent heat exchange: (Eckert, Fig. 17-24)

Above the thermal neutral zone
• Evaporative cooling

5. Active thermostatic regulation of body temperature in mammals.

• Anterior Hypothalamus - (Eckert, Fig. 9-15) (Eckert, Fig. 17-27)
• Overview of nervous system control of human themoregulation
• A rise in core body temperature of only 0.5 degrees C causes extreme peripheral vasodilation (flushing of the skin in humans).
• The hypothalamus is much more sensitive to changes in core body temperature than to surface temperature.
• Normal human body temperature is 37 degrees C. Convulsions occur at 41 degrees and there is no survival above 43 degrees. The thermoneutral zone is from 25 to 30 degrees C.
• Fever resets the hypothalamic thermostat. Endogenous pyrogens are released from macrophages in the presence of bacteria. Endogenous pyrogens (mostly interleukins) stimulate release of prostaglandins from various cells (including muscle). Prostaglandins act on thermoreceptors in the hypothalamus by altering the rate of nerve impulses produced by the thermoreceptor cells. Aspirin lowers fever by inhibiting prostaglandin synthesis (COX-1 pathway). This link describes the role of COX and prostaglandins in human physiology. (REQUIRED reading. You will be examined on its contents.)
• High temperature of fever results in some molecules working better, such as interferon, reduced growth rates of some microorganisms, often due to partial denaturation of their proteins.
• Some ectotherms respond to pyrogens from bacterial infection by increasing their exposure to sun, thereby giving themselves a fever by behavior!

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2005, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.