1Extreme Temperatures and Thermal Tolerance• All organism have a range of tolerable body temperatures– Homeothermic endotherms – narrow range– Poikilothermic ectotherms – broad range• Exceeding limit of thermal tolerance– DEATH!!!!!Extreme Temperatures and Thermal ToleranceFactors influencing lethal exposure:• Exposure Temperature– Degree to which temperature exceeds limits of tolerance• Exposure Duration– Length of time to which organism is exposed to lethal temperature• Individual VariationProblems With High Temperature• Denaturization of proteins– Structural and enzymatic• Thermal inactivation of enzymes faster than rates of activation• Inadequate O2supply to meet metabolic demands• Different temperature effects on interdependent metabolic reactions (“reaction uncoupling”)• Membrane structure alterations• Increased evaporative water loss (terrestrial animals)Problems with Low Temperatures• Thermal inactivation of enzymes faster than rates of activation• Inadequate O2supply to meet metabolic demands• Different temperature effects on interdependent metabolic reactions (“reaction uncoupling”)• Membrane structure alterations• FreezingFreezing• Drastic reduction in gas diffusion – liquid water vs. solid water• Drastic reduction in enzyme function – Reduced molecular mobility• Structural disruption of enzymes• Mechanical disruption of cell membranes• Osmotic dehydration due to freezing of extracellular water– Most important factorDealing with Subfreezing Temperatures• Supercooling– Freezing point depression• Use of antifreeze• Freeze tolerance2Supercooling• Water does not usually freeze at 0 °C– Freezing involves ice crystallization– Can occur spontaneously below 0 °C– Water can remain liquid until crystallization occursSupercooling• Supercooling can be enhanced by addition of solutes to an aqueous solution– ↑ [solutes], ↓ freezing point• Freezing point depression– E.g. insects• Produce high levels of glycerol• Lowers freezing point • Willow gallfly larvae can supercool to –60 °CAntifreeze• Antifreeze – substance that prevents ice crystal formation– thermal hysteresis -lowers freezing point but not melting pointFreeze Tolerance• Ability to tolerate freezing of extracellular fluid– Must cope with…• potential mechanical damage• effects of dehydration• Cryoprotectants– Substances that help animals avoid damage from freezing of body tissues– E.g. glycerol• appears to stabilize cell membrane and protein structureFreeze Tolerance• Many freeze tolerant organisms have ice-nucleating agents– Promotes ice-crystal formation in the extracellularfluid• Draws water out of the cells, ↑ intracellular concentrations and ↓ freezing point– Helps prevent crystal formation inside the cells• Prevents mechanical damageThermal Adapation• Different species have adapted to differences in temperature between species ranges3Thermal Acclimatization• Acclimation and acclimatization are physiological changes in response to previous thermal history• Exposure to warm temperatures increases heat tolerance, decreases cold tolerance• Thermal tolerance of many species changes with seasonal changes in temperatureMechanisms of Thermal Acclimatization and Adaptation• Changes in enzyme systems– Changes in enzyme synthesis/degradation– Changes in use of specific isozymes– Modulation of enzyme activity by the intracellular environment• Changes in membrane phospholipids– increase saturation of fatty acids with increased temperature– homeoviscous adaptationTemperature RegulationApproaches to thermoregulation:• Thermal conformity (poikilothermy)– allow body temperature to fluctuate with environmental temperature• Thermoregulation (homeothermy)– Maintain body temperature at relatively constant levels largely independent of mean environmental temperatureThermoregulation Methods• Behavioral control – Controlling body temperature by repositioning body in the environment• Physiological control– Neural responses (immediate)• E.g. modification of blood flow to skin, sweating/panting, shivering, etc.– Acclimation responses (long-term)• Changes in insulation, increased capacity got metabolic heat generation, etc.Ectothermy• Obtain body heat from external environment• Environmental heat availability subject to change– Some thermally stable environments • vary only 1-2 °C/year– Some highly variable environments • 80 °C variation in one year– Most ectotherms must deal with some degree of temperature variationEctotherms and Cold• Inactivity of enzyme systems– Cold-adapted species have enzymes that function at higher rates at lower temperatures• Subfreezing Temperatures– Supercooling– Antifreezes– Freeze Tolerance4Ectotherms and Heat• Problems associated with heat– Enzyme denaturization and pathway uncoupling– Elevated energy requirements– Reduced O2delivery • affinity of Hb for O2decreases with increased temperature• Critical Thermal Maximum (CTM)– Body temperature over which long-term survival is no longer possibleEctotherms and Temperature Regulation• Behavioral Regulation– Reposition body relative to heat sources in the thermal environment– Most widely used method• Physiological Regulation– Redirect blood flow for increased heat gain-heat loss– Pigmentation changes • absorb/reflect radiant heatEctothermy vs. EndothermyEctothermy – low energy approach to life•Pros– Less food required– Lower maintenance costs (more energy for growth and reproduction)– Less water required (lower rates of evaporation)– Can be small – exploit niches endotherms cannot.• Cons– Reduced ability to regulate temperature– Reduced aerobic capacity – cannot sustain high levels of activityEctothermy vs. EndothermyEndothermy – high energy approach to life•Pros– Maintain high body temperature in narrow ranges– Sustain high body temperature in cold environments– High aerobic capacity – sustain high levels of activity• Cons– Need more food (energy expenditure 17x that of ectotherms)– More needed for maintenance, less for growth and reproduction– Need more water (higher evaporative water loss)– Must be bigEndotherms• Generate most body heat physiologically• Tend to be homeothermic– regulate body temperature (Tb) by adjusting heat productionRegional Homeothermy• Core body