Extreme Temperatures and Thermal Tolerance All organism have a range of tolerable body temperatures...
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Transcript of Extreme Temperatures and Thermal Tolerance All organism have a range of tolerable body temperatures...
Extreme 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 Tolerance
Factors 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 Variation
Problems With High Temperature
• Denaturization of proteins– Structural and enzymatic
• Thermal inactivation of enzymes faster than rates of activation
• Inadequate O2 supply 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 O2 supply to meet metabolic demands
• Different temperature effects on interdependent metabolic reactions (“reaction uncoupling”)
• Membrane structure alterations• Freezing
Freezing
• 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 factor
Dealing with Subfreezing Temperatures
• Supercooling– Freezing point depression
• Use of antifreeze
• Freeze tolerance
Supercooling
• Water does not usually freeze at 0 °C– Freezing involves ice crystallization– Can occur spontaneously below 0 °C– Water can remain liquid until crystallization
occurs
Supercooling
• 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 °C
Antifreeze
• Antifreeze – substance that prevents ice crystal formation– thermal hysteresis - lowers
freezing point but not melting point
Freeze 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 structure
Freeze Tolerance
• Many freeze tolerant organisms have ice-nucleating agents– Promotes ice-crystal formation in the extracellular
fluid• Draws water out of the cells, intracellular
concentrations and freezing point
– Helps prevent crystal formation inside the cells• Prevents mechanical damage
Thermal Adapation
• Different species have adapted to differences in temperature between species ranges
Thermal 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 temperature
Mechanisms 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 adaptation
Temperature Regulation
Approaches 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 temperature
Thermoregulation 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 variation
Ectotherms and Cold
• Inactivity of enzyme systems– Cold-adapted species have
enzymes that function at higher rates at lower temperatures
• Subfreezing Temperatures– Supercooling
– Antifreezes
– Freeze Tolerance
Ectotherms and Heat
• Problems associated with heat– Enzyme denaturization and pathway uncoupling
– Elevated energy requirements
– Reduced O2 delivery
• affinity of Hb for O2 decreases with increased temperature
• Critical Thermal Maximum (CTM)– Body temperature over which long-term survival is no
longer possible
Ectotherms 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 heat
Ectothermy vs. Endothermy
Ectothermy – 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 activity
Ectothermy vs. Endothermy
Endothermy – 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 big
Endotherms
• Generate most body heat physiologically
• Tend to be homeothermic – regulate body temperature (Tb) by adjusting
heat production
Regional Homeothermy
• Core body temperature– Temperature at the interior of the
body (thoracic and abdominal cavity, brain, etc.)
– Maintained within narrow margins
• Peripheral body temperature– Temperature of integument,
limbs, etc.– Tends to vary considerably
Metabolism vs. Ambient Temperature
• Thermal Neutral Zone – basal rate of heat production balances
heat loss– No additional energy required to
regulate temperature, just modification of thermal conductance
• Lower Critical Temperature– Temperature below which basal
metabolism does not produce enough heat to balance heat loss
• Upper Critical Temperature– Temperature above which modifying
thermal conductance cannot balance net heat gain
Below the Lower Critical Temperature…
• Zone of Metabolic Regulation– Increase in metabolism to increase
heat production to balance increased heat loss
– Shivering, BAT, etc.
• Hypothermia– Increased metabolic production
cannot compensate for heat loss
– Tb decreases (as does metabolism)
Above the Upper Critical Temperature…
• Zone of Active Heat Dissipation– Animal increases activity to
increase heat loss
– Evaporative cooling
• Hyperthermia– Evaporative cooling cannot
counteract heat gain
– Tb rises (as does metabolism) towards CTM
Endothermic Homeothermy in the Cold
• Endotherms respond to low ambient temperatures by:– Increasing heat production (thermogenesis)– Limiting heat loss
Thermogenesis
• Shivering– Rapid contractions in groups of antagonistic muscles
– No useful work generated
– Heat liberated by hydrolysis of ATP
• Non-shivering Thermogenesis– Enzyme systems activated that oxidize fats to
produce heat
– Virtually no ATP production
Non-shivering Thermogenesis
• Brown Adipose Tissue (BAT)– Highly vascularized, with large
numbers of mitochondria– Inner mitochondrial membranes
contain thermogenin• Allows H+ to bypass ATP synthase• Protons re-enter mitochondrial matrix
and bind to O2, generating heat and water
– Heat absorbed by blood in vasculature and distributed throughout the body
Body Heat Retention
• Insulation– Fur/hair/feathers (pelage)
• Reduce effects of convection
– Fat/blubber• Lower thermal conductivity of integument
• Low metabolic activity (low perfusion needed)
– Aggregration• Reduce convection effects
Body Heat Retention
• Increased body size surface area/volume ratio– Generally thicker coats – Bergmann’s Rule
• size w/ latitude
Body Heat Retention
• Circulation– Reduced skin perfusion
• Limit heat loss from blood
– Countercurrent Exchange• Heat transferred from arteries to veins
• Limit heat loss from extremities
Endothermic Homeothermy in the Heat
• Endotherms respond to high ambient temperatures by:
1. Limiting heat gain
2. Increasing heat dissipation
Limiting Heat Gain
• Increased Size– Large animals have large heat capacities and
low surface area/volume ratios• Take longer to heat up
– Large animals tend to have thicker pelage• Insulate body from external heating
Increasing Heat Dissipation
• Specific heat exchange surfaces– Enable heat loss through
conduction/convection/radiation
– Thin cuticle
– Highly vascularized
– Lightly insulated
– Large surface areas
– Allen’s Rule• The warmer the climate, the larger the
size of appendages
Evaporative Cooling
• Sweating– Extrusion of water through sweat
glands onto the skin
• Panting– Evaporative cooling through the
respiratory system surfaces
Sweating vs. Panting
• Sweating– Passive (little energy expenditure)– High salt loss– No convection– No effect on blood pH
• Panting– Active (requires muscle contraction)– No salt loss– Convection – increases cooling– Increased ventilation pH
Panting and Brain Cooling
• Panting can cool brain during high levels of activity– Rete mirabile
• heat exchange between warm arterial blood and cooled venous blood from nasal cavity
– Maintain brain temperature despite abnormally high body temperature