Lecture 12.1: Thermal Biology and Metabolic Rate Flashcards
What is the temperature coefficient (Q10)?
allows to determine the effect of temperature on the rate of reaction
- think of it as ‘thermal sensitivity’ – different Q10 values means the reaction has different temperature sensitivities
- Q10 = 1: reaction is NOT temperature-sensitive – increasing temperature does not affect rate of reaction
- Q10 = 2: reaction rate doubles with each 10ºC rise in temperature
- Q10 = 3: reaction rate triples with each 10ºC rise in temperature
Equation for Temperature Coefficient (Q10)
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What are Arrhenius effects?
increase in temperature usually results in an increase in reaction rate
Do increases in temperature result in an increase in reaction rate forever?
NO
- temperature reaches a point where there is a maximum reaction rate – rate starts to decrease after this point
- this is because proteins and lipids start losing their physiological shape and function – proteins begin to denature, and membranes switch from gel to liquid (not as fluid)
What forms do membranes take at lower vs. higher temperatures? What does this affect?
- at lower temperatures, membranes have gel-like form
- at higher temperatures, membranes have liquid form
- this affects the fluidity of the membrane
- increase in fluidity at higher temperatures affects the function of proteins embedded in the membrane, permeability of the membrane, and the function of the cell – as a result, reactions would not occur properly
What is the transition temperature (‘melting point’)?
temperature at which more than 50% of lipids is in the form of liquid crystal (form between liquid and crystal), and membrane will transition from gel-like to liquid-like
What is the tolerance thermal strategy?
Tbody allowed to vary with Tambient
- if surrounding temperature increases or decreases, temperature of organism would also increase or decrease
What are the two main thermal strategies?
- tolerance
- regulation
What is the regulation thermal strategy?
Tbody does not vary with Tambient
- regulate body temperature to be able to to deal with environmental temperature
What are the two terms that describe the origin of thermal energy?
- endothermy: use internally generated heat to maintain body temperature
- ectothermy: do not generate heat, body temperature changes with the temperature of the environment
What are the two terms that describe the degree of constancy (how often a temperature is made)?
- poikilothermy: varies internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature
- homeothermy: maintains constant internal temperature
What are the two main types of animals in terms of thermal strategies?
- homeothermic endotherms: constantly keep temperature constant
- poikilothermic ectotherms: their temperature fluctuates, and they do not make their own temperature
What is the thermoneutral zone?
range of temperatures that are optimal for physiological processes
- lower critical temperature: lower boundary of TNZ
- upper critical temperature: upper boundary of TNZ
- metabolic rate is minimal (basal metabolic rate)
- different organisms have different TNZs – different living environments, and dealing with different situations (ie. tropical animals > arctic animals)
Describe metabolic rate below TNZ.
metabolic rate is high because more work is being done to produce heat
- the further away from TNZ, the higher the metabolic rate
- as temperature gets warmer and body gets closer to TNZ, metabolic rate decreases
Describe metabolic rate above TNZ.
metabolic rate increases to be able to cope with higher temperature
How does body temperature change below TNZ, in TNZ, and above TNZ?
stays relatively constant until ambient temperature is above TNZ – starts to slowly increase
How does heat production (metabolic rate) change below TNZ, in TNZ, and above TNZ?
- decreases as it approaches the lower critical temperature of TNZ
- lowest (basal metabolic rate – BMR) during TNZ period
- increase once ambient temperature is above TNZ
How does the capacity for heat loss (thermal conductance) change below TNZ, in TNZ, and above TNZ?
very low until it reaches TNZ, increases at TNZ, then remains constant above TNZ
- below TNZ, endotherms do not need to lose heat, therefore not much heat conductance – low constant
- at TNZ, there is some heat dispersion – some of the heat that has been produced is being lost
- above TNZ, heat dispersion is constant
What are the 3 mechanisms for losing heat?
- vasoconstriction/vasodilation
- piloerection (goosebumps)
- posture
Metabolic Costs of Thermoregulation in Humans
- metabolic rate increases as temperature decreases
- affected by acclimation conditions
Thermoregulation in Endotherms
What is thermoregulation coordinated by in mammals?
coordinated by the hypothalamus
- sensory neurons monitor core temperature and also receive sensory input about ambient temperature – information is sent to hypothalamus
- responds to changes in temperature by inducing changes in perfusion, heat production, heat dissipation, and piloerection
Thermoregulation in Endotherms
What is thermoregulation coordinated by in birds?
coordinated by the spinal cord
- similar mechanisms as mammals
Thermoregulation in Endotherms
What happens when cold?
- vasoconstriction of skin blood vessels
- brown adipose tissue (BAT)
- shivering
Thermoregulation in Endotherms
What happens when hot?
- vasodilation of skin blood vessels
- sweating
- panting
How do endotherms minimize heat loss to the environment?
by using insulation
- birds: fluffy down feathers trap a layer of air close to the skin, providing insulation
- mammals: hair traps a layer of air close to the skin
- some birds and mammals: layer of adipose tissue below the dermis (subcutaneous fat) provides an insulating layer
- aquatic birds and mammals: this layer is often thickened and forms a dense insulating layer of blubber
What does piloerection do?
increases insulation
- attached to each hair is a tiny erector muscle (smooth muscle) that pulls the hair upright, trapping a thicker layer of air next to the skin
- similar process occurs in birds
Mechanisms of Heat Production/Dissipation – Vasomotor Responses
How can altering blood flow to the body surface affect heat exchange?
when blood travels close to the surface of the animal, heat is lost across the skin
Mechanisms of Heat Production/Dissipation – Vasomotor Responses
What happens when cold?
- do not want to lose heat
- dilated AV shunt prevents blood from going close to the surface
- blood in arteries is redirected back to veins instead
Mechanisms of Heat Production/Dissipation – Vasomotor Responses
What happens when warm?
- want to lose some heat
- constricted AV shunt allows blood to continue flowing through arteries to the surface/skin where heat is lost
Mechanisms of Heat Production/Dissipation – Vasomotor Responses
Normal Body Temperature
- sympathetic nervous system constricts arterioles – tonic constriction
- mediated by adrenergic signals
Mechanisms of Heat Production/Dissipation – Vasomotor Responses
Higher Body Temperature
- decrease in tonic constriction
- increase in constriction of AV shunt
Mechanisms of Heat Production/Dissipation
What is shivering thermogenesis?
heat production with muscle contraction
- motor neurons stimulate myofibers of skeletal muscle to contract, but contraction of motor units is uncoordinated, such that there is no net shortening of the muscle
- as with exercise, shivering can deplete fuels leading to muscle fatigue
Mechanisms of Heat Production/Dissipation
How is low intensity shivering accomplished?
primarily with type I fibers oxidizing lipids
Mechanisms of Heat Production/Dissipation
How is high intensity shivering accomplished?
primarily with type IIb fibers using mainly muscle glycogen
Mechanisms of Heat Production/Dissipation
What is non-shivering thermogenesis?
heat production without muscle contraction
Mechanisms of Heat Production/Dissipation – Non-shivering Thermogenesis
What do endotherms have more of than ectotherms that is helpful in terms of heat generation?
endotherms tend to have leakier cellular and mitochondrial membranes than ectotherms, which is helpful in terms of heat generation
- leaky membranes create a futile cycle of ion gradient dissipation and ion pumping to reestablish these ion gradient, which creates lots of heat
- increasing membrane leakiness increases metabolic rate
- similar to the differences between small and large-bodied birds in Paper 3
Mechanisms of Heat Production/Dissipation – Non-shivering Thermogenesis
What do endotherms have more of than similar-sized ectotherms?
endotherm membranes have greater % unsaturation
- polyunsaturated fatty acids (double bonds, kinks, space between lipids, not as tightly packed) exert more lateral pressure (due to kinks) within a membrane than do saturated fatty acids due to molecular movement
- membranes with higher lateral pressure have higher Na+/K+ ATPase activities to counteract passive ion movement through leakier membranes – higher Na+/K+ ATPase activity leads to higher ATP turnover/greater energy transformations, and greater heat production
Mechanisms of Heat Production/Dissipation – Non-shivering Thermogenesis
What is brown adipose tissue in some endotherms?
specialized thermogenic tissue that generates heat using highly uncoupled mitochondria
- characterized by a large blood supply and numerous mitochondria
- present in many small mammals or neonates (high surface to volume ratio)
- environment can influence expression of brown adipose tissue – ie. more BAT in person with more cold exposure
- ie. bats: when signals are received from central nervous system via their receptors, BAT cells break down triglyceride and sends fatty acids to mitochondria, where heat is generated
Mechanisms of Heat Production/Dissipation – Non-shivering Thermogenesis
What is the mechanism of brown adipose tissue?
uncoupling is due to ionization of fatty acids, and uncoupling protein 1 (UCP1) may act as a fatty acid transporter to mitochondria
- after receiving signals from nervous system, signaling pathways are activated which results in breaking down triglyceride, producing free fatty acids
- fatty acids can be turned into acetyl-CoA in mitochondria OR can enter mitochondria via UCP1
- uncoupling is due to ionization of fatty acids
- low pH outside the mitochondria
- fatty acid and available H+ could turn into fatty acid (no charge)
- fatty acid is ionized (negative charge) and goes through UCP1, which generates heat
Temporal Heterothermy
What is temporal heterothermy?
changing the body temperature at different times
- at some times, they use metabolic heat to maintain their body temperature higher than the environmental temperature
- at some times, they maintain a lower body temperature, but the lower body temperature requires less energy to maintain
- ie. hibernating ground squirrels, hummingbirds, or willow tits in daily torpor
Temporal Heterothermy
What is torpor?
like hibernation, but occurring for less than 24 hours
- metabolic rate is decreased, allowing the body temperature to decrease (although it is still not allowed to fall below a certain temperature)
- ie. in some hummingbirds, body temperature can drop as low as 3.3ºC during daily torpor
- ie. mammals that undergo daily torpor include many bats, shrews, some rodents, tenrecs
Temporal Heterothermy
Some endotherms employ temporal heterothermy, such as hibernation or daily torpor. How does this affect metabolic rate?
decreases metabolic rate
- need to maintain their body – maintain muscle, density, body functions
- overall, because there are some periods where metabolic rate and temperature are low, endotherms can save energy
- for the same mass, hibernating animals have much lower metabolic rate compared to non-hibernating animals
Regional Heterothermy in Ectotherms
What is regional heterothermy?
regions in the body where temperature increases or decreases, or is different from surrounding environment
Regional Heterothermy in Ectotherms
Why is endothermy in water-breathing organisms difficult?
- water is around 25x more conductive than air – difficult to generate heat to compete with surrounding temperature
- marine endothermic mammals have very thick layer of blubber and breathe air
Regional Heterothermy in Ectotherms
Describe some examples.
- in different animals, confirmed regional endotherm in the muscle, cranium, viscera – modified O2-Hb binding characteristics
- in sharks (and tuna fish), red muscle, eye, and brain have higher temperature in low ambient temperature
- heater organs may be why ectotherms can maintain higher temperature in certain areas
Regional Heterothermy in Ectotherms
What does regional heterothermy in fish require?
- mechanism to generate heat
- mechanism to retain heat
Regional Heterothermy in Ectotherms
Describe red muscle in tuna.
- mammalian-like red muscle in tuna was found to be around 10ºC warmer than the environment
- spatial orientation of red muscle in tuna is unique compared to other teleosts – red muscle is moved from the surface to closer to the spinal column, which helps preserve heat that is produced by the muscle (while red muscle in salmon is closer to the surface)
Regional Heterothermy in Ectotherms
How does red muscle help fish?
tuna and other heterothermic fish can achieve higher sustained swimming speeds, but at higher aerobic cost
- circulatory system of tuna and other warm-muscled fish is designed to retain and localize heat in red muscle
Regional Heterothermy in Ectotherms
What is the rete mirabile?
complex of arteries and veins lying very close to each other
- allows for exchange between arteries and veins
- countercurrent exchange via rete mirabile are critical for many physiological functions – ie. kidney function, inflation of swim bladder
- retention of heat – many fish use this structure to maintain regional heterothermy
Regional Heterothermy in Ectotherms
What is the swordfish heater organ?
- swordfish selectively warm the brain, eye, and muscle
- source of heat is the superior rectus muscle – devoid of contractile filaments, but is rich in mitochondria and is highly aerobic, rich in sarcoplasmic reticulum and transverse tubules (T-tubules)
Regional Heterothermy in Ectotherms
How does the superior rectus muscle of swordfish (heater organ) produce heat?
- muscle cycles Ca2+ out and in of the sarcoplasmic reticulum at a high rate
- results in a high tissue O2 consumption rate to support Ca2+-ATPase (to return Ca2+)
- futile cycling of Ca2+ accomplishes no work, but produces heat
- as blood passes through the superior rectus on route to the eye, it is warmed
Regional Heterothermy in Ectotherms
What is the impact of increased eye temperature in swordfish?
warm eye improves visual acuity
- the study assessed the flicker fusion frequency of swordfish retina
Whole-Body Endothermy in Opah fish (Ectotherm)
What suggests whole-body endothermy in this fish?
visceral cavity, heart, muscle, and head are 3-6ºC warmer than ambient temperatures
Whole-Body Endothermy in Opah fish (Ectotherm)
How does body temperature vary throughout the day?
- maintain steady high level of body temperature over different times of the day
- when water temperature decreased (depth they were swimming at decreased) throughout the day, body temperature is maintained
Whole-Body Endothermy in Opah fish (Ectotherm)
Do these fish have rete mirabile?
yes – in the gills
- rete mirabile are surrounded by adipose tissue
- heat generated by continuously contracting pectoral fin muscle is retained using rete mirabile
Whole-Body Endothermy in Opah fish (Ectotherm)
What helps prevent heat loss in this fish?
layer of fatty connective tissues overlaying the pectoral musculature that isolates the fish and prevents heat loss
Ectotherms
How does resting or standard metabolic rate change with temperature?
increases with temperature as expected
- increases in temperature raise the average kinetic energy within a system, affecting reaction rate
- as temperatures get warmer, metabolic costs associated with basic metabolic functions increases
Ectotherms
How does maximal metabolic rate change with temperature?
increases with temperature, but often peaks and then declines as temperatures increase
What is aerobic scope?
metabolic capacity to carry out all physiological functions beyond maintaining cellular and whole-animal homeostasis
- differences between RMR/SMR and MMR
- excess capacity to deliver oxygen, supporting activity, growth, and reproduction
- magnitude of aerobic scope is heavily influenced by temperature, and is smaller at temperature extremes
