PNB Chapter 5

Why do we have Metabolism and Temperature Control

Consequence of biochemical processes (ingesting food to provide energy and molecules to maintain life) is generation of heat - must be dealt with by all animals

Energy Metabolism

All animals produce + consume ATP from food they eat to maintain internal homeostasis against effects of entropy

Biochemical processes that support life are not 100% efficient + release of energy is not all captured in the form of ATP - some lost as heat

Calculating rate at which animals consume energy

Simplify estimation: assuming that all biochemical reactions that occur within body are either:

Used to build complex structures from single molecules - anabolic reactions

Or degrade complex structures into single molecules - catabolic reactions

metabolic rate

Estimates all reactions happening within an animal

Can calculate metabolic rate (M_R) or the summation (E) of all the catabolic and anabolic reactions that occur within an animals body

Educated guess? - main assumption: Fuel + O₂ —> CO₂ + heat (what happens when food is consumed within an animal’s body)

• Assumes:

  • All animal cells are completely aerobic (in presence of O2) when consuming fuel - all animals must consume O₂ to survive, but individual cells with body can be temporarily anaerobic and still survive

  • Assumes all fuels (carbohydrates, lipids, proteins) metabolized in same was - clearly not case even though there are biochemical similarities in metabolism of carbohydrates

  • Assumes that 35-45% of chemical bond energy is converted to ATP, rest converted to heat (quantifiable)

Respiratory quotient

Ability of a fuel source to consume and generate O₂ and CO₂ 

Ratio of CO₂ produced as a function of O₂ consumed

M_R Measurement

Metabolic rate is best measured by total animal heat production

Most accurate way: place an animal within bomb calorimeter (steel container placed inside pool of water) + directly measure heat loss by animal

• Container given O2 and temperature (T°) of water is measured prior to placing animal in tcalorimeter, change in T° of the water is measured after period of time that animal resides with calorimeter

• Used to determine amount of calories food possesses

Homeotherms and poikilotherms

Homeotherm: describes an animals ability to generate sufficient internal heat to maintain its internal temperature at set point\

Poikilotherm: unable to do so

Size vs. M_^R

Animal increases in mass —> equivalent increase in M_R

More cells burn more energy and generate more heat

Similar formula for M_R based on te amount of food animals eat and whether they gain or lose body fat

M_R = energy intake - change in body fat

Change in body fat:

Body mass = BF + Fat-free mass (FFM)

Assumes: if we eat more food than we need, we gain weight, if eat less we lose weight

Concentration effects

Prolonged exposure to severe hypoxia always leads to death

Hypoxic conditions —> before there is permanent damage Mr stays constant —> plummets when exposed to hypoxia for too long.

Oher animals: seemingly capable of quickly decreasing their Mr during hypoxia + survive much longer in periods of hypoxia

Most commonly invertebrates can tolerate hypoxic conditions for long periods of time via anaerobic metabolism

Anaerobic metabolism

Results in formation of lactic acid + ethanol

Does not provide same amount of ATP as is seen in presence of 02 because oxidative phosphorylation is blocked by hypoxia

Internal parasites and sea naemones

Can tolerate long periods of hypoxia

Sea anemones survive hypoxia by producing lactic acid + ethanol at cost of increased energy absorption from seawater

Carp: can survive very hypoxic conditions during dry season in Africa when buried within mud

under 100m depth in open ocean

Anaerobic metabolism is necessity

Filled with animals that migrate upwards and downwards —> adapt to anaerobic conditions - slower metabolism + consumption of bacteria that consume H2 + CO

Surface of ocean + fresh water lakes + ponds is in equilibrium with atmosphere - winds and circulation

• Decreases in depth —> bacterial decomposition consumes available 02

Physics of heat transfer

Heat moves between animals by radiation, conduction, and convection

Heat: form of energy that changes the T° of a body - amount of energy required to raise T° of 1 liter of water 1°C (Calorie/4.2 kJ)

T°: measurement of steady state motion of molecules within body

Heat can be transferred from one body tho another by processes

• Conduction - heat transfer through physical contact between two bodies.

• Convection - heat transfer through mass movement of water or air past an animal, basis of the wind chill.

• Radiation - heat transfer through electromagnetic radiation.

  • Shorter wavelength —> hotter object, opp for longer object

• Evaporation - heat transfer through volatilization of water from the surface of a body.

Animal's absorption and emission of infrared radiation: wavelength dependent + dependent on the properties of skin, fur, feathers, less so on skin coloration

• Objects as

  • black boxes: absorb all infrared radiation

  • mirrors: reflect all of it

• Animals fall within middle of extremes

Internal temperatures

Animals work to maintain their core temperature at an optimal temperature

Core temperature

Not all animals produce heat at same rate or have same internal temperature Core temperature (Tc): temperature in ideally center of animal's body, as far away as possible from outer surface of bodies

• Measure commonly in or near heart or lungs

• Vary greatly among animals

Vertebrate vs invertebrate Tc

Vertebrates have much narrower range of Tc than invertebrates

In some animals Tc does not change significantly with ambient To while in other animals it does

Animals active during day —> Tc is highest during day

Nocturnal animals —> highest T° during night

Likely few metabolic differences if Tc varies by less than 2°C

Great difference: animals who have higher Tc are capable of greater metabolic activity

Tc dramatically affects the reaction rates of enzymes

All biochemical processes are T° dependent

10°C increase will commonly increase enzymatic activity 2 to 3-fold - Q10

Vertebrates with more stable Tc have more consistently higher level of activity Invertebrates activity is proportional to internal temperature —> can spend greater percentage of day unable to be fully active

Temperature Detection

Body temperature is detected by membrane cation channels

To control T°, animals need to detect own body To

• Express in their cell membrane transient receptor potential (TRP) cation channels - open and close as function of T°

• Channels gated by chemicals - can inform animals about changes in temperature, cooler or hotter, but unable to detect specific differences in T°

Heterotherms

Cannot self-generate enough heat to maintain core temperature

• Has characteristic of Tc regulation that falls between poikilotherms and homeotherms

• Wide collection of animals

Homeotherms

Can self-generate enough heat to maintain core temperature

Tc does not vary with environmental T° changes - mammals and birds

Poikilotherm

Tc varies considerably with environmental T° changes - insects and reptiles

Endotherms

Capable of self-generating all heat needed to maintain Tc at set point - mammals and birds.

Ecotherms

Cannot self-generate all heat needed to maintain their Tc - require external heat sources

ex: reptiles, snakes, butterflies, insects

Mesotherms

Can elevate Tc above ambient T° but not as well as endotherms - tuna, sharks, leatherback turtles

Giganotherms

Have low metabolic rates - shouldn't be able to maintain Tc above ambient T° but are able to do so because of high body mass + capacity to contain internal heat - large turtles, icthyosaurs, mosasaurs

Basoendotherms

Endothermic, but cannot maintain high Tc - tenrecs
Tenrecs: lowest Tc of any mammal, primarily nocturnal

Advantages and disadvantages of being endothermic

Disadvantages: animals spend less of day at preferred Tc - mercy of weather, animal needs to eat more food (metabolic rate is higher), they are less tolerant of Tc changes, need more sophisticated way to regulate Tc

Advantages: greater tolerance of T° extremes, need less energy to survive, can generate enough heat through metabolism to maintain Tc at preferred T^O, able to prosper in greater environmental range, able to always operate at preferred Tc

Thermoneutral zone

All animals have thermoneutral zone: expend least amount of energy for thermoregulation - outside zone all animals must expend extra energy to thermoregulate

What determines if an animal is endothermic, ectothermic or falls somewhere in between?

Genetics + proteins and physiological processes

Endotherms: much higher density of mitochondria per cell —> able to maintain higher level of cellular metabolism + heat production

Factors that affect Tc

Affected by genetics, activity, size, external coverings

Metabolic activity

More metabolically active —> more heat generated - true for homeotherms, heterotherms, endotherms, ectotherms

Endothermic homeotherms: higher innate capacity to generate heat —> maintain higher Tc

Homeotherms: need for more sophisticated regulation of Tc - relies on equally sophisticated nervous system

• 40-50% of all ATP consumed by homeotherms used to maintain Na+ and H+ gradient across cell membranes - maintenance of Na+ gradients essential for nervous system function

• Cell membranes generally less permeable in homeotherms compared to heterotherms

• Energy expenditure in homeotherms is critical - T° control depends on sophisticated nervous system

Environment

Aquatic animals, not mammals, have hard time keeping Tc above T° of water around them - large thermal mass of water in comparison to mass of fish

• Exception; Tuna

Tissue placement

Not all animal tissues are at same T°

Heat moves from one tissue to another in an animal's body by blood - actslike the heated water in furnace - moves through copper pipes in house delivering heat to all rooms

Adaptation: arrangement of blood vessels within legs of birds, especially penguins - acts as counter-current heat exchanger

• Warm arterial blood heats returning venous blood from foot - small amount of heat is lost from feet through conduction

• Counter-current arrangement of blood vessels used by tuna - increase Tc by 14°C above sea water T° in which they swim

Fever

Temporary increase in Tc as result of exposure to special compounds - pyrogens - commonly found on surface of bacteria and fungi

Liposaccharides - sugar and lipid combinations - resets Tc of homeotherms to higher T° - response to foreign infection, acts to improve ability of immune system to fight off infection

Size

Size matters when animals try to regulate their Tc.

• Larger animals: lower surface/volume ratio - each cell inside body is farther away from skin and outside environment

  • Consequence: danger of overheating because of inability to lose heat to the environment

Specific heat capacity (K)

Amount of heat (kilojoules) needed to raise T° of 1 kg of any material 1°C

Specific heat capacity of 1 kg of water (1 L): a Calorie

K affects how quickly internal T° is affected by external environment

Energy (joules) = Mass * K * delta T° or K = Energy (joules)/Mass •delta Т°

K is very different for skin + feathers

K: also ability of material to transfer heat

K changing

Can be changed covering feathers with oil (reducing ability to collect water), fluffing feathers, posture (minimizing the surface/volume), changing skin color (light vs. dark), seasonal hair (winter coat), swimming

• Changes affect both homeotherms and heterotherms

Genetics

Directly determines Tc in homeothermic animals + affects Tc in heterothermic animals but not in same direct way.

Mammalian Tc: determined genetically, less affected by environment

Lizard: optimal internal T° also affected by genetics but is affected by environment to greater degree

Genetics of homeotherms affects density of mitochondria within cells, ability of hearts to pressurize blood to move it quickly though out body, carrying with it heat, expression, properties of cell membrane transporters + other factors

How core body temperature is maintained

Balancing heat losses and gains

He+ Hc + Hr (heat losses) = Ht+Hc+ Hr + Hs (heat gains)

• He = heat lost by evaporation (not all animals)

• Hc = heat loss or gain by convection and conduction

• Hr = heat loss or gain by radiation

• Ht = heat gain by metabolic heat production (shivering and brown adipose tissue)

• Hs = heat gain by heat stored in the body

He

Unique way of losing body heat through evaporation of water from surface

Most effective animals that use evaporation: humans - evolved to lose most of surface hair + greatest number of sweat glands (> 5 million/person) —> likely consequence of enormously sophisticated nervous system

Ht

Generation of heat by all catabolic and anabolic reactions in all animals + shivering and thermogenesis by brown adipose tissue

• Shivering: uncoordinated contraction of antagonistic muscles —> no work and 100% of energy in ATP consumed lost as heat, only carried out by endothermic animals, requires sophisticated control of muscle activity, increases heat production from muscle activity by 20-fold

• Many animals possess brown adipose tissue + mitochondrial uncoupling proteins

Brown adipose tissue

High concentration of mitochondria - extensively vascularized, carry out usual oxidative phosphorylation but instead of using NADH + FADH2 to produce proton gradient to generate ATP, express mitochondrial anion transporters that dissipate proton gradient without producing ATP

All of energy transferred by NADH and FADH lost as heat

Uncoupling proteins

Wide range of animals + single cell organisms

Only in some animals (mammals) used to generate heat through classic gain-of-function mutation

Convert energy transferred by NADH + FADH2 —> oxidative phosphorylation —> heat not ATP

Other animals: increase metabolic activity by beating wings on cool mornings/general physical activity

Tropical animals: increase Ht much faster than non-tropical animals - reduced tolerance for cooler temperatures

Hc

Measurement of heat flow from or to environment to animals

Dramatically affected by insulation - fur, feathers, subcutaneous fat, trapped air (decrease convection and conduction)

Adaptation: arrangement of blood vessels within the legs of birds, especially

penguins - acts as countercurrent heat exchanger

• Warm arterial blood heats returning venous blood from food —> small amount of heat is lost from feet through conduction.

Other adaptations to reduce or gain heat through Hc: penguins huddling, large amounts of subcutaneous fat - blubber in mammals, counter-current heat exchangers - tuna or sweating

Tolerance to To Extremes

Tolerance to temperature extremes is not fixed

Mr and environmental T° - How external T° affects Mr in homeotherms and heterotherms

Outside an optimal environmental T° animals must expend energy to survive Difference in response of both types of animals to changes in Mr as function of environmental T°

Homeotherms: less affected by environmental T° than heterotherms - Mr does not change as rapidly with environmental T° as it does in heterotherms

Heterotherms: operate at peak efficiency at narrower temperature range, but can tolerate greater T° extremes than homeotherms

Lethal Temperature (T_L)

T° at which 50% of the animals die - generally 6°C above or below Tc in homeotherms, more variable in heterotherms

Affected by genetics (homeotherm vs. heterotherms) + prior exposure to temperature changes (acclimation)

Causes of death by lethal T^o

• Protein denaturation - irreversible loss of structural and enzymatic proteins both by elevated and reduced T°

• Enzyme inactivation - reversible decrease in enzymatic rates as a function of T° (Q10)

• Inadequate 02 supply - inability to supply 02 for increased metabolic demand

• Disruption of key biosynthetic pathways - causing cascade failure

• Changes in membrane fluidity

Protein denaturation

Protein denaturation requires elevated T° (>60°C) or freezing (<0°C), while animals die from far less T° extremes

Ex: homeotherms die at >43°C or <32°C and arctic heterotherms die at > 6°C and <2°C

• Protein denaturation is less likely cause of T_L

Enzyme inactivation

Real possibility as cause of a T_L - because of Q10 - 10°C change in body T° can affect the enzymatic rate by 2-to 3-fold

Cascade failure

Likely possibility is a T°-related disruption of key biochemical pathway - changes in key enzymatic activities —> failure of pathway

Change in membrane fluidity as a function of T° as a possibility

Powerful effects on membrane transport processes —> affect nervous systems, hormone signaling pathways, enzymatic activities

Tolerance Curves and Acclimation

T° tolerance curve: specific to animal, very different for homeotherms and heterotherms

• Changes in response to prior T° exposure

• Change requires gene expression, protein synthesis, availability of adequate 02

subzero temperature

Ice: occupies 9% larger volume when freezes because of ordered structure - important that animals prevent or regulate formation of ice crystals inside bodies

Formation of ice crystals can have osmotic consequences to cells beside physical destruction of cell membrane

Three approaches to control ice formation within tissues

Super cooling water through freezing point depression, preventing ice crystal formation, directing where ice crystals form and limiting their size

Super cooling ice

Ice formation: crystallization of water molecules into ordered structure - commonly requires seed crystal or surface that facilitates formation of ice

All seed crystals removed → water cooled to -40°C before freezes into ice

Enhanced by the presence of solutes to water - almost any solute will depress freezing point of water

• Better solutes: have non-colligative properties

• Colligative properties: depend on number of dissolved particles in water, not on identity

• Non-colligative properties: depend on chemical and physical properties of dissolved particles + concentration

antifreeze proteins

Most effective solute for suppressing freezing point of water

500-fold more potent in suppressing freezing point than glycerol

Fewer osmotic effects, bind directly to small ice crystals preventing their growth

Suppress freezing without affecting melting temperature of water - thermal hysteresis

Come in many different forms and have independently arisen multiple times during evolution

Minimize osmotic damage caused by ice crystal formation

limit the growth of ice crystals outside cells and to limit their size

Ice crystal growth inside cells is most damaging -- limiting formation to extracellular space is advantageous but still causes osmotic imbalances

Direct where ice crystals form - mostly aquatic invertebrates

Change osmolarity of extracellular and intracellular pools to prevent osmotic imbalances

Only invertebrates tolerate + control exposure to ice crystals

Hot temps

Increase heat loss by increasing surface/volume ratio, changing their surface coloration (lighter colors), to sweat if they can, decrease their metabolic activity

Cannot find the food to support metabolic acclimations to elevated temperature —> estivation - decrease HT production

expression of chaperone proteins

Acclimation process

Protect key proteins from denaturation - large family of proteins that aid new proteins into correct folding + protect existing proteinst agains denaturation

Torpor - Estivation and Hibernation

Animals use estivation and hibernation to avoid adapting to temperature extremes

extreme T° places

torpor: temporary suspension of normal T° regulating systems + allowing substantial change in Tc without any lasting harm

migrate: warmer or colder climate until season of T° extremes passes

• those that remain are capable of waiting out till season becomes warming - have access to enough food + make adaptations that allow them to

migration

Many animals choose to migrate - commonly isn't enough available energy in usual environment when season changes to much hotter or colder

torpor

Some animals: might not be enough available energy along way, or trip might involve crossing mountains or rivers or other obstructions,

trip might open them up to predation —> evolved ability to experience torpor to effectively decrease

needed energy to survive while tolerating either an elevated or decreased Tc

Costt: euthermia - energy needed to heat body when in cold climate or cool in hot

Cold climate: hibernation

• Tc decrease

Warm climates: estivaton

• Tc increase (estivation)

• Both terrestrial and aquatic animals

• Summer sleep —> reduce metabolic costs associated with cooling + prevents over heating and desiccation

Saves animals from finding food

Hibernation phases

• Thermal dormancy - continue to operate essential physiological processes at a lower than optimal Tc.

• Behavioral changes - looking for places (like burrows and caves for example) where they will protected as they hibernate.

• Metabolic changes - slowing breathing, movement, all anabolic reactions

Can allow just some portions of their bodies to decrease Tc - regional hypothermia or more whole body hypothermia

Humans: CNS-mediated hypothermia due to circadian rhythm

Deep hibernation

• Drop in metabolic rate

• Drop in Tc

• Decreased breathing

Black bears

Hibernates for 5 to 7 months

Decreasing Tc to 30°C, Mr by 75%, heart rate to 9 beats per minute, respiration to one breath per minute

Changed position, shivered to keep Tc above 30°C, gestated (allowed for fetal development), lactated (produced breast milk)

Don't eat, drink, urinate, defecate for up to 7 months

Prep: late summer - consume >50,000 calories of food, gain 20 lbs. of adipose tissue, shut down urine production

vertebrates

Hibernation triggered by signals sent to hypothalamus (a main region of the CNS that collects internal and external information)

Not clear what tissues and signals are responsible for hibernation in invertebrates

Liver produces hibernation protein complex (HPC) - found in plasma

Correct conditions —> HPC passes through blood-brain barrier into cerebral spinal fluid + induces hibernation

Aquatic animals

Move to waters of different T°

camel

Daytime T° exceeds 50°C and night time T° drops below 0°C

Can sweat but have limited access to water

• Drink up to 40 gallons of water in one sitting

• Lose very little water by breathing through their nostrils

• Possess oval, not circular red blood cells that are more stable and easier to move through desiccated blood

• Only sweat when their Tc exceeds 42°C, and allow their Tc to range from 34°C at night to 42°C during the day.

• Have fur that reflects sunlight

• Lose up to 25% of body weight during the day

• Urine with a consistency of syrup

• Feces so dry its commonly used as fuel