BIOB34 Module 2 - Metabolism Lecture Slides PDF

Metabolism (and metabolic rate) Energy: The capacity to do work (SI unit: Joule, J) 𝐽 𝑘𝑔 𝑚 𝑠 Work: The transfer of energy by a force acting on an object as it is displaced (SI unit: Joule, J) 𝑊𝑜𝑟𝑘 𝐹𝑜𝑟𝑐𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐹𝑜𝑟𝑐𝑒 𝑚𝑎𝑠𝑠 𝑘𝑔 𝑎𝑐𝑐𝑒𝑙....

Metabolism (and metabolic rate) Energy: The capacity to do work (SI unit: Joule, J) 𝐽 𝑘𝑔 𝑚 𝑠 Work: The transfer of energy by a force acting on an object as it is displaced (SI unit: Joule, J) 𝑊𝑜𝑟𝑘 𝐹𝑜𝑟𝑐𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐹𝑜𝑟𝑐𝑒 𝑚𝑎𝑠𝑠 𝑘𝑔 𝑎𝑐𝑐𝑒𝑙. 𝑎𝑐𝑐𝑒𝑙. 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑡𝑖𝑚𝑒 𝑠 So, both work and energy have units: 𝐽 2 © K. Welch - Do not distribute Energy: The capacity to do work (SI unit: Joule, J) 𝐽 Work: The transfer of energy by a force acting on an object as it is displaced (SI unit: Joule, J) Power: The rate at which work is done (SI unit: Watt) 3 © K. Welch - Do not distribute Metabolism: The set of processes by which cells and organisms acquire, rearrange, and void commodities (e.g. elements or energy) in ways that sustain life Metabolic rate: An animal’s rate of energy consumption; the rate at which it converts chemical-bond energy to heat and…work (“Animal Physiology” by Hill, Wise, and Anderson) 4 © K. Welch - Do not distribute Animals are capable of astounding variation in metabolic rate (power) What is animal physiology? Integrated function Metabolism is cellular, biochemical… integrates up to whole animal metabolism/metabolic rate Protein structure 6 © K. Welch - Do not distribute  Biochemistry Metabolic pathways Series of reactions that convert substrates to products Catalyzed by enzymes Synthesis (anabolic) Degradative (catabolic) Metabolic pathways are linked by intermediates Metabolism – sum of metabolic pathways for the synthesis and breakdown of molecules. © K. Welch - Do Not Distribute 7  Cellular chemical energy Cells store energy in two main forms Reducing energy High energy bonds © K. Welch - Do Not Distribute 8  High Energy Bonds Energy can be “stored” in covalent bonds Energy is released when bonds are broken ATP is the most common “high energy” molecule © K. Welch - Do Not Distribute 9  Carbohydrates “Hydrates of carbon” Many hydroxyl (–OH) groups Glucose is the most common carbohydrate in animal diets Energy metabolism Biosynthesis – precursor to many other carbohydrates © K. Welch - Do Not Distribute 10  Monosaccharides Used for energy and biosynthesis Small carbohydrates have three to seven carbons – six is most common © K. Welch - Do Not Distribute 11  Complex Carbohydrates Polysaccharides Long chain of monosaccharides Energy storage Example: glycogen, starch, inulin Structural molecules chitin, hyaluronate, cellulose (in plants) Amylose + amylopectin = starch © K. Welch - Do Not Distribute 12  Glycogen Metabolism Main carbohydrate storage form in animals (vertebrates) Glycogen synthesis (glycogenesis) Glycogen breakdown (glycogenolysis) Note reciprocal regulatory enzymes which act on glycogen synthase or glycogen phosphorylase © K. Welch - Do Not Distribute 13  Glucose Metabolism Glucose breakdown (glycolysis) Produces reducing equivalents Releases energy Glucose + 2ADP + 2NAD+ 2ATP + 2 pyruvate + 2NADH + 2H+ Takes place in cytoplasm Does not require oxygen Produces intermediates for synthesis of various molecules Carbohydrates, nucleic acids, amino acids, and fatty acids End product, pyruvate, can be used in further catabolic processes © K. Welch - Do Not Distribute 14  Oxidation of Pyruvate in the Presence of O2 Glycolysis Converts carbohydrates to pyruvate within the cytoplasm Lactate and amino acids can also be converted to pyruvate Pyruvate is carried into the mitochondria Pyruvate dehydrogenase (PDH) Pyruvate is oxidized by PDH to form acetyl CoA + NADH © K. Welch - Do Not Distribute 15  Oxidation of NADH in the Presence of O2 Glycolysis can only continue if NADH is oxidized to NAD+ Two “redox shuttles” carry reducing equivalents from cytoplasm mitochondria -glycerophosphate shuttle Malate-aspartate shuttle Figure 2.29 © K. Welch - Do Not Distribute 16  Oxidation of NADH in the Absence of O2 NADH cannot be used rapidly by mitochondria when oxygen is not present NADH is oxidized in the cytoplasm Buildup of NADH (in cyto) means drop in [NAD+] This would inhibit glycolysis (since NAD+ is an important substrate) pyruvate + NADH + H+ lactate + NAD+ Catalyzed by the enzyme lactate dehydrogenase (LDH) Also reversible Other anaerobic pathways form less toxic end products and more ATP than lactate (2 ATP) For example, succinate (4 ATP) and proprionate (6 ATP) More common in invertebrates (we can’t make these compounds) Figure 2.30 © K. Welch - Do Not Distribute 17  Lipids All are hydrophobic (do not dissolve in water) Carbon backbone Linear – aliphatic Ring – aromatic Examples: fatty acids, triglycerides, phospholipids, steroids Lipids are used for energy metabolism, cell structure (e.g. membranes), and signaling © K. Welch - Do Not Distribute 18  Fatty Acids Chain of carbon atoms ending with a carboxyl group Usually an even number of carbons Saturated No double bonds between carbons Unsaturated One or more double bonds between carbons Figure 2.31 © K. Welch - Do Not Distribute 19  Fatty Acid Oxidation (-Oxidation) Fatty acids are more dense form of energy storage than carbohydrates Water associates with carbs, not with oils/fats More reduced form of carbon Take more O2 (more oxidation) to unlock energy No significant anaerobic ATP production possible Breakdown of fatty acids -oxidation Takes place in mitochondria Results in formation of Acetyl CoA Acetyl CoA is then oxidized © K. Welch - Do Not Distribute 20  Ketones Some tissues cannot metabolize fatty acids, but they can metabolize ketones For example, vertebrate brain, shark muscle – Ketogenesis Fatty acids converted to acetyl CoA Acetyl CoA converted to ketones Ketone bodies can move through circulation – Ketolysis Ketones are broken down to acetyl CoA Which can then participate in oxidative phosphorylation © K. Welch - Do Not Distribute 21  Mitochondrial (Oxidative) Metabolism Energy-yielding reactions that require oxygen Enzymes convert nutrients into metabolites Metabolites enter mitochondria Many metabolites are converted to acetyl CoA Acetyl CoA enters the tricarboxylic acid cycle (TCA or Krebs cycle) Acetyl CoA is oxidized to form reducing equivalents Reducing equivalents are oxidized to release energy O2 is final electron acceptor © K. Welch - Do Not Distribute 22  Oxygen and animals Metazoans = “animals” Carl Linnaeus (1707-1778) Joseph Priestley (1733-1804) © K. Welch - Do Not Distribute 23 All animals are ultimately dependent on O2 except….. © K. Welch - Do not distribute 24  Oxidative Metabolism Acetyl CoA  Tricarboxylic acid (TCA) cycle: acetyl CoA  CO2 + reducing equivalent (NADH and FADH2) and GTP  Electron transport system (ETS): reducing equivalents are oxidized to release energy  Oxidative phosphorylation: ATP synthesis (phosphorylation) © K. Welch - Do Not Distribute 25  Tricarboxylic Acid (TCA) Cycle Generates reducing equivalents within the mitochondria Acetyl CoA + 3NAD+ + GDP + Pi + FAD  2CO2 + 3NADH + FADH2 + GTP – Amphibolic pathway Some intermediates are broken down (catabolic) Some intermediates are used for syntheses (anabolic) © K. Welch - Do Not Distribute 26  Electron Transport System (ETS) Electrons from NADH and FADH2 are transferred to the ETS Found within the inner mitochondrial membrane Composed of four multisubunit proteins (complexes I, II, III, IV) and two electron carriers (ubiquinone and cytochrome c) Oxidation: 4e– + 4H+ + O2  2H2O Generates a proton gradient, heat, water, and reactive oxygen species (ROS) © K. Welch - Do Not Distribute 27  ATP Synthesis Phosphorylation: ADP + Pi  ATP Proton motive force (p) pH gradient and the membrane potential () F1F0ATPase uses energy in p to produce ATP There is no physical linkage between oxidation and phosphorylation Two processes are functionally coupled through p http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2003/Bennett/protein1.htm © K. Welch - Do Not Distribute 28  Phosphocreatine Figure 2.19 Alternative high-energy phosphate compound Creatine + ATP  ADP + phosphocreatine Creatine phosphokinase (CPK) Reaction is reversible so relative rate of ATP versus phosphocreatine production depends on ratio of concentration of substrates/products Phosphocreatine can also move throughout cell (like ATP) Thus, it can enhance flux of high energy phosphate molecules from site of synthesis (e.g. mitochondria) to site of hydrolysis (e.g. muscle sarcomeres) Figure 2.41 © K. Welch - Do Not Distribute 29  Integration of Metabolic Pathways Fluctuations in nutrient availability, energy demand, and environmental conditions Reciprocal regulation avoids simultaneous synthesis and degradation (futile cycles) Use of appropriate metabolic “fuel” Carbohydrate vs. lipid Energetic intermediates regulate balance between anabolism and catabolism © K. Welch - Do Not Distribute 30  Measuring metabolic rate: 31P-NMR Spectroscopy Measures ATP turnover Detects change in NMR spectra as Pi groups shift between ATP and inorganic phosphate Pros: Measures cellular energy currency Accounts for aerobic, anaerobic metabolism, etc. Accurate over extremely short time scales E.g. A single muscle contraction Cons: Note use of term ATP “turnover” Logistically difficult NOT “consumption” Subject must be restrained, possibly anesthetized [ATP] in cells tightly regulated, thus Equipment not portable, and complicated no large changes in amount at any one time © K. Welch - Do Not Distribute 31 Heat is a byproduct of all catabolic (and anabolic) steps © K. Welch - Do Not Distribute 32  Hess’s Law Hess’s Law: Total amount of energy released (eventually as heat) for breakdown of given amount of fuel always the same Regardless of intermediate chemical steps (e.g. particular ATP synthesis pathway) © K. Welch - Do Not Distribute 33 Measuring metabolic rate: direct calorimetry  Calorimetry:  Measurement of heat of chemical/physiological processes (unit can be ‘calorie’)  Pros: Quite accurate under many conditions Accounts for aerobic and anaerobic energy production  Cons: Subject must be restrained Equipment heavy and complicated Makes assumptions about anabolic versus catabolic activity © K. Welch - Do Not Distribute 34 Measuring metabolic rate: direct calorimetry  Calorimetry:  Measurement of heat of chemical/physiological processes (unit can be ‘calorie’) 1. Animal held in central chamber surrounded by two concentric chambers filled with ice or ice water 2. Exterior of the 2 is to buffer influx of heat from outside  Melts ice, but inner edge stays ice cold 3. Interior of the 2 melts due to animal heat production only 4. Water collected, volume measured  If we know how much heat it takes to melt a given amount of ice:  Calculation of metabolic rate © K. Welch - Do Not Distribute 35  Measuring metabolic rate: indirect calorimetry Indirect calorimetry Inferring metabolic heat production E.g. through respiratory gas exchange - Respirometry  Pros Relatively user friendly and easy Equipment can be portable Can be very accurate if assumptions met Can be easily used on active animals  Cons Dependent upon certain assumptions Aerobic metabolism only E.g. Known relationship between O2 (or CO2) and ATP Must sample gases effectively All relevant gases, limit leaks Animal ‘tied’ to equipment, at least © K. Welch - Do Not Distribute 36  Respiratory Quotient Type of fuel being used can be monitored by measuring the RQ (similar to Respiratory Exchange Ratio – RER) Respiratory quotient (RQ) = rate of CO2 production/O2 consumption What is actually used/produced by mitochondria? RQ = 0.7 for lipids = 1.0 for carbohydrates ≈ 0.85 for proteins Under many circumstances, catabolism of protein is negligible RQ can directly reveal ratio of carb/fat oxidation in such cases RER is measured at respiratory interface (actual breathing animal) – can be ‘uncoupled’ from what’s happening at mitochondria © K. Welch - Do Not Distribute 37  RQ and ATP turnover If the relative amounts of O2 and CO2 consumed and produced, respectively, differ depending on which fuel is being oxidized Does # of ATP produced per unit molecular O2 consumed vary as well? How does the ATP/O stoichiometric relationship vary with fuel type? © K. Welch - Do Not Distribute 38 Molecular stoichiometry of ATP production and O2 consumption O2 CO2 Isolated cells To produce a given # of ATP molecules: ADP + Pi ATP 14.9 – 18.7% more O2 required when oxidizing fats, compared to carbs Brand, 2005 © K. Welch - Do Not Distribute 39  Fuel use and oxygen consumption in the hummingbird O2 CO2 ADP + Pi ATP If the “cost” of hovering flight (i.e. amount of ATP turnover needed) is constant, but the fuel being oxidized varies… Hypothesis: Hovering VO2 will be 15-19% greater when hummingbird is fasted (burning fat) than when fed (burning carbs) © K. Welch - Do Not Distribute 40 VO2 vs RQ in the hovering hummingbird fat carb Welch et al. (2007) © K. Welch - Do Not Distribute 41 Sugar is a more “O2-efficient” fuel Anna’s 14.9 - 18.7% 16.4 - 18.0% Rufous Brand (2005) 16.2 - 16.8% Welch et al. (2007) © K. Welch - Do Not Distribute 42 Chamber respirometry  Animal enclosed in chamber  Pros  Easiest approach  More sure that all expired gases accounted for  More accurate  Quality of air provided (environment) easier to control  Cons  Animal constrained, less natural behaviour  Risk of asphyxiation  Can be messy  Animal may do all its functions in chamber © K. Welch - Do Not Distribute 43 Metabolic (chamber) treadmill  Chamber respirometry: closed system  Animal enclosed in chamber  Pros  More sure that all expired gases accounted for  Quality of air/water provided (environment) easier to control Animal consumes O2 in chamber  Cons O2 level (in air or water) drops  Risk of asphyxiation if O2 level gets too low, CO2 Typically, measured by detecting O2 % level gets too high or conc.  More acurate with longer time scales  Activity state must be known or constant  Switching between rest and activity complicates calculations © K. Welch - Do Not Distribute 45  Mask respirometry A variant of flow-through chamber respirometry in which the chamber only covers the mouth/nose/head Pros: Small volume, faster flow rates mean even greater temporal resolution; animal can behave nearly naturally Cons: Poorer signal to noise ratio; composition of gases harder to control; must assume gas concentrations in surrounding ambient air © K. Welch - Do Not Distribute 46 Mask respirometry example: hummingbird hover-feeding  Energy and changes in activity state There is not necessarily instantaneous matching between ATP turnover rate and rate of fuel (carb, fat, O2) delivery to, or supply in, the cell For example, in muscle cells, as we transition between rest and activity, the maintenance of [ATP] occurs via activation of three main pathways, each with different kinetics © K. Welch - Do Not Distribute 48  O2 consumption and metabolic rate O2 consumption/CO2 production measured at the respiratory interface with the environment (e.g. mouth/nose, gills, etc.) does not reflect instantaneous O2 consumption/CO2 production in tissues Nor does it necessarily reflect totality of ATP turnover – even if, integrated through time, all ATP turnover is essentially aerobically powered For example, during transition between exercise (high ATP turnover rate) and rest (low ATP turnover rate), there can be offset in tissue level O2 consumption or metabolic rate, and VO2 © K. Welch - Do Not Distribute 49  Integrating power and metabolic fuel source Inverse relationship between max power (rate of energy production) and sustainability (i.e. the size of the available energy store) © K. Welch - Do Not Distribute 50  O2 consumption and metabolic rate O2 consumption can remain high after return to lower activity level as muscle glycogen stores are replenished and anaerobic end products (e.g. lactate and creatine) are dealt with, using aerobic metabolism Lactate Cori Cycle: Used in gluconeogenesis in liver (glucose returned to tissues to rebuild glycogen) Lactate Shu le: Used as substrate (via conversion → pyruvate → acetyl CoA) in oxida ve phosphoryla on in aerobic ssue Creatine Rebuilding phosphocreatine at expense of ATP (now largely being generated aerobically) © K. Welch - Do Not Distribute 51  O2 consumption, metabolic rate, and diving O2 consumption (at ventilatory tissue) may not be possible Diving animal must cease/dramatically reduce exchange of O2/CO2 with environment Hypoxic/hypercapnic environment can pose similar challenges There can be compensatory hyperventilation Recover O2 “Blow off” stored CO2 Uncoupling of RER from RQ © K. Welch - Do Not Distribute 52 Metabolic rates: some definitions supramaximal metabolic rate 160 (anaerobic, unsustainable) Basal Metabolic Rate (BMR) 150 maximal (peak) metabolic rate Metabolic rate of homeothermic 140 (aerobic, sustainable) animal at rest (typically 130 quiescent phase), post- Metabolic rate or Power (W) 120 absorptive, at a temperature 110 within thermal neutral zone 100 Homeotherms only 90 FMR could be anywhere along this spectrum Standard Metabolic Rate 80 (SMR) 70 Same as BMR, except for 60 poikilothermic animal, at a defined environmental temperature resting metabolic rate basal/standard metabolic rate Poikilotherms, only torpid metabolic rate © K. Welch - Do Not Distribute 53 Metabolic rates: some definitions supramaximal metabolic rate 160 (anaerobic, unsustainable) Resting Metabolic Rate (RMR) 150 Metabolic rate of animal at rest maximal (peak) metabolic rate 140 under defined conditions (aerobic, sustainable) 130 Not necessarily during quiescent Metabolic rate or Power (W) 120 phase, or totally post-absorptive, or 110 within TNZ 100 Maximum Aerobic Metabolic 90 FMR could be anywhere Rate (VO2max) along this spectrum 80 Maximum sustainable VO2 70 Such as: 60 During intense aerobic exercise When homeotherm is exposed to very cold temps resting metabolic rate Supramaximal Metabolic Rate basal/standard metabolic rate Burst only torpid metabolic rate © K. Welch - Do Not Distribute 54 Metabolic rates: some definitions supramaximal metabolic rate 160 (anaerobic, unsustainable) Field Metabolic Rate 150 maximal (peak) metabolic rate The actual, realized metabolic 140 (aerobic, sustainable) rate of an animal behaving 130 Metabolic rate or Power (W) naturally in the wild 120 110 100 Daily Energy Expenditure 90 FMR could be anywhere along this spectrum Total energetic cost of a day of 80 life 70 NOT a metabolic RATE, an 60 energy amount Useful for considering ecology, resting metabolic rate survival, etc. basal/standard metabolic rate torpid metabolic rate © K. Welch - Do Not Distribute 55 Isometric scaling of Metabolic Rate 1 10 0.9 9 Mass-specific Metabolic Rate (W/kg) Whole-animal Metabolic Rate (W) 0.8 8 0.7 7 0.6 6 0.5 5 0.4 4 0.3 3 0.2 2 0.1 1 0 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0 0.02 0.04 0.06 0.08 0.1 0.12 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 56 Isometric scaling of Metabolic Rate 10000 10 9000 9 Whole-animal Metabolic Rate (W) Mass-specif Metabolic Rate (W/kg) 8000 8 7000 7 6000 6 5000 5 4000 4 3000 3 2000 2 1000 1 0 0 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 57 Allometric scaling of Metabolic Rate 0.6 10 9 Mass-specific Metabolic Rate (W/kg) Whole-animal Metabolic Rate (W) 0.5 8 7 0.4 6 0.3 5 4 0.2 3 2 0.1 1 0 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0 0.02 0.04 0.06 0.08 0.1 0.12 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 58 Allometric scaling of Metabolic Rate 600 10 9 Mass-specific Metabolic Rate (W/kg) Whole-animal Metabolic Rate (W) 500 8 7 400 6 300 5 4 200 3 2 100 1 0 0 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 59 Allometric scaling of Metabolic Rate 600 10 9 Mass-specific Metabolic Rate (W/kg) Whole-animal Metabolic Rate (W) 500 8 400 MR = 2.75Mb0.75 7 MR/kg = 2.75Mb-0.25 6 300 5 4 200 3 100 2 1 0 0 0 200 400 600 800 1000 1200 0.01 0.1 1 10 100 1000 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 60 Allometric scaling of Metabolic Rate 600 14 Mass-specific Metabolic Rate (W/kg) 12 Whole-animal Metabolic Rate (W) 500 MR/kg = 2.75Mb-0.33 10 400 MR = 2.75Mb0.75 8 300 6 200 4 MR = 2.75Mb0.67 MR/kg = 2.75Mb-0.25 100 2 0 0 0 200 400 600 800 1000 1200 0.01 0.1 1 10 100 1000 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 61  Scaling exponent? Scaling of metabolic rate is non-linear Best fit equations: Whole-animal MR 𝒃 𝑴𝑹 𝒂 𝑴𝒂𝒔𝒔 Mass-specific MR 𝑴𝑹 𝒂 𝑴𝒂𝒔𝒔𝒃 𝒂 𝑴𝒂𝒔𝒔 𝒃 𝟏 𝑴𝒂𝒔𝒔 𝑴𝒂𝒔𝒔 a = scaling coefficient b = scaling exponent © K. Welch - Do Not Distribute 62  Scaling exponent? Non-linear equations are harder to work with Log transform Best fit equations: Whole-animal MR 𝒍𝒐𝒈 𝑴𝑹 𝒍𝒐𝒈 𝒂 𝒃 𝒍𝒐𝒈 𝑴𝒂𝒔𝒔 Mass-specific MR 𝑴𝑹 𝒍𝒐𝒈 𝒍𝒐𝒈 𝒂 𝒃 𝟏 𝒍𝒐𝒈 𝑴𝒂𝒔𝒔 𝑴𝒂𝒔𝒔 Equation is ‘linearized’ © K. Welch - Do Not Distribute 63 Why scaling exponent < 1 for BMR in mammals? BMR is metabolic rate of maintenance function in homeothermic animal Perhaps maintenance of body temperature is paramount (match body heat production rate to rate of heat loss) Rate of heat production should be a function of volume The size of all the cells consuming energy and producing heat Rate of heat loss should be a function of surface area Heat is lost to environment across surfaces © K. Welch - Do Not Distribute 64  Scaling relationships D C B A Dimension Unit (e.g.) Equation Value A Value B Value C Value D Length cm = length 1 cm 2 cm 3 cm 4 cm Surface =6x cm2 6 cm2 24 cm2 54 cm2 96 cm2 area length2 Volume cm3 = length3 1 cm3 8 cm3 27 cm3 64 cm3 © K. Welch - Do Not Distribute 65 Scaling relationships: A sphere © K. Welch - Do Not Distribute 66  Prediction of scaling exponent for BMR in mammals Modeling mammals of different sizes as roughly spherical in shape: 𝟐 𝟑 Surface area ( ) scales with volume/mass ( ) to the power So, BMR can be predicted to scale with an exponent of 0.667 © K. Welch - Do Not Distribute 67 Kleiber’s Law Max Kleiber (1893-1976) “The Fire of Life” Kleiber M., Body size and metabolism. Hilgardia 1932; 6:315-53; http://dx.doi.org/10.3733/hilg.v06n11p315 © K. Welch - Do Not Distribute 68 Allometric scaling of Metabolic Rate 600 14 12 Whole-animal Metabolic Rate (W) Whole-animal Metabolic Rate (W) 500 MR/kg = 2.75Mb-0.33 10 400 MR = 2.75Mb0.75 8 300 6 200 4 MR = 2.75Mb0.67 MR/kg = 2.75Mb-0.25 100 2 0 0 0 200 400 600 800 1000 1200 0.01 0.1 1 10 100 1000 Mass (kg) Mass (kg) © K. Welch - Do Not Distribute 69  Scaling exponent for BMR in mammals Actually, surface area scales with an exponent of 0.63 Pretty close! In some cases, BMR scales with similar exponent comparing WITHIN a species © K. Welch - Do Not Distribute 70  Scaling exponent for BMR in mammals Surface area to volume scaling relationship should affect a lot more than just heat balance. E.g…. Gas exchange Across respiratory surface, but needed by a volume of tissue Nutrient absorption Across gut surfaces, but used by all tissues in volume © K. Welch - Do Not Distribute 71  Interspecific scaling exponent for BMR in mammals But BMR scales interspecifically with an exponent closer to 0.75 And, this scaling exponent seems to hold for more than just mammals E.g. comparing BMR or SMR Different groups have different scaling coefficients Why this scaling exponent? No clear answer! The debate rages on! © K. Welch - Do Not Distribute 72  Different scaling exponents for different activity states VO2max scales with exponent significantly greater than that for BMR 0.80-0.85 versus 0.75 But, VO2max clearly limited by O2 absorption, distribution No simple answer available © K. Welch - Do Not Distribute 73