The Battle Against Entropy: Energy MetabolismEnergy Metabolism is the overall process by which animals use anabolic and catabolic reactions to

acquire energy use energy to perform biological functions lose energy to their environment

Animals store and use potential energy as chemical energy in the covalent bonds of the molecules they build and break down

totipotent - can be used to perform any of these functions electrical energy in the voltage potentials across cell membranes

not totipotent - but is used for many functions we already have seen mechanical energy as the ability to do work due to the position or motion of a body component

not totipotent - is used for common functions such as locomotion or other movementsAnimals expend energy as

cellular work (metabolism, homeostasis, osmoregulation, etc.) mechanical motion (organized, simultaneous movement of molecules in one direction) heat (random molecular motion, unavailable to do work in biological systems)

Physiological work increases order decreases entropy

...in an animal's own systems. Energy for Biological FunctionsAnimals use energy for three main biological functions:

biosynthesis - manufacturing molecular and cellular body componentso some of this is maintained for the animal itselfo some is lost to the environment (examples?)

maintenance - metabolically maintaining body integrityo this energy is irreversibly lost as heat within the body

external work - applying forces to external environment objectso some of this is irreversibly lost as heat within the bodyo some of this is converted to potential energy (examples?)

Animal metbolism --> entropy. So animals must eat for energy to maintain order.

Units of EnergyRecall units of energy measure. We'll be referring to them.

One calorie (cal) is the amount of energy needed to raise 1 gram (=1 mL) of water by 1oC. One Calorie (kilocalorie, or kcal) is equal to 1000 calories. One Joule (J) is the Systeme International (SI) unit of energy One calorie = 4.186J One Watt (W) is equal to 1 Joule per second. (It's a rate)

Metabolic RateMetabolic rate is the rate at which an animal converts chemical energy to heat and external work.Metabolic rate of an animal

is a quantitative measure of its physiological mechanisms' energy cost determines how much food it must consume for maintenance determines its "drain" on its ecosystem: it procures energy from its environment is (usually) measured via its O2 consumption

Factors that Affect Metabolic RateA homeotherm's thermoneutral zone (TNZ) is the range of ambient/environmental temperatures within which

heat loss to the environment = basal metabolic heat productionIn the TNZ, animals need not expend extra energy to maintain a constant body temperature.Metabolic rates do not remain constant, of course.Various internal and external factors can affect how quickly animals use energy to maintain body temperature.

Basal Metabolic Rate (BMR)BMR is the metabolic rate of a homeotherm

at rest (lying down, but awake) unstressed fasting in its thermoneutral zone

(photo by Tanto Yensen)

Standard Metabolic Rate (SMR)SMR is the metabolic rate of a poikilotherm

at rest (awake, unmoving) unstressed fasting at a particular environmental temperature

And now for a QUESTION.

Routine Metabolic Rate (RMR)What does "at rest" mean in different species?Some animals are never truly motionless. (Can you think of examples?)RMR is the metabolic rate of a poikilotherm animal in its most motionless state.

at rest (awake, occasional movement) unstressed fasting at a particular environmental temperature

Metabolic ScalingSome physiological functions (e.g., gestation) vary predictably with size in a given taxon.Similarly, energy needs are not proportional to body size.Variation in metabolic rate with size is known as

metabolic scaling or metabolism-size relation.

Animals that eat the same type of diet but of different sizes

... have very different proportional caloric needs. (left)Whole body BMR is a function of body weight, but it is not proportional to body weight. (right)

As size increases, weight-specific BMR (Joules/g x hour) decreases.

Weight-specific BMRMetabolic scaling also can be visualized as weight-specific BMR. (left)Poikilotherms exhibit the same BMR dynamic, though their overall BMR is lower. (right)Relatively speaking, it takes more energy to run a mouse than to run an elephant.

Y = aMb

AllometryAllometry is the study of the effects of differential growth rates of the parts of a living organism's body.Animal metabolic rates are an allometric function of body size.Allometric equations are expressed as Y = aMb

Where: Y is a particular biological variable

o organ sizeo rate of a particular physiological processo circulatory system sizeo etc.

M is a measure of body size a is a constant b is a constant scaling exponent

Values of a and b constants are determined statistically: metabolic rate data are collected for multiple species constants that give the best fit to the relationship between MR and weight are calculated

MR = M0.75

Kleiber's LawMax Kleiber proposed that animal metabolic rate (MR) should be proportional to the 3/4 power of the animal's mass (i.e., b = 0.75)According to Kleiber's Law, the metabolic rate of a 5kg (5000g) catshould be only 32 times that of a 50g mouse.He attributed this difference in (adult) animals to the fact that

A large animal devotes more of its body mass to storage than a small animal does.

A small animal must provide relatively more body structure than a large animal.

Body structures require more maintenance energy than storage tissues do.

MR = aWb

Kleiber's Law RevisitedKleiber's Law does not take into account metabolic differences among species,such as between homeotherms and poikilotherms, etc.The allometric equation can be used to predict metabolic rate scaling with more accuracy.Where:

MR is metabolic rate W is body weight a is a constant (a measure of metabolic "intensity") b is a constant (but not constant across species)

The value of b tends to range between 0.6 - 0.8 (not static at 0.75). The value of a, is far more variable than the value of b.

Allometric Scaling of Metabolic RateFor example, birds have a much higher metabolic rate than amphibians.Therefore, in every case,

abird > afrog

This relationship holds true among different species among individuals of the same species

...and is represented at the left.Many physiologically significant items scale allometrically.

o brain sizeo heart sizeo heart rateo lung sizeo respiration rateo circulatory systemo energy requirement

Proximate reasons for allometric metabolic scaling are not fully understood. A century ago, many scientists believed it was simply a matter of relative surface area:

Small animals (high surface area : volume) should expend more energy to maintain heat.

Large animals (lower surface area : volume) should expend less energy to maintain heat.

But the actual phenomenon is more complex. The heat loss hypothesis does not apply to poikilotherms. b (allomtric equation) is highly variable among species. In fact, b can change in a single individual

o at resto performing different activities

The b "constant" appears not to be constant at all.

Ecological Application of MR ScalingAllometric scaling of metabolic rate can aid in predicting an ecosystem's carrying capacity, based on the identities of its interacting populations.Consider a woodland that harbors both

White-footed Mice (Peromyscus leucopus)o average weight 20g

White-tailed Deer (Hemionus virginianus)o average weight 70kg (70,000g)

Would 3500 mice (total weight = 70,000g) use the same amount of energy as one deer?

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Measuring Metabolic RateDifferent methods can be used to measure the metabolic rate of an individual.In many cases, care must be taken to account for factors that could lead to error.Direct measures are generally more accurate than indirect measures.

Direct Measure of Metabolic Rate: CalorimetryDirect calorimetry records the total heat produced by an animal

at rest while doing work

This is a direct measure of rate of metabolism.

Caveat Diet must be taken into consideration. CO2 released will equal O2 consumed only if the animal has been eating

only carbohydrates. CO2 released and O2 consumed will be significantly different for lipids

and proteins.The table shows conversion factors for calculating the amount of heat generated by consumption of one mL of O2.

(crickets are 65% protein)

Diet Matters: An ExampleIn a cricket that has been fed only glucose (100% carbohydrate):

If the cricket is consuming O2 at a rate of 10mL/min Then metabolic rate = 10mL/min x 21.1 J/mL O2 = 211 J/min

In a cricket that has been fed only protein: If the cricket is consuming O2 at a rate of 10mL/min Then metabolic rate = 10mL/min x 18.7 J/mL O2 = 187 J/min

In a cricket that has been fed only lipids: If the cricket is consuming O2 at a rate of 10mL/min Then metabolic rate = 10mL/min x 19.8 J/mL O2 = 198 J/min

Unless the exact diet is known, accuracy of respirometer metabolic rate calculations are uncertain.

Respiratory Quotient (RQ)The Respiratory Quotient (RQ) provides a metabolic signature of the macromolecules an animal is oxidizing.

moles of CO2 produced/unit timemoles of O2 consumed/unit time

RQ = 1.0 indicates animal is metabolizing only carbohydrates RQ close to 0.7 indicates the animal is metabolizing only lipids. RQ close to 0.8 indicates the animal is metabolizing only proteins.

Another caveat: Outside the LabIn animals not fed a strictly controlled diet,values other than the above are common.Many researchers use a standard conversion factor of 20.2 J/mL O2,understanding that the measurement may not be 100% accurate.Measurement of only O2 consumption (no attempt to quantify energy expended)

is a possible end run around this complication but will be inaccurate if anaerobic respiration is occurring anywhere in the body

Indirect Measure of Metabolic Rate: Material BalanceWhat goes in must come out, unless it's used for metabolic processes.Comparison of

food input vs.

fecal output urinary output shedding output (skin, hair, feathers, mucus, etc.)

...over a specified time interval should yield the amount of energy used for metabolism.This method is suitable only for long-term measurements of metabolic rates.

And then this happens...

SDA and DITIf an animal has not eaten for a given period of time, ingestion of food causes a short-term increase in metabolic rate.Specific Dynamic Action (SDA) is the energy expenditure above basal metabolic rate due to processing food for use and storage.It is a short-term response to a meal.SDA magnitude is approximately proportional to the amount of food eaten.The mechanisms of SDA are not fully understood. However... There is evidence that SDA is caused by intracellular work after absorption. This is at least partly the work required for cellular disposal of nitrogenous waste.Diet-Induced Thermogenesis (DIT) is the response of some animals to long-term, chronic high calorie intake.In some lucky individuals long-term increase in caloric intake induces a long-term increase in metabolic rate This prevents weight gain. The mechanism is not fully understood Individuals with higher proportions of brown adipose tissue have higher DIT levels.

Individuals with lower proportions of brown adipose tissue have lower DIT levels. Nature? Nurture? We don't know yet.SDA and DIT are interrelated, but not the same thing.Research in this area is ongoing.

Energy and GrowthEnergy absorption efficiency is a measure of how efficiently an animal can absorb nutrients from its food.

energy absorption efficiency = absorbed energy/ingested energy

For example... A human cannot digest cellulose. It passes, unchanged, from mouth to anus. A ruminant can absorb about 50% of the energy in cellulose. This is used for various metabolic processes.

Point: Cow

Gross growth efficiency and Net growth efficiency are measures of how efficiently an animal can turn energy into biomass.

gross growth efficiency = chemical bond energy of new tissue/ingested energy

net growth efficiency = chemical bond energy of new tissue/absorbed energy

Net growth efficiency declines with age.This explains why animals raised as food are slaughtered while still young.(But it does not explain why Dr. Krempels can have one beer and gain five pounds.)