1. Metabolism
Chapters 5-7

2. Chemical energy → (Electrical or Mechanical Energy) → Heat
Metabolism
Sum of all chemical reactions occurring in a living organism
Energy Conversion
Chemical energy → (Electrical or Mechanical Energy) → Heat
Material Conversion
Catabolism - break down of complex substances
Anabolism - build up of complex substances

3. Uses of Energy Biosynthesis Maintenance (homeostasis)
Replacing body structures
Growth
Reproduction
Storage (Fat, Glycogen)
Exported Materials
Maintenance (homeostasis)
External Work (e.g, movement)

4. Energy Metabolism Fig 5.1 Law of Entropy (2nd Law of Thermodynamics)
all metabolic processes involve a loss of free energy (organized energy  disorganized energy)
Animals require a constant input of organized energy (organic chemical bonds)
All energy involved in metabolism is eventually lost in the form of heat
Fig 5.1

5. Energy Metabolism Fig 5.4, Box 5.4 Energy usage by an organism
Rate at which organized energy is converted into heat
Calculation of Metabolic Rate
Direct calorimetry
measure heat production (kJ or Cal)
Indirect calorimetry
measure chemical changes
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (673 Cal, 2820 kJ)
Fig 5.4,
Box 5.4

6. Indirect Calorimetry: Oxygen Consumption
Almost synonymous w/ metabolism
NOTE: only indicates energy usage through aerobic respiration
Accurate measure of energy expenditure through aerobic respiration
Roughly equal heat generation per liter O2 by carbohydrates, fats and proteins
Table 5.1

7. Indirect Calorimetry: Carbon Dioxide Production
Amount of CO2 formed does not always equal amount of O2 consumed
Respiratory Quotient (RQ)
Amt CO2 produced/O2 consumed
Varies for different energy sources
Table 5.2

8. O2 vs. CO2
CO2 production is not as effective a measure of energy metabolism as O2 consumption
Energy yield per ml CO2 produced varies greatly
CO2 production can change easily through non-metabolic processes
e.g. hyperventilation

9. What Affects Metabolic Rate?
Physical Activity
Environmental Temperature
Digestive Processing (Specific Dynamic Action)
Body Size
Age
Gender
Endocrine Activity
Circadian Rhythms
Aquatic Salinity (Osmoregulation)
Fig 5.5

10. Measuring “Apples and Apples” Metabolic Rate
Define physiological conditions under which metabolism is measured
Basal metabolic rate (BMR) – homeotherms
Temperature in thermal neutral zone
Fasting
Resting
Standard metabolic rate (SMR) – poikilotherms

11. Metabolism and Body Size
Kleiber’s Rule
For eutherian mammals
Oxygen Consumption (VO2) = 0.676(Mass)0.75
Specific Oxygen Consumption (VO2/kg) = 0.676(Mass)-0.25
Small animals have relatively higher metabolic rates
E.g. shrews have 100x the per-gram VO2 as an elephant
Figs

12. Metabolism and Body Size
Marsupial Mammals
VO2 = 0.409(Mass)0.75
Passerine Birds
VO2 = 1.11(Mass)0.72
Non-Passerine Birds
VO2 = 0.679(Mass)0.72
Other Organisms
Ectothermic vertebrates
Invertebrates
Protozoa
Plants

13. Why Does Metabolic Rate Scale to Mass0.75?
Max Rubner – study on dogs
Small and large dogs have same body temperature
Heat must be produced in relation to heat loss
Heat production per square m2 surface area equal in small and large dogs
Large dogs have relatively lower surface areas
Rubner’s Surface Rule
Metabolic rate (heat production)  surface area

14. What’s Wrong With This?
If metabolism was directly related to scaling of heat loss, it should scale to Mass0.67
If related to heat generation and body temperature maintenance, why is it seen in ectothermic organisms?

15. Why Does Metabolic Rate Scale to Mass0.75?
O2 delivery mechanisms function  Mass0.75
Lung Ventilation  Mass0.75
Lung Volume  Mass1.0
Breathing Rate  Mass-0.25
Cardiac Output  Mass0.75
Heart Mass  Mass1.0
Heart Rate  Mass-0.25
Do these cause metabolism’s scaling, or does metabolism cause their scaling?

16. Why Does Metabolic Rate Scale to Mass0.75?
West et al. model - space filling fractal model
Biological distribution networks have a fractal design (branching)
Delivery of volumes of material to tissues approximated as spheres
Account for number of branchings needed to fill a given body volume (mass) , change in diameter, and delivery, flow to tissues α mass0.75
Supply limitation
Fig 5.12a

17. Why Does Metabolic Rate Scale to Mass0.75?
Darveau et al. model - allometric cascade
Overall MR = Σ various contributors to ATP turnover (materials supply and energetic demand)
b = Σ scaling exponents of these contributors
Scaling differs depending upon particular biochemical and physiological pathways activated
e.g., SMR - scaling dominated by demand (ATP usage)
e.g., MMR - both supply and demand influence scaling (O2 delivery vs. ATP usage)
Fig 5.12b

18. Other Explanations
Related to the noncoding DNA content of cells (Koslowski et al. 2003)
Larger organisms have more noncoding DNA
More noncoding DNA produces larger cells
Larger cells have relatively lower MR
Mitochondrial function (Porter 2001)
Relatively less inner mitochondrial membrane surface in the cells of larger animals
Less ATP turnover
Less proton leak

19. Why Does Metabolic Rate Scale to Mass0.75?